1. Introduction

Projet title is “Automatic Room Light Controller”. The objective of this project is to make a model to count number of person visiting particular room and accordingly light up the room. Here we can use sensonrs and can know present number of persons.

In today’s world, there is a continuous need for automatic appliances with the increase in standard of living, there is a sense of urgency for developing circuits that would ease the complexity of life.

Also if at all one wants to know the number of people present in room so as not to have congestion. This circuit proves to be helpful. The project is a reliable circuit that takes over the task of controlling the room lights as well as counting number of persons/visitors in the room very accurately. When somebody enters in the room then the counter is decremented by one. The light will be onlye switched OFF until all the persons in the room go out. The total number of persons inside the room is also displayed on the seven segment displays.

The 555 timer does the above job.It receives the signals from the sensors, and this signal is operated under the control of software which is stored in 555 timer continuously monitor the Infrared Receivers. When any object pass through the IR’s Receivers then the IR rays falling on the receivers are obstructed, this obstraction is sensed by the 555 timer.

2. Working Principle

2.1 Circuit Diagram

There are two main part of the circuits.1. Transmission Circuit(Infrared Led’s)2. Receiver Circuit

1. Transmission Circuit

This circuit diagram show how a 555 timer IC, configured to function as a basic astable multivibrator. The astable multivibrator generates a square wave, the period of which is determined by the circuit external to IC 555. The astable multivibrator does not require any external trigger to change the state of the output. Hence the name free running oscillator. The time during which the output is either high or low is

determined by the two resistors and a capacitor which are externally connected to the 555 timer.

IR Transmission circuit is used to generate the modulated 36 kHz IR signal. The IC555 in the transmitter side is to generate 36 kHz square wave. Adjust the preset in the transmitter to get a 38 kHz signal at the output. Then you point it over the sensors and its output will go low when it senses the IR signal of 38 kHz.

2. Receiver Circuit

The IR transmitter will emit modulated 38 kHz IR signal and at the receiver we use TSPO1738(Infrared Sensor). The output goes high when there is an interruption and it return back to low after the time period determined by the capacitor and resistor in the circuit i.e. around 1 Second. CL100 is to trigger the IC555 which is configured as monostable multivibrator. Input is given to the port 1. Port 0 is user for the 7-Segment

display purpose. Port 2 is used for the relay Turn on and Turn off purpose. LTS 542(Common Anode) is used for 7-Segment display and that time relay will get voltage and triggered, so light will get voltage and it will turn on and when counter will be 00 and at that time relay turned off.

2.2 PCB Layout

2.3 Components Lists

2.4 Working

This system is designed by using two sets of IR transmitters and receivers. These IR sensors are placed in such a way that they detect a person entering and leaving the room to turn the home appliances. In this optimum energy management system, a microcontroller is the central processing unit of this project which is of 89S51

controller from the 8051 family. This system facilitates a bidirectional visitor counter for displaying the number of persons inside the room.

When a person enters into the room, an IR beam is obstructed between the IR transmitter and the receiver. This IR obstruction from the sensor-1 gives the corresponding signal to the microcontroller. The microcontroller in such a way that by the reception of the signal from the sensor-1 it turns on the fans and lights inside the room. Thus, the microcontroller gives command signals to a relay driver which turns the rellays such that all these appliances turn on.

When the person leaves from this room, another set of IR sensors enable and give control signals to the microcontroller. Furthermore, similar to the above process, this system turns off the appliances like fans and lights. Apart from this, the system also takes account of the number of persons inside the room so that this control operation is varied depending on the persons’ availability in the room.

For every person entering and leaving the room, the microcontroller reads the digital input from two receivers, and calculates the number of persons inside the room, and then displays it on the LCD. When the persons’ count is greater than one, the microcontroller turns on the room light and when the persons’ count is zero, it turns off all the lights and fans.

3. DESIGN PROCEDURE

3.1 Multivator

A multivibrator circuit oscillates between a “HIGH” state and a “LOW” state producing a continuous output. Astable multivibrators generally have an even 50% duty cycle, that is that 50% of the cycle time the output is “HIGH” and the remaining 50% of the cycle time the output is “OFF”. In other words, the duty cycle for an astable timing pulse is 1:1.

Sequential Logic Circuits that use the clock signal for synchronization are dependant upon the frequency and and clock pulse width to activate there switching action. Sequential circuits may also change their state on either the rising or falling edge, or both of the actual clock signal as we have seen previously with the basic flip-flop circuits. The following list are terms associated with a timing pulse or waveform.

Active HIGH - if the state change occurs from a “LOW” to a “HIGH” at the clock’s pulse rising edge or during the clock width.

Clock Signal WaveformActive LOW - if the state change occurs from a “HIGH” to a “LOW” at the clock’s pulses falling edge.

Duty Cycle - this is the ratio of the clock width to the clock period.

Clock Width - this is the time during which the value of the clock signal is equal to a logic “1”, or HIGH.

Clock Period - this is the time between successive transitions in the same direction, ie, between two rising or two falling edges.

