Atomic Structure (a review)

Why can't we see an atom if we have a powerful enough microscope?It's almost improper to say we see matter. What we see is reflected energy. When

you hit a bell, it resonates at a frequency depending on the properties of the bell. When radiated energy (photons) hit an atom, the atom resonates at a frequency (or frequencies) also. There is a narrow band of frequencies that we can see from about 400 nm to 700 nm in wavelength. Below this frequency is Infrared, energy we can perceive as heat, and radio waves. Above this frequency is Ultraviolet, and beyond.

When a wave of photons hit a group of atoms, the electrons are raised to higher energy levels. Energy is stored in the electron at a higher energy level. Eventually the electron acquires an excessive amount of energy than can be retained. The electron gives off the excess amount of energy, which gets radiated out (as photons or phonons), and falls back to the lower energy level it should be. This process is repeated endlessly. The rate of absorption and radiation of energy gives the radiated energy a frequency. Often that frequency falls into the spectrum of frequencies we see as visible light. Now that we know how we see, let's see what there is to see.

An atom is made up of a cloud of electrons surrounding a nucleus. The nucleus is made up of Protons and Neutrons, which are made up of quarks, which are made up of... (We may never know the answer to this endless question). An atom so small, it is on the edge of imagination. The nucleus of the atom is smaller, and electrons are smaller still. The electrons are made of not much more than energy, themselves. They have almost no mass, but a predictable amount of energy. The electrons are in motion around the nucleus, traveling at about 300,000 km per second. When you take into consideration how small the atom is, the electron is making a phenomenal number of revolutions per second. The question of where is the atom at any given instant is meaningless because we can't define an instant small enough to say when the electron is at any instant. The Uncertainty Principle may as well be a poem.

Most books show a picture of an atom with the electron in close proximity to the nucleus. A more realistic drawing would be difficult to put on a page. Speaking in non-specifics, if the nucleus were the size of a quarter, the first electron shell would be about a hundred meters away. Specifics would depend on Temperature, Pressure, and Gravity. The orbit (not the best word to choose) of one electron around an atom is roughly spherical, as with Hydrogen. In Helium, with two electrons in the first shell, the orbits of the two electrons take on the shape of a fat ice cream cones, opposite on another. The atom takes on a shape somewhat like an hourglass. This atom is constantly tumbling in a random pattern, influenced by external forces. The nucleus also is doing a random tumble with the protons rotating to chase the position of the electrons. In the next more complex atom, Lithium, the electron shell has three electrons. Since the first shell can only have two electrons, the third electron starts another shell. This orbit takes a toroid shape around the middle of the hourglass. This shell is also influenced at any given instance by where the electrons are in the inner shell, as well as external forces. As more complex structures are formed, the shape takes on more complex configurations.

The atom is primarily nothing but empty space, made of unimaginably small particles with relatively great distances between them.

These great distances are filled with absolutely nothing we can perceive as stable

1

matter or energy.What is there to see?

Conductors, Insulators, and SemiconductorsWhether an atom is a good conductor of electricity depends on how many

electrons are in the outer shell of the atom (the Valence Shell). One, two, or even three electrons in the valence shell make good conductors. The electrons are easily pulled away by external force. If the outer shell has seven or eight electrons, it's hard to free an electron. These elements make bad conductors, or good insulators. In between conductors and insulators is a group of elements called Semiconductors. They are neither good conductors, nor are they good insulators. Carbon, Silicon, and Germanium, are popular semiconductor materials.

These statements are true whether we are talking about atoms or molecules. Even a good conductor can make a bad conductor when included in a molecule. To put it in a simple case, Iron is a good conductor. When formed into molecules with Oxygen (as in Iron Oxide, or rust) it becomes a bad conductor. The valence electrons of the Iron are tied up by the Oxygen, and are no longer available to conduct electricity. Likewise, a semiconductor may be doped with another poor conductor to make a reasonably good conductor.

ConductionI have read some appalling stories about how electricity flows in basic electronics

books. I agree simplification is necessary, but it shouldn't be misleading. One such story says that electrons are at rest (not moving, shown sleeping) until a voltage is applied, and then suddenly take off at 186,000 miles per second. Another describes AC as electrons at rest, slowly increasing in speed until they reach a maximum, then decrease in speed to zero, then increasing slowly in the other direction to a maximum speed, finally slowing down to zero again to make a complete cycle. That such things should ever be taught, especially in the 1990's, is frightening.

Of course, that someone in the future may look back at my descriptions, and being equally appalled by my words, is also a possibility. Nonetheless, I continue...

Electrons (at standard temperature, pressure and gravity) are never at rest. They are constantly in motion from atom to atom, or molecule to molecule, at a speed of 300,000 km per second (or 186,000 miles per second, if you prefer), or closely at that speed anyway. I don't have any instrument that would measure the difference. Since this motion is random, there is no perceptible current.

When a voltage is applied to a conductor, the electrons of the conductor are both repelled by the more negative potential, and attracted to the more positive potential. The movement is almost instantaneous, although, it may take an individual electron a little longer to move from point A to point B (at normal temperature, pressure and gravity, anyway). At extremely low temperatures, super conduction becomes a factor, resistance disappears, magnetism does weird things, but that's another story.

2

If we could imagine what is happening somewhere along the conductor at the atomic level, we may see something like this:

As an electron feels the applied negative voltage, and the attracting positive voltage, it is motivated to leave its present orbit around the nucleus, and move to an atom closer to the more positive charge. This action leaves a hole (an absence of an electron) in the atom it just left. The atom with the hole now has a more positive charge than it used to, and attracts another electron from an atom closer to the more negative charge. This tension between the free electrons and the positively charged nucleus is the source of that quality we call voltage.

What we can imagine happening is electrons (negative charges) moving from negative to positive, and holes (positive charges) moving from positive to negative.

If you take a narrow necked bottle filled with water, and pour it out, do you see water coming down, or bubbles coming up? The same is true for electric current, with the electrons flowing in one direction, and holes flowing in the opposite direction. The concept of hole flow becomes important when we get to the study of semiconductors.

ElectromagnetismAn electron has a magnetic field. As long as all the electrons are moving in a

random direction, the magnetic fields cancel one another out, and no perceptible field is present. When a voltage pushes the flow of electrons in a unified direction, these magnetic fields add to one another and a magnetic field is present around the wire.

If we wind the wire into a coil, these magnetic fields add to one another, and a strong magnetic field develops around the coil. We can simulate a permanent magnet by applying a DC voltage to the coil. The negative side of the coil, takes on a polarity equal to the North Pole of a magnet. There is no notable difference between the magnetic field of a permanent magnet, and that produced by a current through a coil.

The electromagnet has the advantage of being one we can control. We can use this electromagnet phenomenon to make an electric motor, or an electric generator. Large electromagnets are used to move cars and scrap steel around in junkyards. There are countless uses for electromagnets.

InductionAn electromagnetic field crossing a conductor induces a voltage in the conductor.

Likewise, a wire crossing a magnetic field gets a voltage induced into it. As long as there is a difference of motion between the wire and the magnetic field, there will be an induced voltage in the wire. This happens at the component level, as well as at the atomic level. At the atomic level, the electron moving from atom to atom creates a magnetic field, which crosses the electron structure of neighboring atoms. The electrons of the neighboring atom are affected by this magnetic field, and the electrons are motivated to move also (but in the opposite direction as the electron that caused the magnetic field). This induced current is an opposition to a change in incoming current, and this effect we call inductance. Any conductor has an inductance of some kind (at standard temperature, pressure, and gravity). In superconductor environments this world is upset, see Bose-Einstein Condensate).

