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Gary Plimer 2005Electrical Circuits / Electronics
Electricity is one of the most important forms of energy available to man. It affects everyone’s lives in many ways. If you take time to think about your everyday life you will realise that our lives are full of devices that depend upon electricity.
Some important terms:
Electric current
Electric current is the name given to the flow of negatively charged particles called electrons.
Current is measured in amperes, usually referred to as ‘amps’ (A). Current is the rate of flow of electrical charges (called electrons) through a circuit.
Gary Plimer 2005
Electrical CircuitsVoltage
In most circuits a battery or voltage supply is used to drive the electrons through the components. Voltage is measured in volts (V).
R+
_V C o nve ntio na l
C urre nt
ResistanceAll materials conduct electricity. The materials that conduct electricity well are called conductors and those that are poor conductors are called insulators. Metals are good conductors while rubber and glass are good insulators.Resistance is therefore a measure of how much voltage is required to let a current flow. Resistance is measured in ohms ().
Gary Plimer 2005Batteries & Voltage Supplies
Single battery or cell
Multiple batteries or cells
6 volts Voltage supply-ve +ve
Gary Plimer 2005
Components - Resistors
Fixed Resistor Symbol
Resistors are basic components in electrical and electronic circuits. They limit the amount of current flowing in circuits or parts of circuits. Resistors are roughly cylindrical and have coloured stripes. They also have connection wires sticking out of each end.
The stripes indicate the value of the resistors. The colours represent numerical values according to a special code.Although the symbol for ohms is ‘’ it is often shown as a capital R; that is, 270 ohms can be expressed as either 270 or 270 R.
Gary Plimer 2005
Resistor Colour Code
First and second colour band Digit Multiplier
Black 0 x 1
Brown 1 x 10
Red 2 x 100
Orange 3 x 1000 or 1 K
Yellow 4 x 10 000 or 10 K
Green 5 x 100 000 or 100 K
Blue 6 x 1 000 000 or 1 M
Violet 7 Silver means divide by 100
Grey 8 Gold means divide by 10
White 9
Tolerances: brown 1% red 2% gold 5% silver 10% none 20%
Gary Plimer 2005Resistor Value Calculation
4 Band Resistor Colour Code Layout
4th bandtolerance
3rd bandmultiplier
1st band1st digit
2nd band2nd digit
If the colours on the resistor are:1st band red2nd band violet3rd band brown4th band gold
Then its value is: 2(red) 7(Violet) x 10(Brown) with a 5% tolerance (Gold) i.e. 270ohms 5% tolerance.
Gary Plimer 2005
Pupil Assignment
Calculate the value of the following resistors:
1) blue – violet – brown – silver
2) orange – white – brown – gold
3) brown – black – red – gold
4) brown – black – green – brown
What colours would the following resistors have?
1) 270 R
2) 1K5
3) 33 K
Gary Plimer 2005
Diodes
Current can pass this way only
Anode Cathode
Symbol for Diode
Diodes are devices that allow current to flow in one direction only.
Current will flow through the diode only when the anode (positive side) is connected to the positive side of the circuit and the cathode (negative side) is connected to the negative side of the circuit.
Gary Plimer 2005
Light Emitting Diode
A light-emitting diode is a special diode that gives out light when current is flowing though it. LEDs are used as indicators to tell when a circuit (or part or a circuit) is working. You can tell the cathode of an LED as it is the short leg and there is a ‘flat’ on the plastic casing.
-ve
Gary Plimer 2005
Switches
Toggle Slide
Key Tilt
Rocker
Reed
Switches are useful input devices (or transducers) that have metal contacts inside them to allow current to pass when then they are touching. There are several ways in which the contacts in mechanical switches can be operated. The main types are push-button, toggle, key, slide, magnetic (reed) and tilt. These switches are ‘digital’ input devices as they can only be on or off.
Gary Plimer 2005
Switch Contacts
SPST
SPDT
DPST
DPDT
Gary Plimer 2005
Pupil Activity
We have now seen a number of common electronic components. Lets now try and combine some of these into a working circuit.