Clock Frequency - the clock frequency is the reciprocal of the clock period, frequency = 1/clock period

Clock pulse generation circuits can be a combination of analogue and digital circuits that produce a continuous series of pulses (these are called astable multivibrators) or a pulse of a specific duration (these are called monostable multivibrators). Combining two or more of multivibrators provides generation of a desired pattern of pulses (including pulse width, time between pulses and frequency of pulses).

There are basically three types of clock pulse generation circuits:

• Astable – A free-running multivibrator that has NO stable states but switches continuously between two states this action produces a train of square wave pulses at a fixed frequency.

• Monostable – A one-shot multivibrator that has only ONE stable state and is triggered externally with it returning back to its first stable state.

• Bistable – A flip-flop that has TWO stable states that produces a single pulse either positive or negative in value.

One way of producing a very simple clock signal is by the interconnection of logic gates. As NANDgates contains amplification, they can also be used to provide a clock signal or timing pulse with the aid of a single Capacitor and a single Resistor to provide the feedback and timing function.

These timing circuits are often used because of there simplicity and are also useful if a logic circuit is designed that has unused gates which can be utilised to create the monostable or astable oscillator. This simple type of RC Oscillator network is sometimes called a “Relaxation Oscillator”.

3.1 Operation:

Figure 1: Basic BJT astable multivibrator

The circuit has two astable (unstable) states that change alternatively with maximum transition rate because of the "accelerating" positive feedback. It is implemented by the coupling capacitors that instantly transfer voltage changes because the voltage across a capacitor cannot suddenly change. In each state, one transistor is switched on and the other is switched off. Accordingly, one fully charged capacitor discharges (reverse charges) slowly thus converting the time into an exponentially changing voltage. At the same time, the other empty capacitor quickly charges thus restoring its charge (the first capacitor acts as a time-setting capacitor and the second prepares to play this role in the next state). The circuit operation is based on the fact that the forward-biased base-emitter junction of the switched-on bipolar transistor can provide a path for the capacitor restoration.

3.2 State 1 (Q1 is switched on, Q2 is switched off):

In the beginning, the capacitor C1 is fully charged (in the previous State 2) to the power supply voltage V with the polarity shown in Figure 1. Q1 is on and connects the left-hand positive plate of C1 to ground. As its right-hand negative plate is connected to Q2 base, a maximum negative voltage (-V) is applied to Q2 base that keeps Q2 firmly off. C1 begins discharging (reverse charging) via the high-value base resistor R2, so that the voltage of its right-hand plate (and at the base of Q2) is rising from below ground (-V) toward +V. As Q2 base-emitter junction is backward-biased, it does not conduct, so all the current from R2 goes into C1. Simultaneously, C2 that is fully discharged and even slightly charged to 0.6 V (in the previous State 2) quickly charges via the low-value collector resistor R4 and Q1 forward-biased base-emitter junction (because R4 is less than R2, C2 charges faster than C1). Thus C2 restores its charge and prepares for the next State C2 when it will act as a time-setting capacitor. Q1 is firmly saturated in the beginning by the "forcing" C2 charging current added to R3 current; in the end, only R3 provides the needed input base current. The resistance R3 is chosen small enough to keep Q1 (not deeply) saturated after C2 is fully charged.

When the voltage of C1 right-hand plate (Q2 base voltage) becomes positive and reaches 0.6 V, Q2 base-emitter junction begins diverting a part of R2 charging current. Q2 begins conducting and this starts the avalanche-like positive feedback process as follows. Q2 collector voltage begins falling; this change transfers through the fully charged C2 to Q1 base and Q1 begins cutting off. Its collector voltage begins rising; this change transfers back through the almost empty C1 to Q2 base and makes Q2 conduct more thus sustaining the initial input impact on Q2 base. Thus the initial input change circulates along the feedback loop and grows in an avalanche-like manner until finally Q1 switches off and Q2 switches on. The forward-biased Q2 base-emitter junction fixes the voltage of C1 right-hand plate at 0.6 V and does not allow it to continue rising toward +V.

3.3 State 2 (Q1 is switched off, Q2 is switched on):

Now, the capacitor C2 is fully charged (in the previous State 1) to the power supply voltage V with the polarity shown in Figure 1. Q2 is on and connects the right-hand positive plate of C2 to ground. As its left-hand negative plate is connected to Q1 base, a maximum negative voltage (-V) is applied to Q1 base that keeps Q1 firmly off. C2 begins discharging (reverse charging) via the high-value base resistor R3, so that the voltage of its left-hand plate (and at the base of Q1) is rising from below ground (-V) toward +V. Simultaneously, C1 that is fully discharged and even slightly charged to 0.6 V (in the previous State 1) quickly charges via the low-value collector resistor R1 and Q2 forward-biased base-emitter junction (because R1 is less than R3, C1 charges faster than C2). Thus C1 restores its charge and prepares for the next State 1 when it will act again as a time-setting capacitor...and so on...