If we wind turns upon turns of wire around one another, this induction characteristic is magnified, and we create strong magnetic fields. If we place another

3

winding of wire close to the original coil of wire, we get an induced current in the second winding. This makes a transformer with primary and secondary windings.

TransformersIf we take one winding of wire, and wind a second winding around it, the

magnetic field produced by one winding will induce a voltage in the second winding. The voltage induced in the second winding will depend on the ratio of windings. If the first winding, we'll call it the primary, has the same number of turns as the second winding, we'll call it the secondary, the voltage on the secondary will be equal to that of the primary. (Neglecting any loss. In the classroom, all our transformers are perfect. In the real world, transformers are less than 100 percent efficient. It depends on the design of the transformer.)

If the secondary has more windings than the primary, we have a step-up transformer. That is, the voltage on the secondary will be higher than the primary. If the secondary has fewer windings than the primary, we have a step-down transformer. That is, the voltage on the secondary will be lower than the primary. An isolation transformer is designed to have the same voltage on the secondary and primary. In all cases, the voltage on the secondary has no reference to ground. That is, it provides isolation from ground on the primary circuit.

Please note than we cannot gain power in the transformer. If we step up voltage on the secondary, we have less current available to draw. If we step down voltage, we have more current available. Watts available in the primary (Volts times Amps) will always be the same in the secondary. That is, the same maximum Watts available. How much current we have flowing in the primary depends on how much current we have flowing in the secondary, which depends on the characteristics of the load.

Step-down transformers will have fewer turns of larger wire on the secondary. Fewer turns means lower voltage. The larger wire is to accommodate higher current.

So, how does drawing current from the secondary of a transformer result in more current flowing through the primary? What’s the connection? Are the electrons in the secondary linked to electrons in the primary by the magnetic field?

Yea! Right! And the universe is one infinitely inter-linked universal entity. Let’s get out of the dark ages, shall we? The answer is Permeability.

When we pass an AC current through a coil, it creates a magnetic field around the coil. The inductance (AC resistance) of the coil depends on the size of the wire, the number of windings, and the nature of whatever the coil is wound around. Is it air, ferrite, iron, lead? Each type of core material has a quality called permeability. If we bring a piece of metal close to a coil, it effects the permeability of the coil just as changing the type of core material would. This changes the inductance of the coil, which changes the current through the coil. (Somewhat like a metal detector, right?) The secondary winding, and its load, also affects the permeability of the primary winding. When we pull more current from the secondary of a transformer, the permeability of the transformer changes, which changes the inductance of the primary, which causes more current to flow.

4

Keep in mind, in a transformer, the magnetic field must be constantly moving. Transformers work on AC, not DC. If we apply DC to a coil, we get an inductive reactance only on the rising and falling edges of the signal. While the DC level is constant, the only effect the coil has on the circuit is the resistance of the wire. When the signal rises the magnetic field expands, and we have some degree of energy stored in the magnetic field around the coil. When the DC level drops, the magnetic field collapses, inducing a current flow in the wire in the opposite direction as the original signal. This inductive kick can be a hazard to the components of the circuit if it is not taken into consideration. Note the presence of protective diodes across coils, and transistors designed to drive inductive loads.

CapacitanceA capacitor is two (or more) plates of conductive material, separated by an

insulator. In a capacitor, the insulator is called a dielectric. Since there is no actual electrical contact between the two plates, it would seem that current would not be able to flow through a capacitor. The electrostatic pressure caused by the voltage being applied to the plates can cause electrons to be pushed off the more positive plate, resulting in a charge between the capacitor plates. The closer the plates are together, or the larger the plates, the higher the capacitance. The material of the dielectric also plays a role in capacitance. Different materials have a different dielectric constant (k). Air and a vacuum have a dielectric constant of 1 (the reference value by which all other materials are compared). To make a short list:

Material Dielectric ConstantAir 1Vacuum 1Waxed Paper 3.5Mica 6Glass 8Ceramic 100+ (depending on structure and type)Metal Oxides (higher)

What this means is that, roughly speaking, a capacitor made of mica one thousands of an inch thick would have six times as much capacitance as one of similar size of air or a vacuum. Many other materials are popularly used. Tantalum and Aluminum oxides are popular. Plastic films are good for making capacitors for audio circuits, or RF applications, or where high reliability is required. Consult a parts distributor's catalog for all the possibilities and applications.

I remember reading somebody's description of a capacitor's operation as saying that the charge of a capacitor was stored in the distorted field of the electrons in the dielectric. It would seem from that, a vacuum could not be used as a dielectric in a capacitor. I don't think that is so.

Another story I have read is that a capacitor stores electrons. I can’t let that one pass either. For every electron that goes into a capacitor, another electron leaves. The number of electrons stays the same. Capacitors store an electrical charge (not electrons).

5

When a capacitor charges, the electron entering the negative side pushes an electron off the positive side, storing a charge equal to one electron. But, the capacitor stores charges, not electrons.

The charge is stored in the area of the dielectric, between the plates, but it is improper to say the charge is stored in the material of the dielectric. This subject leaves a need for a better explanation. It can't be stored in the material of the dielectric, because even a vacuum may be used to make a capacitor. The material of the dielectric, as described above, in deed, affects capacitance but there is something missing to this story. Some would say that the charge is stored in the plates of the capacitor. This is a good concept. We can make a capacitor without material for a dielectric, but we can't make one without plates.

The best story I would repeat about capacitor operation concerns electrostatics. The charge is stored in the electrostatic field, between the plates. All the formulas work, and dielectric constants work into the formula.

To get up to date on capacitors, I remember my High School electronics teacher talking about capacitors and how large a Farad was. In his world of paper and plastic capacitors, a one Farad capacitor would "fill this whole room". Today, a one Farad capacitor is about a half cubic inch, and is used on CPU boards in place of a battery to keep power to CMOS RAM when power is removed. (High School was a long time ago for me.)

Questions I've never answered about capacitorsNot being one of those to claim to know everything, or make up answers that are

incorrect, just to have an answer (I hate when people do that). "An educated person is firmly aware of what he doesn't know," quote me on that one.

By what force does the capacitor actually transfer the energy from one plate to the other? Some books describe it in the same terms they use for magnetism, but, to me, magnetism is not at work here, is it? If we put two metal plates in a magnetic field, we don't get them to take on a charge. I have never done any experiments concerning capacitors in a magnetic field, and having it affect the capacitance, or charge the capacitor.

Some books say it is electrostatic forces at work, as opposed to electromagnetic. I have the same questions concerning this story. Never having done any experiments, I couldn't support or deny the story. It just doesn't sound like a complete theory to me.

We can readily show that the capacitor does work, but all our stories are empirical logic, not rational theories, supported by solid explanation of cause and effect.

Most of the explanation are only explanations-by-comparison, and say "it kind of works, like ..." Don't give me this type of explanation. It sounds like "The principle of Correspondence" from Hermetic philosophy. Such obsolete ways of thinking should have been tossed aside long ago. If you don't really know what you are talking about, say so. Don't make up a bunch of BS just to justify the letters behind your name, in hopes that nobody realizes you don't know what you are talking about.

If anybody has a better explanation of how capacitors work, I'm seriously interested in listening.

6

Voltage and CurrentVoltage is the motivating force behind the flow of electrons. Current is a matter of

how many electrons are in motion passed a given point in a given time.

To keep up with a current trend (no pun intended) to avoid the term "current flow" to describe the flow of electrical charges, I will try to avoid the use of the term here, also. Current is already defined as the flow of electrical charges, so the term current flow is like saying the flow of current flow.