390Ror390
Switch
LED
6V
ICopy the circuit into your workbook
Build the circuit using the modular boards
Your teacher will demonstrate how to connect the boards
Gary Plimer 2005
Series Circuits
When components are connected end to end, as in the last activity, we say they are connected in series.
This leads to an important law, Kirchoff’s 2nd Law
The sum of voltages dropped across each component (V1, V2 ) is equal to the total voltage supply in the circuit.
18 V
6 V 6 V 6 V
VT = V1 + V2 + V3 + …
Gary Plimer 2005
Measuring Voltage Drops
50V
10V
1V
100m V
200k20k 2k 200
200A
2 Am
20 Am
10A
10A m A V C O M
Ohms
Amps VoltsOn/Off a.c. d.c.
To measure d.c. voltage:
Connect the black lead to the ‘COM’ socket
Connect the red lead to the ‘V’ socket
Make sure that ‘d.c.’ is selected
Place the lead probes on the points where the voltage is to be measured
Digital Multimeter
Gary Plimer 2005
Measuring Voltage Drops
V
Note how voltage is measured over the components
Make sure you take a note of the symbol for VOLTMETER
Gary Plimer 2005
Pupil Activity (Voltage Drops)
Task:
Measure the voltage drop over the 2 bulbs. Enter your findings into a table.
Bulb No. Voltage (v)
1
2
9V
Gary Plimer 2005
Pupil Activity (Voltage Drops)
Task:
Measure the voltage drop over the 2 bulbs and resistor. Enter your findings into a table.
Gary Plimer 2005
Prototype Board
M ETALLIC STRIPC O NN EC TO R
Prototype Board is used to test circuits prior to manufacturing the circuit in large numbers.
Build a series circuit using 2 resistors of different values as shown by your teacher.
Using the multimeter, check the voltage drop over each resistor.
Do the results confirm Kirchoff’s law?
Gary Plimer 2005
Circuit Simulation
As in Pneumatics, it is possible to simulate electrical circuits. In this case we will use a program called Crocodile Technology. Your teacher will demonstrate the use of Croc Clips to simulate the circuit shown below..
Gary Plimer 2005
Measuring Current
6V
A
Current is measured through components or parts of circuits, as shown in the circuit diagram opposite.
Note that it is necessary to ‘break’ the circuit and connect the meter in series with the components.
Take a note of the symbol for an Ammeter
Gary Plimer 2005
Current measurement
Using circuit simulation, measure the current flowing through all three components in the LED circuit.
In a series circuit the current flowing through all components is the same. Try placing the meter at different parts of the circuit to prove this. In parallel circuits the same current does not always flow through each component you will find out about this later.
Gary Plimer 2005
Measuring Resistance
COMmA10A V
R1
R2
Connect two resistors in series on a prototype circuit board and measure the overall resistance.
You should find that
Rtotal = R1 + R2
And the general rule for finding the sum of any amount of resistors in series is
Rtotal = R1 + R2 + R3 + Rn
Gary Plimer 2005
OHMS LAW
R = V
I
Ohms law can be used to calculate theoretical Voltage drops, Current and Resistance in circuits.
V
I R
Using the triangle shown opposite, we can rearrange the formula to obtain V or I.
Gary Plimer 2005
Ohms Law in Practice
La mp
C urrent 0.06 a mp s
6 vo lts
R =V
I
R = 6
0.06
R = 100
The task is to calculate the resistance of the lamp.
Gary Plimer 2005
Worked Example
c
S
For the series circuit shown, calculate:
a) The total resistance (RT)b) The circuit current (IC)c) The potential difference across both resistors (V1
and V2)
Gary Plimer 2005
Worked Example
R = R + R
= 6 + 18
R = 24
T 1 2
T
V = I R
I = V
R
12
24I = 0.5 A
S C T
CS
T
C
V = I R
0 . 5 1 8
V = 9 V
2 C 2
2
a) b)
V = V + V
3 + 9
V = 12 V
T 1 2
T
c)
Gary Plimer 2005
Pupil Problems
12V
For the circuit shown below calculate:
a) The total resistance of the circuit
b) The circuit current
Gary Plimer 2005
Pupil Problems
6V
For the circuit shown below calculate:
a) The total resistanceb) The circuit currentc) The voltage drop across each resistor.d) Use Kirchoff’s second law to verify your answers to
(c).