Normally an atom has an equal number of electrons (with a negative charge), and protons (with a positive charge). In a conductor, when an electron is pulled away, it leaves a hole (an absence of an electron) and the forces between the electron structure and the nucleus result in a positive charge on that atom. This force is the basis of that quality we call voltage. This is a static force being exerted on the circuit at any given point with reference to another point. The presence of this voltage is what causes current to flow (whether you view current flow as positive or negative charges). Voltage doesn't flow, current flows.

Voltage is measured in Volts, and given the symbol V, or sometimes E, or EMF for ElectroMotive Force (literally, the force that motivates the electrons).

Current is measured in Amps, or Amperes, and is given the symbol A, or sometimes I, for Intensity of current flow. This is a measure of how many charge carriers (electrons or holes) pass a given point in the circuit in a given time.

ResistanceResistance is the opposition to the flow of charge carriers (electrons or holes) in a

circuit. It is a matter of how good a conductor the material is, as well as temperature, pressure and gravity. Temperature plays an important factor in resistance. Pressure and gravity have an effect in extreme conditions. This statement is true for any place in this paper that mentions STPG (Standard Temperature, Pressure and Gravity).

Resistance is measured in Ohms, and usually given the symbol R, or sometimes Omega, from the Greek alphabet. Since this text will be converted to ASCII, I couldn't give the actual symbol here; it wouldn't stay with the text.

Many texts still reference resistors as having tolerances of 20%, 10%, and 5%. I haven't seen a 10% or 20% resistor since I saw a vacuum tube. Most resistors today are 5%, with 2% values coming into popularity at a reasonable price.

The color code follows in later text, but is not worth elaborate discussion. It is worth getting to know. You can find it in any book on Basic Electronics. What is usually omitted is the fact that this same color code scheme is used for capacitors, and even diodes in some cases. Some manufacturers also use this same color code to refer to color-coded wires. A Blue wire with a red stripe would be called wire 62, and would be unique in that circuit. This makes troubleshooting a lot easier if you have to trace a wire down.

Another point not mentioned enough is that resistors don't come in all possible values you can make up with the color code. 5% resistors only come in certain values.

7

OHM’S LAW

Many of the "Grand Old Men of Electricity" got their name tied to an aspect of electronics to which they made contributions. The "Electromotive Force" of Volta's days is now called Volts. The "Intensity of Electrical Current" of Ampere's days is now called Amps. In a way it is a shame to getaway from descriptive names of these characteristics of electricity and use the non-descriptive terms Volts and Amps. Having said that, we will move the conversation along to Ohm's Law. Ohm states that:

I = E/R

In words, the formula states, “The current in a circuit is proportional to the applied voltage and inversely proportional to the resistance.” What he was trying to say is that current in a circuit increases with increasing applied voltage, and decreases with increasing resistance. (My kids would say "well, duh!")

E is in Volts. E is for EMF (Electromotive Force); the force that motivates the electrons. This is the electrical pressure in the circuit that pushes (-), or pulls (+) the electrons through a circuit.

“I” is in Amps. This is the actual amount of current flowing through a circuit, or a part. “I” for Intensity.

R is in Ohms. This is how much opposition to current flow a component has.

As long as we know two of these factors, we can find the other.

E = I x R

Amps times Ohms gives us Volts.

EI = ---- R

Volts divided by Ohms gives us Amps.

ER = ----

I

Volts divided by Amps gives us Ohms.

8

Power, measured in Watts, is an indication of how much heat a part, or a circuit, will give off, or consume. This is strictly a mathematical computation, but does equate to other units of heat or power in other sciences.

P = E x IVolts times Amps gives us Watts.

PE = ---- I

Watts divided by Amps gives us Volts.

PI = ---- E

Watts divided by Volts gives us Amps.

9

Basic Circuits 1

The objective of this session is to get you familiar with the fundamentals of reading a schematic, and applying Ohm’s Law. We will introduce you to a few schematic symbols and build simple circuits.

Referring to schematic document number “BC 1”, these are the schematic symbols of a battery, a switch, an incandescent lamp, and a resistor. Batteries and switches you may already be familiar with, we’ve all handled both.

ResistorsResistors are devices made of some member of the semiconductor family, usually

carbon, but not in all cases. We use resistors to tailor the amount of current we want to flow in a circuit. The schematic symbol is, as shown in the drawing.

For basic electronics classes we use resistors to symbolize “any component, in general”. All components have some degree of resistance, and can be substituted by a resistor for the purpose of learning the basics of Ohm’s Law. For Ohm’s Law, it doesn’t matter if the 100 Ohms in the circuit is a real resistor, a lamp, a heater, a motor, or what ever. It is “some component” that has a certain resistance.

Later we will get around to using real parts and learn their characteristics. For basic Ohm’s Law, we are only concerned with the resistance of these devices, and use resistors to symbolize their presence in a circuit.

FlashlightThe top drawing is a schematic of a flashlight. We have a battery, a switch, and a

lamp. The other side of the lamp returns to the other side of the battery, completing our circuit. We must have a complete circuit for current to flow.

We apply power from a 6 Volt battery. Our switch is either On (zero resistance and allows current to flow), or it is Off (infinite resistance, preventing current from flowing). (In the real world these extremes are not found, but this is basics, so our components work perfectly and simple.)

When we turn the switch on electrons leave the negative side of the battery (the smaller line), travel through the switch, through the light, and to the positive side of the battery. When the switch is off no current can flow, and the light goes out.

We have a 6 Volt battery and a light that runs on 6 Volts, so our world is correct. How much current do we have flowing? The manufacturer of the lamp states that at 6 Volts, the lamp should draw 0.20 Amps. Ohm’s Law states that E/I = R, so we have 6 V divided by 0.20 Amps, or 30 Ohms.

10

Roll your own flashlightConsider that you are McGyver, trapped in a room of electronic components and

needed a flashlight. Looking around you, you find a 9 Volt battery, a 6 Volt lamp (rated at 0.20 Amps), and an assortment of resistors. If you try to run the lamp off of 9 Volts, it will exceed its ratings and blow out. You need to limit the current through it to 0.20 Amps, from a 9 Volt source. What resistor do you need?

You are applying 9 V. You know the lamp will drop 6 V at 0.20 Amps. A quick calculation (9V – 6V) shows that the resistor will have to drop 3 Volts across it. Ohm’s Law, again, 3 V / 0.20 A = 15 ohms. We look through our drawers of resistors and pull out a 15-Ohm resistor, and build our circuit.

11

What component do we use to select the amount of current we want to flow in a circuit?

1) What is the schematic symbol for a resistor?

2) What is the schematic symbol for a battery?

3) What is the schematic symbol for an incandescent lamp?

4) What is the schematic symbol for a wire?

5) What is the schematic symbol for a switch?

6) In order for current to flow we must have _________________.

12

COMPONENTS

ResistorsThe purpose of resistors is to determine the amount of current we want to flow in

a circuit. Resistors (perfect resistors, anyway) have the same resistance to AC or DC. If you have had the chance to play with components, one of our basic exercises is a series circuit made up of a resistor and an LED. The voltage drop across the LED is fairly constant over a range of operating current. We select the resistor to get the current we desire to flow through the LED.

A resistor is a semi conductive material, typically carbon or some metal oxide, that is neither a good conductor, nor a good insulator.

Reactors (capacitors and inductors)In contrast to resistors, there is a group of components called reactors. They

exhibit a different resistance to AC than to DC. These are basically capacitors and inductors. Reactance is resistance to an AC signal. Capacitors have Capacitive Reactance. Inductors have Inductive Reactance.

ResistorsResistors are made of a semiconductor material, usually carbon, and are rated

according to how much resistance it has, in ohms. Leaded components are typically cylindrical in shape, with leads coming out the ends (axial case). Colored bands show the rating of the resistor, in ohms. The physical size of the resistor indicates its wattage.

The color code used for resistance is an industry standard, used for many other components. This color code is worth getting to know.

The first three bands follow this system:

Black 0Brown 1Red 2Orange 3Yellow 4

Green 5Blue 6Purple 7Gray 8White 9

The first two colors are interpreted as numbers (significant digits). The third is the multiplier (how many zeros should be added to come up with the resistance). For instance, a resistor with yellow, violet, and red stripes would be 4700 ohms.

To get resistor values below 10 ohms, the third band may be Gold or Silver. A Gold third band indicates a value between 1.0 and 9.1 ohms. A Silver third band indicates a value between 0.1 and 0.91 ohms.

The forth band gives the percentage value:

Gold 5%Silver 10% (seldom seen anymore)(none) 20% (really old, you are not likely to ever see these)

13

Many schemes have been created to aid in remembering the color code.

Black (Bad) 0Brown (Boys) 1Red (Race) 2Orange (Our) 3Yellow (Young) 4Green (Girls) 5Blue (But) 6

Violet (Violet) 7Gray (Generally) 8White (Wins) 9

(the)(Gold) 5%(and)(Silver) 10%

14

Not all possible values are represented. The pattern they follow is another thing worth spending some time with, and getting to know. 5% values are 5% values, whether they are resistance, capacitance, voltage, or anything else. The pattern formed is the same.

1011121315161820

2224273033363943

4751566268758291

Surface mount resistorsSurface mount components are small rectangular, flat packages. The top surface is

usually black and the sides are white. The resistance is stated in numbers, as if they were colored bands. A resistor of 4700 ohms would say “472”.

Surface mount resistors, being smaller packages, cannot dissipate the heat a larger package can, and typically have ratings of 1/8th, 1/10th, or 1/16th of a Watt. These are a ceramic substrate with the resistive material covering the top side. A coating covers the resistive material, usually black. The ends are covered on three sides (top, end, and bottom) with a metal cap that makes the connection to the board.

2% and 1% devicesOn occasion you may find resistors with five colored bands, instead of four. The

fifth band will usually be Brown (1%), or Red (2%). These values have three significant digits, followed by the multiplier, and Brown or Red. A resistor with Brown, White, Brown, Red, Brown stripes would be 19,100 ohms, 1%.

Most resistors you will encounter in the gaming industry will be the 5% variety.

Another thing you may want to watch out for. On schematics, decimal points often become blurred, or faded out. There is a trend in schematic creators to label resistor values, avoiding decimal points. Instead of labeling a resistor as 1.2K ohms, it will say 1K2. In either case the resistor is 1200 ohms.

ConstructionMost resistors today are “Carbon Film” construction. Physically, these are a

ceramic rod with carbon film coating it, and metal caps on each end to connect the leads on. A plastic coating covers the resistor, and colored stripes show the rated value.

Higher wattage resistors are usually “Metal oxide” instead of carbon. If you remember from earlier lessons, metal oxides are not good conductors, and the oxides may be tailored to give a specific resistance.

Older resistors were “Carbon Composition” construction. These devices are a rod of carbon with leads attached, and covered with a plastic (brown) coating. These are the ancient devices that came in 20%, or 10%, and seldom 5%, ratings. Avoid these devices. Carbon Composition devices offer no improvements, and have a few design flaws.

Just to give you an idea of how old these devices are, the last time I saw a 20% resistor in a circuit, it was a hand wired chassis with vacuum tubes. The last time I bought them, they were manufactured in 1971.

Carbon Composition devices change effective resistance at higher frequencies. At high frequencies we get a phenomenon called “Skin Effect”. The current tends to stay close to the surface of the device. For carbon composition devices, this means that the whole resistor no longer affects the current.

Metal oxide, and metal film devices are a ceramic rod with all the resistor material on the surface. Higher frequencies do not get this skin effect.

Failures in resistors

Other than suffering physical damage, the only way resistors fail is to burn up, and open. Resistors do not short out. A given physical size case is rated at a certain wattage, typically ¼ W. As you pull current through it, we get a voltage drop across the resistor (I x R). When the voltage drop and current exceeds the rated wattage value (E x I), the body can no longer dissipate the heat, and it gets hot. In extreme conditions the resistor may get so hot it actually bursts into flame. Metal film devices are capable of dissipating more heat for a given size package. Metal film resistor also are covered with a colored ceramic coating, instead of plastic. These devices are flameproof. They are guaranteed not to burst into flame. They may get hot enough to burn another nearby component, but the resistor will not burst into flame.

Wire Wound resistorsLower wattage resistors are typically 1/4 W. Some are 1/8 W, or 1/2 W, or 1 W.

Above 1 Watt, we need a design that can tolerate higher temperatures. The resistive element is usually a metal wire that is not a good conductor, like tungsten, or a compound like Nickel-chromium (nichrome). The case is ceramic material, round or square, to tolerate the high heat that may be encountered. Usually these devices have their values printed in numbers, or as a coded part number. Colors tend to change under heat, so colored bands are avoided.

Resistance

Continuity is (theoretically) zero Ohms. No resistance. An open is (theoretically) infinite resistance. Maximum resistance. The real world seldom actually sees either extreme. Continuity is any resistance low enough to allow current to flow easily. An open is a break in the circuit that allows no meaningful current to flow. Resistance is what the real world sees as an opposition to current, measured in Ohms.

Wire has some resistance, usually measured in Ohms per Foot, and is usually some number close to zero, a small fraction of one Ohm for most measurements. For long runs, even this resistance can be meaningful.

CapacitorsA capacitor is two (or more) conductive plates separated by an insulator. They

have a very high resistance to DC, and a resistance to AC that changes with frequency. Capacitors have less resistance as the frequency gets higher.

The forces at play here are the electrostatic charges that build up on the plates. Even if there is no complete path through the capacitor we can still pass a change in voltage through the capacitor. As we apply a voltage to one plate of a capacitor it builds up a charge on that plate. The insulator between the plates is so thin that the electrostatic field on one plate can be felt through the insulator and move affect electrons on the other plate, leaving a charge between the plates.

For the purpose of capacitors (and batteries) this insulator is called a dielectric. A capacitor can store a charge between its plates, just like a battery. This is another characteristic of capacitors, the ability to store a charge.

Capacitance is measured in Farads. One farad is a relatively large value. Most capacitors are rated in micro-Farads (millionths of a Farad). This value indicates the capacitors ability to store a charge. A capacitors resistance to AC is called Capacitive reactance, and changes with the applied frequency according to the formula:

Xc = 1/(6.28 x F x C)

Xc is Capacitive reactance, measured in Ohms6.28 is actually 2 x pi (3.14159…)F is the frequency, in HertzC is the capacitance, in Farads

InductorsAn inductor is basically a winding (or windings) of wire. Inductors have very

little resistance to DC, and a resistance to AC that also changes with frequency. Just opposite that of capacitors, as frequency gets higher inductors have a higher resistance.

Light Emitting Diodes

w - wavelength of the light emitted, in nm (nano-meters), approximate valuesV - Typical forward voltage of the junction

w Color V940 IR 1.3 to 1.7 (lowest frequency)900 IR 1.2 to 1.6880 IR 2.0 to 2.5690 Red 2.2 to 3.0640 Red 1.6 to 2.0615 Orange1.8 to 2.7590 Yellow2.2 to 3.0565 Green 2.2 to 3.0430 Blue 3.6 to 5.0370 UV (recently developed and available for sale)

The white 2-leaded LEDs are really blue LEDs with a phosphor surface that glows white.