Gary Plimer 2005
Pupil Problems
24V
For the circuit shown below calculate:
a) The total resistance of the circuit
b) The circuit current.
Gary Plimer 2005
Pupil Problems
A circuit has three resistors in series. Their values are 15 R,
24 R and 60 R. Calculate the total resistance of the circuit.
Two resistors are connected in series. Their values are 25 R and 75 R. If the voltage drop across the 25 R resistor is 4 volts, determine the circuit current and the supply voltage
Gary Plimer 2005
Series Circuits
One of the problems with series circuits is if a component fails, then the whole circuit fails. Consider a set of bulbs connected in series.
If one of these bulbs fail, then current cannot flow through the circuit, hence the remaining bulbs will fail to light also.
Gary Plimer 2005
Parallel Circuits
Parallel circuits are circuits where there is more than one path for electricity to flow along or that have more than one ‘branch’. Each branch receives the supply voltage, which means that you can run a number of devices from one supply voltage. A good example of a simple parallel circuit is a set of Christmas-tree lights where all the bulbs require a 230 volt supply.
240 volts
Gary Plimer 2005
Parallel Circuits Activity
12 volts
Parallel circuits can be arranged in many ways, but are normally set out so that you can easily see the parallel ‘branches’. A simple parallel car-alarm circuit is shown below with the switches wired up in parallel.
Simulate the circuit shown below, then describe its operation in your note book.
Gary Plimer 2005
Resistors in Parallel
COMmA10A VR1 R2
Connect two resistors in parallel on a prototype circuit board and measure the overall resistance
1
R =
1
R +
1
RT 1 2
The formula to calculate the theoretical value of resistors in parallel is shown below.
Gary Plimer 2005
Worked Example
12 volts
R2
R1
The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.
Calculate the resistance of the parallel branch and the total circuit resistance.
Gary Plimer 2005
Pupil Activity (Parallel Circuits)
Task:
Build the circuit, Measure the voltage over each of the bulbs. Enter your findings into a table.
Gary Plimer 2005Current in Parallel Circuits
I I
I
I
T T
2
1
There are two important points to remember about resistors in parallel.
1) The voltage drop across each resistor is the same.
2) The sum of the currents through each resistor is equal
to the current flowing from the voltage source.
Gary Plimer 2005
Worked Example
12 volts
R2
R1
The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.
Your teacher will work through this problem on the white board.
Gary Plimer 2005
Pupil Problems
9V
For the circuit shown below calculate:
(a) The total resistance of the circuit
(b) The circuit current.
Gary Plimer 2005
Pupil Problems
110V
For the circuit shown below calculate:
(a) the total resistance of the circuit(b) the circuit current(c) the current flowing though R1 (10 R)(d) the current flowing through R2 (24 R).
Gary Plimer 2005
Pupil Problems
240 V
For the circuit shown below calculate:(a) the total resistance of the circuit(b) the circuit current(c) the current flowing through R1 (660 R).(d) the current flowing through R2 (470 R).
Gary Plimer 2005
Pupil Problems
A 6 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 12 V. Calculate the circuit current.
A 440 R resistor is connected in parallel with a 330 R resistor. The current through the 440 R resistor is 300 mA. Find the current through the 330 R resistor
Gary Plimer 2005Combined Series & Parallel
Consider the combined series and parallel circuit shown in the figure below.
You can see that R2 and R3 are connected in parallel and that R1 is connected in series with the parallel combination.
Gary Plimer 2005Combined Series & Parallel
Some points to remember when you are dealing with combined series and parallel circuits are:
The voltage drop across R2 is the same as the voltage drop across R3 The current through R2 added to the current through R3 is the same as the current through R1 The voltage drop across R1 added to the voltage drop across R2 (which is the same as across R3) would equal the supply voltage Vs.