As with any diode, the forward conduction voltage depends on the chemistry of the semiconductor material. All diodes emit light somewhat. The light comes from electrons combining with holes in the junction, and releasing excess energy as radiation. With proper construction and materials, we can design Light Emitting Diodes with radiation in specific frequencies we can see. In doing so, the forward conduction voltage raises to 1.2 V, to 3.0 V, or more. The reverse breakdown voltage also becomes dangerously low (around 4 to 6 Volts), and needs to kept in consideration when designing. In a Germanium diode, this forward voltage would be closer to 220 mV. In Silicon, it may be 600 mV to 1 V. Most LEDs are single junction devices with unusual forward voltages. Some of the materials used in LEDs are Indium Phosphide, Gallium Arsenide, Gallium Arsenide Phosphide, Gallium Phosphide, Gallium Nitride, just to name a few.

In contrast, your typical incandescent tungsten light covers 400 nm to 950 nm, and is more yellow than white. It lacks somewhat in the UV range. A good white light would cover from UV to IR. (Maybe a Mercury Vapor or Halogen bulb might be closer to white.)

The sensitivity of CdS cells covers a range from 500 nm to 650 nm, and lacks sensitivity in the IR range. I just thought I'd throw that in here. It didn't seem to fit anywhere else.

SemiconductorsSemiconductors are materials that have (typically) four electrons in the valence

shell. As such they are neither good conductors, nor are they good insulators. We have here the opportunity for creativity. Silicon, and Germanium, are among the more popular semiconductors we use to make diodes and transistors. In their pure form they take on a crystalline structure.

Pure silicon, or any other semiconductor, has a specific resistance. Depending on purity that resistance may be in the tens of thousands to hundreds of thousands of ohms. Impurities play a major role in the resistance of the crystal. Heat is also a factor. As the crystal gets hot more current carriers (electrons or holes) are freed from their bonds, allowing resistance to flow easier, lowering resistance.

If we deliberately add impurities to the semiconductor material (called doping), we can control how conductive the material is. By doping the Silicon with some element that has three valence electrons we make a silicon material that we call “positive-type, or P-Type”, meaning it is capable of accepting an electron for each impurity atom that we add. If we dope the Silicon with some element that has five valence electrons we make a silicon material we call “negative-type, or N-Type”, meaning that it has an excess of electrons. If we melt these two types together, the junction where they bond forms a region that will allow the extra electrons in the N-type to pass through to the P-type if we apply a voltage across the junction, negative to N-Type and positive to P-type. Resistance drops down to hundreds, tens, or even a fraction of an ohm. If we reverse our voltage the electrons are pulled away from the junction, toward the positive voltage, and the holes are pulled away from the junction, toward the negative voltage, and our resistance of the junction is in the millions of ohms.

This is our basic semiconductor device, called a Diode, meaning that it has two electrodes. If we apply Forward Bias to the diode (Negative voltage to N-type and Positive voltage to P-type) we conduct electricity. If we reverse the voltage (Reverse Bias) we do not conduct electricity.

The terminal on the N-type side we call the Cathode, meaning something that emits electrons. The terminal on the P-type we call the Anode, meaning something that accepts electrons (as viewed from the inside of the device).

Our doping need only be in the parts-per-million to be effective. By changing the doping material, doping percentage, size and structure of the junction, and material of the substrate (in our example, the Silicon), we can have different results.

A closer lookWhen a diode is forward biased the electrons from the N-type material move

toward the junction, and holes from the P-type material move toward the junction. The hole and the electron combine in the area of the junction, allowing current to flow. Our supply of electrons and holes are supplied by our battery, or power supply, or whatever our source of power is.

The combining of the electron and hole releases an electromagnet radiation of some sort, usually in the frequency of heat, and in some cases, light. Silicon produces a radiation in the infrared region. Gallium Arsenide, a semiconductive molecule, produces radiation in the red region.

Before the junction can conduct we have to overcome the resistance of the junction to conduct at all. This voltage and current we must apply makes a Threshold level that must be attained before we see conduction. In Silicon we must have at least 0.60 volts, or so, before the junction will break into conduction. Germanium requires less, about 0.40 volts. Gallium Arsenide requires about 1.20 volts.

Real devicesDiodes used in low level signals (below 100 mA, and below 100 V) are called

signal diodes, and we use them in various ways we will discover in circuits to be introduced later.

Diodes designed to work at higher power levels we use to change AC to DC in power supplies. This process of changing AC power to DC power is called rectification, and diodes used in these type of circuits we call rectifiers.

Signal diodes and rectifiers both work in the same way, the only difference is their physical size of the substrate and the case they must be in to dissipate the power.

Silicon signal diodes we see most often are of the part numbers 1N914, 1N4148, and 1N4448. The “1N” at the beginning indicates that the device is a diode (one junction). The numbers that follow have no special meaning that indicates characteristics. A 1N914 made by any company has about the same characteristics. All will be made of Silicon (and thus have a forward voltage of around 0.6 volts). All will carry about 100 mA and still be within a safe operating range. All will be able to withstand about 100 volts when reverse biased before the junction breaks down are the device self destructs.

Silicon Rectifiers we often find are numbered 1N400x series of numbers. No matter who makes them, they will all pass 1 Amp of currently safely. All 1N4000’s will have a breakdown voltage of 50 V. 1N4001’s have a breakdown of 100 V. 1N4002’s have a breakdown of 200 V. The list continues as standard part numbers up to 1N4007 with a breakdown of 1000 volts.

1N540x devices are capable of operating at 3 Amps. 1N5400 has a breakdown voltage of 50 V. 1N5401, 100 V. 1N5402, 200 V. And so on.

These part numbers are Industry standards, and are used in home computers, microwaves, stereos, as well as slot machines both reel and video.

The cases are standard sizes also. You can’t tell much by looking at the case, other than judging that is works within a certain power range. Larger cases are required at higher current levels.

The DO-35 case is a standard size for signal diodes. The case is usually clear, or white glass. The banded end is the Cathode. !N914, 1N4148, and such typically look like these.

The DO-41 and DO-15 cases are deices that are rated at around 1 Amp, maximum, and less than 1,000 volts. 1N400x devices look like these.

The DO-201AD size case is physically larger than DO-41 and are capable of handling up to 3 Amps and 1,000 Volts.

The R-6 size case is usually a device rated at 6 Amp, and less than 1,000 volts.

The main difference between the DO-41 and DO-15 cases are not visible on the outside of the device. Between the actual silicon diode substrate itself and the case is a layer of insulation material called Passivation. In DO-15 devices this passivation is plastic. In DO-41 devices this passivation is glass.

Glass passivated devices are capable of working at higher operating temperatures than plastic passivated devices before failure occurs. Typically plastic passivated devices can operate at up to 125 degrees centigrade before failure. Glass passivated devices are rated at 150 degrees centigrade. That is “ambient” temperature. The temperature of the silicon substrate inside the case, not the outside temperature.

In all these circumstances the silicon substrate inside is enclosed in a hard plastic case. The part number is printed on the case. If you want to know the characteristics of

the device, you look up the part number in a reference book, or catalog. There are only a few dozen different types of diodes in use in the gaming industry. It isn’t a difficult task getting to know the major players by part number.

Part numbersThe manufacturer of the diode assigns an Industry Standard part number to the

device, like 1N4004. When IGT buys these devices they assign their own part number to it (48402190). When Bally uses it they assign their part number (E-587-40). When Wells Gardner uses t in their monitors they assign their number (066X0071-001). Distributors assign their part number (900-2869 for Radio Shack). Happ Controls assigns them their number, as does Aristocrat, Konami, or any other person who uses or distributes the device.

In purchasing the parts for our shop we need not buy a different supply of 1N4004s from each vendor. We can shop around and buy a 1N4004 from whoever has it at the best price. They may all, in the beginning, come from the same manufacturer. We can be sure that all diodes marked 1N4004 will have the same characteristics, and be compatible.

To make stocking even easier, we can realize that a device rated for a maximum of 200 volts will work fine at all lower voltages as well. So why not just buy a stock of 1,000 volt rated devices and use them for 1N4000, 1N4001,1N4002, and so on?

This is what companies like Philips does in marketing their line of “Replacement Parts”, like the ECG line of components. NTE Electronics does this also with their NTE brand. They stock the components with the highest voltage rating and list it as a substitute for all equal, or lesser, rated devices.

A word of warning here. SUBSTITUTE does not mean EQUAL. A 1N4007, rated at 1,000 volts, will substitute for a 1N4001 rated at 100 volts. But the 1N4001 will not substitute for the 1N4007.

The higher voltage device may be stocked and used for all lower voltage ratings. In most cases the higher voltage rating may cost you a few cents more per diode. It is a judgment call on the part of the person who purchases the components to choose what is going to be stocked.

Prices can vary greatly. A 1N4004 from one vendor may cost you $0.05, or $1.50 from another vendor or distributor. Shop around. Be aware that a real bargain price may be an older component with oxidized leads. When buying a component from a vendor you don’t know, buy in small quantities first just to sample quality and promptness of service. Any of the companies listed herein I would recommend. I can think of none off hand that I would warn you against.

Purchasing in quantity brings the price down somewhat also when purchasing from a distributor, like Happ Controls, or Radio Shack. Usually manufacturers (like IGT or Bally) do not give discounts, and have higher prices than available from distributors. Not many manufacturers want to get into being parts distributors. If they sell a batch of 100 diodes, they may short themselves on a production run. A major error to a manufacturer. Personal experience is highly valuable. In many cases I can get better prices on common parts from IGT than Radio Shack, for instance. Some things are more available from Radio Shack.

Standard Part NumbersAs mentioned a specific component may be known by various part numbers from

different game manufacturers or gaming component distributors. Some game manufacturers do not even give you a parts list or a tech manual. To combat this a standard part number list is available at the Bench Techs forum at Delphi Forums.

www.forums.delphiforums.com/benchtech

This list is in constant development and will cross part numbers between various gaming manufacturers and distributors, to Industry Standard part numbers. It also provides an often educational description of what the part is.

Delphi forums is free for basic service, or advanced service and capabilities at a modest fee. There is no charge for access to the Bench Techs forum, or for the parts cross list. Typical size of the file is between 500K and 1 MB. It will fit on a floppy disk for convenience. It is distributed as a Microsoft Excel file, but easily converts to text, if you prefer. Don’t try to print it unless you have a lot of paper and time. It is typically between 2,000 and 3,000 lines long (in Excel) and VERY wide.

Suppliers mentioned:

Happ Controls(Wholesale Electronics is a division of Happ Controls)6870 S. Paradise Rd.Las Vegas, NV 89119(702) 891-9116(702) 891-9117 faxwww.happcontrols.com

Radio Shack(Get the Commercial catalog, not the store catalog)AKA Tech AmericaPO Box 1981Fort Worth, TX 76101-1981(800) 877-0072www.techam.com

Others:

Mouser Electronics1000 N. Main St.Mansfield, TX 76063-1511(800) 346-6873www.mouser.com

Allied Electronics7410 Pebble Drive

Fort Worth, TX 76118(800) 433-5700www.alliedelec.com

Diode exerciseOur first exercise with semiconductors will be with diodes. We will use the same

breadboard we used with making previous circuits and hook up various devices, analyzing what we observe in the process. From these observations we will get an understanding of what proper behavior of the components is. Once we understand normal behavior we will deliberately destroy the device and get a familiarity with what it takes to destroy the device, and what characteristics it takes on when it fails.

We will choose from a variety of diode devices (Silicon, Germanium, and a Light Emitting Diode, at least).

Select a diode to do the exercise with.

1N914 Silicon signal diode1N270 Germanium signal diode1N5819 Schottky diode1N400x 1 Amp rectifierZener Any Zener diode of a low voltage (below what ever VCC is used)LED, redLED, yellowLED, greenLED, blue or white

Select a resistor within the safe limits of the diode. Reference a data book that contains operating characteristics for the diode. If no data is available keep the maximum current below 100 mA, but higher than 1 micro-Amp.

Make a simple series circuit using a diode with the cathode connected to ground and a resistor connecting to a positive voltage, VCC (4.5 Volts to 12 volts).

Measure the voltage at VCC.

Measure the voltage across the diode.

Calculate the voltage across the resistor. (VCC – V diode). (This step is for an exercise in Ohm’s Law. You could just measure the voltage across the resistor directly.)

Find the current through the diode. We can find this by finding the current through the resistor (V / R = I). Since this is a simple series circuit, the current through the resistor will be the same as the current through the diode.

Calculate the effective resistance of the diode at that current. (V diode / I = R).

Repeat the exercise using different resistors. Keep the current in the safe operating range of the diode.

Graph the voltage across the diode at different currents.

Graph the effective resistance of the diode at different currents.

Repeat the whole exercise using different diodes.

What can we tell from the exercise?

Different types of diodes have a different pattern of forward voltage.

At different currents the voltage across the diode changes to some degree. These changes reflect the changing resistance of the diode at various currents. Below a certain level the change is major. Once the diode has sufficient operating current the voltage across the

diode stays fairly flat, but its resistance still changes as current through the diode changes.

Bi-Polar Junction transistor exercise, NPN and PNP

Select a transistor for the exercise:Pinout

Transistor Type 1 2 32N2222A NPN E B C2N2907A PNP E B C2N3904 NPN E B C2N3906 PNP E B C2N4401 NPN E B C2N4403 PNP E B CPN2222 NPN E B CPN2907 PNP E B C

(For other possibilities, other reference material may be required.)

Construct the above circuit using appropriate resistors.

Select a base resistor to give about 1 micro amp of base current. (VCC/ 1 micro amp)

Select a collector resistor to give about 100 mA, maximum, if the transistor should turn all the way on. (VCC / 100 mA)

Connect the transistor up in a Common Emitter circuit (as shown).

Measure the voltage on the base.

Measure the voltage on the collector.

Find the voltage across the collector resistor. (Calculate or measure).

Calculate the collector current. (V R3 / R3 = I).

Calculate the gain of the transistor (IC / IB).

Calculate the effective resistance between the Emitter and the Collector of the transistor. (V collector / I = R e-c).

Calculate the wattage being dissipated by the transistor. (V c-e x I).

Repeat the exercise using a different base resistor and the same collector resistor.

Repeat the whole exercise using a different collector resistor.

Note how gain changes under different base and collector currents.

Characteristics of the transistor:

Gain must be stated under a certain condition.

Voltage at the collector during saturation varies with different collector currents.

Wattage rating of the transistor can be exceeded even while within the safe operating range of collector currents and voltages.

Transistor exercise

Select an NPN transistor for the exercise.