Gary Plimer 2005
Worked Example 2
24R
48R
10R
12V
For the combined series and parallel circuit shown, calculate:
The total circuit resistance (RT) The circuit current (IC) The voltage drop across resistor R1 (VR1) The current through resistor R2 (I2).
Gary Plimer 2005
Pupil Problems
7.5 V
For the circuit shown calculate:
The resistance of the parallel combination
The total circuit resistance.
The branch currents
Gary Plimer 2005
Pupil Problems
24 V
For the circuit shown calculate:
The total resistance
The circuit current
The branch current
The voltage drop across each resistor.
Gary Plimer 2005
Pupil Problems
110 V
For the circuit shown calculate:
The total resistance of the circuit The circuit current The current through each resistor The voltage drop across each resistor.
Gary Plimer 2005
Power in Circuits
Electrical power is measured in watts (W).
Electrical power can be converted into other forms of power using electric circuits. For example the power used in overcoming electrical resistance can be converted into heat – this is the principle of an electric fire.
The power in an electric circuit depends both on the amount of current (I) flowing and the voltage (V) applied.
The formula for power in electric circuits is:
Power = Voltage x Current (watts)
P = V x I (W)
Gary Plimer 2005
Worked Example
An electric household lamp consumes 60 watts from a 240 volt supply. Calculate the current drawn by the lamp and the resistance of the lamp.
P = V I
I = P
V
I = 60
240
I = 0. 25 A (or 250 mA)
V = I R
R = V
I
R = 240
0 2. 5
R = 960
Gary Plimer 2005
Pupil Problems
240 volts
M 200 volts
3 amps
In the following simplified circuit for a vacuum cleaner motor, calculate:
The power consumption of the motor
The voltage of the lamp
The total power drawn from the power supply.
Gary Plimer 2005
Pupil Problems
100 ohms
Switch
Bulb
The torch circuit below is supplied with two 4.5 volt batteries connected in series, with the current being 20 mA.
Determine:
The resistance of the bulb
The voltage across the bulb
The total power drawn from the supply
The power drawn by the bulb.
Gary Plimer 2005
Pupil Problems
An electric iron rated at 800 W is connected to a 230 V supply.
Calculate the maximum current drawn by the iron. What is the
power used by the iron at half- heat setting?
A kettle and a toaster use the same double socket. If the
kettle draws a current of 10 A and the toaster 3 A, find the
power used by each of the appliances. The two sockets are
wired in parallel to a 230 V supply.
An electric drill draws a current of 1.5 amps from a 110 volt
supply. Calculate the power rating of the drill.
Gary Plimer 2005
Pupil Problems
An emergency power generator has to drive 80 lamps. Each
lamp takes 60 W at 230 V. Calculate the current through each
bulb if:
They are connected in series.
They are connected in parallel.
How many 150 W lamps can be connected in parallel to a 250 V supply through a 5 A fuse?
Gary Plimer 2005
Pupil Problems
The rear screen heater in a car is connected to the 12 V system and draws a current of 2 A. Find the resistance of the circuit. In reality the 12 V, 0.5 A interior light is on the same circuit. State whether this is a parallel or series circuit and calculate the power and current when both lamp and heater are on.
Gary Plimer 2005
Voltage Dividers
Input transducers are devices that convert a change in physical conditions (for example, temperature) into a change in resistance and/or voltage. This can then be processed in an electrical network based on a voltage divider circuit.
0 V 0 V
Gary Plimer 2005Voltage Dividers Activity
COMmA10A V
R1
R2
VS
0 V
Volts
Build a voltage divider circuit using any 2 values of resistor.
Using the multimeter measure the voltage drop over R2.
This voltage is known as Vo or the output voltage from the divider.
Gary Plimer 2005Voltage Dividers Activity
V = R
R + R VO
2
1 2S
Measure the resistance of the 2 resistors from the last activity.