Select a base resistor to give about 1 micro amp of base current. (VCC/ 1 micro amp)

Select a collector resistor to give about 100 mA, maximum, if the transistor should turn all the way on. (VCC / 100 mA)

Connect the transistor up in a Common Emitter circuit (as shown).

Measure the voltage on the base.

Measure the voltage on the collector.

Find the voltage across the collector resistor. (Calculate or measure).

Calculate the collector current. (V R3 / R3 = I).

Calculate the gain of the transistor (IC / IB).

Calculate the effective resistance between the Emitter and the Collector of the transistor. (V collector / I = R e-c).

Calculate the wattage being dissipated by the transistor. (V c-e * I).

Repeat the exercise using a different base resistor and the same collector resistor.

Repeat the whole exercise using a different collector resistor.

Note how gain changes under different base and collector currents.

Characteristics of the transistor:

Gain must be stated under a certain condition.

Voltage at the collector during saturation varies with different collector currents.

Wattage rating of the transistor can be exceeded even while within the safe operating range of collector currents and voltages.

SCR exercise

An SCR (Silicon Controlled Rectifier) is exactly what the name implies. A rectifier (diode), made of silicon, that we can control with an input. SCRs are a member of a class of components called Thyristors. They all have the characteristic that once triggered on, they stay on until power is removed on the output. The SCR is a thyristor that will pass current in only one direction (DC only).

An SCR is made up of two transistors, as shown below. When we draw current from the base lead of the NPN transistor (called a Gate), we begin to turn the NPN transistor on. The NPN transistor turning on starts drawing current through the emitter and collector of it. This circuit just happens to include the base circuit of the PNP transistor. As the NPN transistor turns on it turns on the PNP transistor, which starts to conduct. The collector of the PNP transistor feeds back to the base of the NPN transistor. As the PNP transistor conducts it turns on the NPN transistor even more.

All we have to do is send a pulse to the base of the NPN transistor, and the SCR latches in the ON condition. It will stay on until we remove power between the Anode (the emitter of the PNP side) or the Cathode (the emitter of the NPN side).

Construct the above circuit. Start with 0V out at the slider of the pot. As we increase voltage at the pot the SCR will come on at some point. We want to note at what voltage and current level the SCR turns on. After it is on, we will return the Gate current we apply back to 0 V, and observe that the SCR stays on until we remove power at the output.

1) Monitoring the voltage at the slider of the pot, increase the voltage out by 0.10 V.

2) Measure the voltage at the gate of the SCR.

3) Calculate the gate current.

4) Is the SCR ON?

5) Repeat steps 1) through 4) until the SCR turns on.

6) Note the voltage on the gate and the current through it when it first turned on.

7) Return the voltage at the slider back to 0V.

8) What is the voltage on the Gate?

9) Did the SCR stay on?

10) Remove one side of resistor R3.

11) The SCR should turn off and stay off after R3 is reconnected.

Triac exercise

A Triac is another member of the Thyristor family. The Triac is essentially two SCRs placed back to back, allowing the device to operate on AC. We have a Gate input, common to both SCRs, and two Main terminals, MT1, and MT2.

While SCRs are used on logic boards to control DC devices, SCRs are used to control AC devices. Operation is the same.

Like any other transistor, SCRs and Triacs may come in various packages. The larger the package, the higher the power it can control. Larger cases allow for more heat to be dissipated, meaning more current may be passed through the device, and higher voltages may be tolerated.

Surface Mount Device Case styles

Typical cases of leaded components (not shown in proportional size).

Construct the circuit below, leaving the jumper shown to be attached in one of three places. It may be attached to D1 to have Q1 pass only the positive sides of the AC signal, to D2 to pass only the negative side of the AC signal, or directly to the gate to pass both sides of the AC signal.

7400 Quad 2 input NAND gate

A following drawing shows the basic internal structure of the 7400, and how it works. This is the basic Totem-Pole output, and basic TTL input. Schottky and Advanced devices are more complex in order to reach higher speeds, and protect inputs against static electricity.

When both inputs of a NAND gate are high, the output will be low. If either input is low, the output will be forced high. This gate may be used as an active high input NAND, or an active low input NOR.

1 Input 1A2 Input 1B3 Output 14 Input 2A5 Input 2B6 Output 27 GND8 Output 39 Input 3A10 Input 3B11 Output 412 Input 4A13 Input 4B14 VCC

Below are some drawings of examples of the circuit inside a basic 7400 gate. Be aware that the drawing is simplified for clarity. The actual device will have more components. These will be included later on in the lessons. Right now, this is still basics.

The first following drawing is an equivalent of the 2-emitter’ed transistor, Q1, in the next drawing. The AND-ing is done in this circuit. In the diode circuit, if either input is low the diode will conduct to VCC through R4. This puts point where D3, D4, and D5 connect one diode voltage above ground. This keeps Q5 from turning on. With both inputs high D4 and D5 are off and Q4 is allowed to turn on through D3 and R4.

Both inputs must be high to operate. This is the basic requirement for a AND operation.

This circuit is equivalent to the 2-Emitter’ed transistor of the following circuit. This is a Common-Base circuit. The Base lead is common to both the Input and the Output. Pulling one of the emitter leads low turns Q1 on. Both inputs must be high to turn Q1’s Emitter-Base junction off.

The output stage in the circuit below does the inverting of the signal to make the AND operation a NAND gate. With two high in Q2 turns on, making it’s emitter more positive, turning on Q3, giving a Low out. With either input low Q2 is off, making the Collector more positive, turning Q4 on, giving a high out.

The examples below show how the 7400 may be used to drive an LED. Do not use this output to drive other logic inputs also. These uses exceed the voltage limitations for a valid High or Low output of the gate.

The following example shows how to make a basic S-R Latch using two NAND gates. Note the inputs are active low inputs. A low at the Reset side resets the latch (Q low, Q\ high). A low on the Set side sets the latch (Q high, Q\ low)

The following example shows the 7400 used in the Active Low 2-input NAND operation. We may view the NAND gate as "All highs in gives a low out", or "Any low in gives a high out". Both are descriptions of a NAND gate.

Most drawings are made using the symbol for a NAND gate that matches operation of the circuit. This is not always true, but most manufacturers are thoughtful enough to do so.

Now that we know the basics, lets get some hands-on experience with some real devices. For this we need a breadboard with switches for inputs and LEDs to hook up and monitor the signals. If you are using our standard Training Kit, you should have a 7400 device included. (7400, 74LS00, or some variation. Avoid using CMOS devices. (74HC00, 74HCT00, or anything with a “C” in it is CMOS.)

The following circuit is an exercise in learning how the 7400 operates. The switches will put in either Highs or Lows. When both inputs are high, the output should be low. The LEDs will be on when that signal is low.

Construct the following circuit. Double check wiring before turning power on.

With either input low, the output should be high. Only with both inputs high should the output go low.

BASIC DIGITAL LOGIC

If you’ve been keeping up with Slot Tech Magazine’s Basic Logic Course you should have a good understanding of basic Gates and Latches. This article picks up from there and continues on emphasizing Hands-on experience. We will review flip flops and latches and build a training station to exercise these devices so we can watch them work and build circuits using them. In my opinion, the best way to understand ICs is to sit down and build a circuit using them. This gives you the base of understanding necessary for troubleshooting them in a circuit.

This part of the course is not merely for academic purposes. The circuits we will use to manipulate the Inputs of the ICs and monitor the Outputs of the ICs will be the same (or similar) circuits we will use in designing test fixtures later on in the series. Now that you know where we’ve been and where we’re going, let’s get started on where we are.