Enter the values into the formula below and calculate Vo.
Simulate the circuit using croc clips and measure Vo.
Hopefully! The value of Vo should be the same in all three cases, (within reason).
Gary Plimer 2005
Worked Example
0 volts
R = 40kV
V = 12 volts
2
2
S
R = 80k 1
V = V R
R + R
V = 12 40
40 + 80V = 4 volts
2 S2
1 2
2
2
Gary Plimer 2005
Pupil Problems
0 volts
R = 810RV
V = 12 volts
2
2
S
R = 270R 1
Calculate Vo in the following exercises
0 volts
R = 10KV
V = 12 volts
2
2
S
R = 390R 1
Gary Plimer 2005
Pupil Problems
0 volts
R = 47KV
V = 6 volts
2
2
S
R = 10K 1
0 volts
R = 2.2KV
V = 9 volts
2
2
S
R = 10K 1
Calculate Vo in the following exercises
Gary Plimer 2005
Switches
Switches are useful input devices (transducers).
There are several ways in which the contacts in mechanical switches can be operated. Such as push button, key, slide, toggle, magnetic (reed) and tilt.
These switches are digital input devices as they can only be on or off.
The contacts on a switch can be NO or NC (normally open, normally closed)
Gary Plimer 2005
Switch ContactsTypes of switch contacts:
SPST (Single Pole Single Throw)
SPDT (Single Pole Double Throw)
Double-pole double-throw switch (DPDT)Double-pole single-throw switch (DPST)
Gary Plimer 2005
Pupil Activity
Copy the circuit into your note book. Simulate the circuit using Croc Clips, then describe in your own words how the circuit operates.
Gary Plimer 2005
Analogue Transducers
A thermistor is a device whose resistance varies with temperature. It is a temperature-dependent resistor.
There are two main types.
Negative temperature coefficient (t or NTC) – where
resistance decreases as temperature increases.
Positive temperature coefficient (+t or PTC) – where
resistance increases as temperature increases.
The circuit symbols for and typical characteristics of the two types of resistor are shown on the next slide.
Gary Plimer 2005
Thermistor
NTC is the most common thermistor
Gary Plimer 2005
Data Charts
Thermistor types
In your Data book you should find a graph which describes how the resistance of a thermistor changes with temperature.
Your teacher will work through the use of the chart.
Gary Plimer 2005
Strain Gauges
Strain gauge
Strain gauges are really load sensors. They consist of a length of resistance wire and when stretched their resistance changes. Strain gauges are attached to structural members (beams, etc.) and as they are loaded, a reading on a voltmeter can be obtained.
Gary Plimer 2005Light Dependent Resistor
The LDR (sometimes called a photoresistor) is a component whose resistance depends on the amount of light falling on it. Its resistance changes with light level. In bright light its resistance is low (usually around 1 K). In darkness its resistance is high (usually around 1 M).
Gary Plimer 2005
Pupil Activity
Use your Data Book to find the resistance of an ORP12 LDR for the following light conditions:
a) 10 Lux
b) 40 Lux
c) 100 Lux
Gary Plimer 2005
Pupil Activity
0 volts
V
V = 9 volts
O
S
R = 10K 1
-t
1) Copy the circuit shown below into your note book.
2) Using the Electrical Modular Boards, construct the voltage divider circuit.
3) Using a multimeter measure Vo.
4) Warm the thermistor up with your fingers and re measure Vo.
5) Describe the operation of the voltage divider.
6) Reverse the position of the thermistor and resistor. Repeat 3,4 & 5.
Gary Plimer 2005
Pupil Activity
0 volts
V
V = 9 volts
O
S
ORP12
10K
1) Copy the circuit shown below
into your note book.
2) Using the Electrical Modular Boards, construct the
voltage divider circuit.
3) Using a multimeter measure Vo.
Cover the LDR up with your hand and re measure Vo.
4) Describe the operation of the
voltage divider.
5) Reverse the position of the LDR and resistor. Repeat
3,4 & 5. Describe what is
happening.
Gary Plimer 2005
Pupil Activity
A potentiometer configured as a variable resistor can be used in a circuit as a voltage or current control device. They are often used in voltage divider circuits to adjust the sensitivity of the input.
Build a voltage divider using a potentiometer. Check its operation by measuring Vo from the voltage divider.
Gary Plimer 2005
Potentiometers
Some more examples of potentiometers.
Gary Plimer 2005Voltage Divider Sensitivity
0 volts
V
V = 9 volts
O
S
ORP12
47K
With an analogue sensor it is normally desirable to adjust the sensitivity of the circuit. Rather than using a fixed resistor we can replace it with a variable resistor (or potentiometer).
This allows us to fine tune the sensitivity of the voltage divider.
Gary Plimer 2005
Pupil Activity
To save money and inconvenience the residents want the outside light to come on when it gets dark. They also want to be able to adjust the sensitivity from summer to winter nights.
Build the following circuit using modular circuit boards.
0 volts
V = 9 volts
S
ORP12
10K
V
Adjust the variable resistor so as Vo goes higher when your hand is about 100mm above the LDR
Gary Plimer 2005
Pupil Problems
9 V 5 V
0 V 0 V
Calculate the voltages that would appear across each of the resistors marked ‘X’ in the circuits below.
6v
0v
Gary Plimer 2005
Pupil Problems
12 V 16 V 12 V
0 V 0 V 0 V
In each of the following voltage divider circuits determine the unknown quantity.
Gary Plimer 2005
Pupil Problems
15 V 20 V
220R
0 V 0 V 0 V
In each of the following voltage divider circuits determine the unknown quantity.
Gary Plimer 2005
Pupil Problems
An NTC (negative temperature coefficient) thermistor is used in a voltage divider circuit as shown below. Using information from the graph shown, determine the resistance of the thermistor and hence calculate the voltage that would appear across it when it is at a temperature of: 80C OR 20C.
Gary Plimer 2005
Pupil Problems
What would happen to the voltage across the thermistor in the circuit shown previously
as the temperature increased?
What would happen to the voltage across the resistor in the circuit shown previously as the temperature increased?
Gary Plimer 2005
Pupil Problems
A thermistor (type 5) is used in a voltage divider circuit as shown below. The characteristics of the thermistor are shown in your Data Book. If the voltage V2 is to be 4.5 V at 100 C, determine a suitable value for R1. State whether V2 will increase or decrease as the temperature drops. Explain your answer
Gary Plimer 2005
Voltage Dividers
We have seen that Voltage Dividers, divide the voltage depending on the value of resistors used. In addition, if we include a variable resistor, we can alter the sensitivity of the voltage divider.
If we include a thermistor, we can measure changes in temperature.
If we include a LDR, we can measure changes in light levels.
If we include a potentiometer, we can measure changes in position.
Gary Plimer 2005
Transistors
The transistor is a semiconductor device. This means that it is sometimes a good conductor of electricity and sometimes a poor one. A transistor is made up of three layers of semiconductor materials that are either ‘n type’ or ‘p type’.There are two types of bipolar transistor available: pnp or npn. Transistors come in many variations and sizes. However, they all are reliable, efficient, small and relatively cheap.
Gary Plimer 2005
Transistors
Collector
Emitter
Base
NPN Bipolar Transistor
A transistor is an electronic switch
Transistors amplify current which enables them to
drive heavy loads such as motors
A voltage of 0.7V will switch on a NPN transistor
Gary Plimer 2005
Transistors Activity9V
0V
BC 108bc
e1K
Lamp
A
VBase resistor
value (K)Base/EmitterVoltage (mV)
Base current (A)
Lampon/off
2200
1000
470
220
100
47
33
22
10
1
Simulate the circuit using Croc Clips. Fill in the table shown below.
Gary Plimer 2005
Transistors
Vin
R
Your teacher will work through a number of problems which will look at calculating:
a) Base current
b) Voltage drop over R
c) Base – Emitter voltage
M
Gary Plimer 2005
Transistors Activity
1k
Build the following transistor circuit using modular boards.