Gates and simple circuits can be tested with simple switch inputs. We need a resistor to pull the input up to VCC when it isn’t pulled to ground through the switch. TTL outputs are capable of driving enough current to power an LED. 74xxx, 74LSxxx, 74Asxxx, anything that doesn’t have a “C” in it. Inclusion of the “C” in the part number means the IC is actually CMOS technology, not TTL, and may only be able to drive a milliamp, or so, of current. Let’s take a circuit to test basic gates with first.

Building the circuits

Figure 1 shows basic switch inputs and an LED output to exercise a 7400 2-Input NAND Gate. As we close one of the switches it inputs a low to the input of the 7400 it is connected to. If the output goes low the LED should light. If we remember the 7400, both inputs must be high (switches off) before the output goes low.

Figure 1 shows basic switch inputs and an LED output to exercise a 7400 2-Input NAND Gate. As we close one of the switches it inputs a low to the input of the 7400 it is connected to. If the output goes low the LED should light. If we remember the 7400, both inputs must be high (switches off) before the output goes low.

Our first decision is how to accomplish building the circuit. Radio Shack, and quite a few other stores, sell small general purpose circuit boards such as Radio Shack’s 276-168, and 276-150A. (See pictures with these titles.) These are circuit boards etched with a general-purpose pattern. On either you will find rows of copper clad 0.300” apart. Exactly the size of a DIP IC. You will also find long strings of holes running lengthwise for power buses. These are one possibility for building the circuits we will use. For a few more dollars you can get a “Huge IC Socket” type of breadboard. (See picture title “Breadboard”.) The breadboard gives you the flexibility to reuse the board many times. Since our “Basic Input Switches” and “Basic Output LEDs” will be repeatedly used, I suggest building them on the circuit boards, and using the breadboard for the circuit unique to the IC we are going to use in the exercise.

The picture above is Radio Shack part 276-150A. This board is suitable for projects that have only one or two ICs and a few discrete components. The picture to the right of it is 276-168. This board is more suitable for a circuit with a few ICs or more complex circuits.

For more flexibility, a breadboard can be used, as shown below. This is one big IC socket that may be reused. It is arranged in rows of five dots. Each row of five are connected together. If you wish to connect two components together you just plug one end into one connector in

that row.

Powering the circuits

Of course we need to power these circuits somehow. We can use batteries or a power supply. Most TTL and CMOS will work fine off of 4.5 Volts (three 1.5 V batteries). For more safety, we can use a pack of four batteries, giving us 6 Volts, and include a series diode. The diode drops the voltage down closer to 5 V, and prevents damage to the circuit if we should ever connect the batteries in reverse order. Radio Shack sells battery packs that will work just fine. If you would rather run off of wall power you can use a “wall

wart” power supply like the kind used to charge cell phones. I have a box full of various kinds I bought from second hand stores for $1.00 each. Ideally, you want one that has a regulated output of 5 Volts at a few hundred milliamps. If you find one that is unregulated you can build a voltage regulator to bring the voltage down to 5 Volts. Unregulated wall warts can be identified by measuring the output voltage and comparing that to what the label says it should be. One that puts out 3.6 Volts and has a current capability of 500 or 600 (or more) milliamps, may measure at up to 8 Volts with no load. As you draw current from it the voltage drops down so that at the rated current you will have the rated voltage. The picture “Power Supply” shows a 7.5 Volt at 800 mA unregulated power supply made for these exercises.

The picture and schematic above give you an idea of the circuit required. Components are not tightly regimented. The LEDs and their resistors are optional. C1 may be anything from a few hundred microfarads to a few thousand microfarads, as long as the voltage rating is higher than the unloaded voltage coming out of the “Wall Wart” supply. C2 also may be anything from 1 microfarad to something less than one hundred microfarads. The wall wart supply must be able to supply more than 7 or 8 Volts at a few hundred milliamps (up to 18 Volts).

The 7805 will regulate the voltage coming out of the wall wart down to +5 Volts, as long as the input is above 7 or 8 volts, but not higher than 24 volts.

Input Circuit

What you use for switches is more a matter of prerogative. Any kind of switch at will do. The cheapest way to go may be a DIP switch. For exercising simple Gates switches are adequate as inputs. When we get to Latches we need cleaner square waves than we can get out of switches alone. When a mechanical switch opens and closes the contacts wipe on one another and bounce. If we could look closely at the signal on an oscilloscope we would see bounces in the signal over milliseconds. For anything besides gates we cannot accept such dirty signals. In order to get clean square waves we can build a “Debouncing” circuit using 7400 gates to build an S-R Latch, or the 74279 is four S-R latches built into one case.

The above would be a suitable circuit for exercising basic gates. The circuit below will be more appropriate for latches and more complex circuits.

A circuit like this provides a clean (glitch-free) signal for latches, counters, and such.

Output circuits

From TTL outputs we can get up to 25 mA of output current. From 74xx we can get 16 mA easily. From 74LSxxx we can get a little less, but as long as we are not driving other logic devices that are sensitive to the low level output voltage, we can still drive an LED directly. For these ICs the “Four LED” circuit will work fine.

CMOS devices may only have drive currents of a milliamp or so. For these we need an LED driver that will work on a milliamp or less and drive an LED requiring 10’s of milliamps. For these we need a “Low Current LED Driver”.

All of these examples were designed for minimal cost. For the cost of lunch you can get started building these circuits. For a more reliable circuit you can use better switches and mount the whole thing in boxes. This is the way we will build the test fixture examples that follow. How you proceed to build your depends on your budget as well as your long term intentions.

Using the setup to test a J-K Flip Flop (74107)

The 74017 is a popular device. They are easy to get hold of and not expensive. You can get the part from Radio Shack, or most any other distributor, or it is IGT part number 32015590. We will exercise the 74107 one section at a time. There are two in a package. We need four (clean) inputs and two outputs.

If we look at the Truth Table for a 74107 we find it to be like this:

J K Clk Reset Q Q-not1 0 / 1 1 0 With J high, K low, reset high, on the positive edge of the clock pulse the flip flop should set (Q high, Q-not low).

0 1 / 1 0 1 With J low, K high, reset high, on the positive edge of the clock pulse the flip flop should clear (Q low, Q-not high).

1 1 / 1 T T With J and K high, reset high, on the positive edge of the clock pulse the flip flop should toggle. If it was set it will clear. If it was clear it will set.

0 0 / 1 (nc) (nc) With J and K low, reset high, on the positive edge of the clock pulse the flip flop should not change. If it was set it should stay set. If it was clear it should stay clear.x x x 0 0 1 With reset low the flip-flop will clear. All synchronous (clock related) operations are inoperative.

To test a 74107 we need four inputs from switches and two outputs. Since we have a “Clock” input we need a glitch-free signal from our switch circuits. We can exercise one section of the 74107 at a time.

From testing ICs to testing boards

The procedure for testing an IC, as we have just done, is little different if we were to test a board. As long as we know what the board or assembly should do we can exercise it for testing and troubleshooting. In following articles we will use these same, or similar, circuits to test simpler boards, and more on to more complicated assemblies. Among these we can do Coin Comparators of all types, Meter boards, Hoppers and Hopper Control Boards… any board or assembly that only requires a few simple inputs and outputs.

Above is an example of a basic, general purpose, four-input and four-output test setup. In order to use the setup to test a certain assembly, all we have to do is make a cable to interface to the assembly to be tested.

In the schematic above, J2 is the jack coming out of the test setup. J1 is the hopper connector. This cable looks, as shown below.

By making different cables we can test different types of hoppers, VFD Display Assemblies, Coin Comparators… most anything with four inputs and four outputs.