Adjust the voltage reaching the transistor base by altering the value the potentiometer.
At what voltage does the transistor switch on?
Measure the current flowing to the base.
Now measure the current flowing in the collector leg.
What is the transistor doing?
5V (B)
10K
5V (A)
Buzzer
Gary Plimer 2005
Relays
Although relays are often considered to be output devices, they are really output switches from electric or electronic circuits.
When an electric current flows into the relay coil, the coil becomes an electromagnet. This electromagnet attracts the armature and moves the contacts. This movement provides the switching, just as the contacts in any other switch do.
Gary Plimer 2005
Relays
The relay is a very useful device because it is the vital link between microelectronics and high-energy systems that require substantial amounts of current. The relay is perhaps the most commonly used switch for driving devices that demand large currents.
Gary Plimer 2005Relays – Protection Diode
As seen earlier, relays have a coil that is energised and de-energised as the relay switches on and off. During this process of switching, the coil can generate a large reverse voltage (called a back e.m.f.). This reverse voltage can cause considerable damage to components, especially transistors.
The transistors and other sensitive components can be protected by the inclusion of a diode that provides a path for the current caused by the reverse voltage to escape.
Gary Plimer 2005
Solenoid
A solenoid is another output transducer that has a coil inside. Circuits containing a solenoid require a protective diode as well.
Gary Plimer 2005
Pupil Activity - Relay
+6 volts
0 volts
BC 108
-t
bc
e10K
1K
Relay
Build the following circuit using modular boards
Change the resistance of the thermistor by heating it up with your hand
Listen carefully, you should hear the relay contacts opening and closing
Now add a bulb and 5V power supply to the output side of the relay and heat the thermistor up again, (your teacher will demonstrate the correct connections). The bulb should light when the thermistor is hot.
It might be necessary to replace the 10K with a potentiometer.
Gary Plimer 2005
DPDT Relay
TO SEN SO RC IR C U IT
0V
+V
As electric motors normally draw larger currents, relays are ideal devices for such circuits. By using DTDP switching, relays can control the direction of rotation of motors.
Simulate a sensing circuit using an LDR in a voltage divider
Add a transistor driving circuit and a DPDT relay
Connect the relay up so as the motor drives clockwise and anticlockwise depending on the amount of light hitting the LDR
Gary Plimer 2005
Motor Reversal Circuit
Gary Plimer 2005
Capacitors
Capacitors are electronic components that store electricity for short periods of time within electronic circuits or networks.
RADIALCAPACITATOR
AXIALCAPACITATOR
ELECTROLYTIC
Electrolytic capacitors are polarity conscious. This means that they must be connected ‘the right way round’. The negative lead must be connected to zero volts with the positive terminal towards the higher voltage side of the circuit.
Gary Plimer 2005
Capacitors
POLYESTER
Polyester capacitors are for small-value uses and can be connected without regard to polarity.
Capacitance in measured in farads, but because this is a very large measurement most capacitors are rated in F (microfarads) or in nF (nanofarads).
Gary Plimer 2005
Pupil Activity
+
100uF
10K
9V
0V
Construct the following circuit
Allow the capacitor to charge up
Connect the end of the LED to 0V
The LED should light up for a short period of time
Gary Plimer 2005
555 Integrated Circuit
An integrated circuit (or IC) is simply an electronic package that contains a number of components on a silicon ‘chip’. The 555-timer IC that you are going to use is a very versatile chip that has many applications.
Gary Plimer 2005Pupil Activity - Monostable
12
34
6
78
IC1555
100 F
100K
1K
1K
+6 volts
390R
0 volts
Simulate the circuit shown, press the switch and observe the circuit operation.
Monostable means one stable state. The light comes on but always goes back to its original state.
Gary Plimer 2005
ASTABLE CIRCUIT
Gary Plimer 2005
Pupil Activity - Astable
0 volts
+ 6volts
555
+
390R
LED
1K
1KLDR
100uF 1uF
Build the following circuit on Breadboard, describe its operation.