3.4 Multivibrator frequency

Derivation

The duration of state 1 (low output) will be related to the time constant R2C1 as it depends on the charging of C1, and the duration of state 2 (high output) will be related to the time constant R3C2 as it depends on the charging of C2. Because they do not need to be the same, an asymmetric duty cycle is easily achieved.

The voltage on a capacitor with non-zero initial charge is:

Looking at C2, just before Q2 turns on, the left terminal of C2 is at the base-emitter voltage of Q1 (VBE_Q1) and the right terminal is at VCC ("VCC" is used here

instead of "+V" to ease notation). The voltage across C2 is VCC minus VBE_Q1 . The moment after Q2 turns on, the right terminal of C2 is now at 0 V which drives the left terminal of C2 to 0 V minus (VCC - VBE_Q1) or VBE_Q1 - VCC. From this instant in time, the left terminal of C2 must be charged back up to VBE_Q1. How long this takes is half our multivibrator switching time (the other half comes from C1). In the charging capacitor equation above, substituting:

VBE_Q1 for

(VBE_Q1 - VCC) for

VCC for

results in:

Solving for t results in:

For this circuit to work, VCC>>VBE_Q1 (for example: VCC=5 V, VBE_Q1=0.6 V), therefore the equation can be simplified to:

or

or

The period of each half of the multivibrator is therefore given by t = ln(2)RC.

The total period of oscillation is given by:

T = t1 + t2 = ln(2)R2 C1 + ln(2)R3 C2

where...

• f is frequency in hertz.• R2 and R3 are resistor values in ohms.• C1 and C2 are capacitor values in farads.• T is the period (In this case, the sum of two period durations).

For the special case where

• t1 = t2 (50% duty cycle)• R2 = R3

• C1 = C2

3.5 Output pulse shape

The output voltage has a shape that approximates a square waveform. It is considered below for the transistor Q1.

During State 1, Q2 base-emitter junction is reverse-biased and the capacitor C1 is "unhooked" from ground. The output voltage of the switched-on transistor Q1 changes rapidly from high to low since this low-resistive output is loaded by a high impedance load (the series connected capacitor C1 and the high-resistive base resistor R2).

During State 2, Q2 base-emitter junction is forward-biased and the capacitor C1 is "hooked" to ground. The output voltage of the switched-off transistor Q1 changes exponentially from low to high since this relatively high resistive output is loaded by a low impedance load (the capacitance C1). This is the output voltage of R1C1 integrating circuit.

To approach the needed square waveform, the collector resistors have to be low resistance. The base resistors have to be low enough to make the transistors saturate in the end of the restoration (RB < β.RC).

3.6 Initial power-up:

When the circuit is first powered up, neither transistor will be switched on. However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on. This will quickly put the circuit into one of the above states, and oscillation will ensue. In practice, oscillation always occurs for practical values of R and C.

However, if the circuit is temporarily held with both bases high, for longer than it takes for both capacitors to charge fully, then the circuit will remain in this stable state, with both bases at 0.6 V, both collectors at 0 V, and both capacitors charged backwards to −0.6 V. This can occur at startup without external intervention, if R and C are both very small.

3.7 Frequency divider:

An astable multivibrator can be synchronized to an external chain of pulses. A single pair of active devices can be used to divide a reference by a large ratio, however, the stability of the technique is poor owing to the variability of the power supply and the circuit elements; a division ratio of 10, for example, is easy to obtain but not dependable. Chains of bistable flip-flops provide more predictable division, at the cost of more active elements.

3.8 Protective components:

While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the Veb breakdown voltage, typically around 5-10 volts for general purpose silicon transistors. In the monostable configuration, only one of the transistors requires protection.

3.9 Astablemultivibrator:

Astable Multivibrators are the most commonly used type of multivibrator circuit. An astable multivibrator is a free running oscillator that have no permanent “meta” or “steady” state but are continually changing there output from one state (LOW) to the other state (HIGH) and then back again. This continual switching action from “HIGH” to “LOW” and “LOW” to “HIGH” produces a continuous and stable square wave output that switches abruptly between the two logic levels making it ideal for timing and clock pulse applications.

As with the previous monostable multivibrator circuit above, the timing cycle is determined by the RC time constant of the resistor-capacitor, RC Network. Then the output frequency can be varied by changing the value(s) of the resistors and capacitor in the circuit.

NAND Gate Astable Multivibrator

The astable multivibrator circuit uses two CMOS NOT gates such as the CD4069 or the 74HC04 hex inverter ICs, or as in our simple circuit below a pair of CMOS NAND such as the CD4011 or the 74LS132 and an RC timing network. The two NAND gates are connected as inverting NOT gates.

Suppose that initially the output from the NAND gate U2 is HIGH at logic level “1”, then the input must therefore be LOW at logic level “0” (NAND gate principles) as will be the output from the first NAND gate U1. Capacitor, C is connected between the output of the second NAND gate U2 and its input via the timing resistor, R2. The capacitor now charges up at a rate determined by the time constant of R2 and C.

As the capacitor, C charges up, the junction between the resistor R2 and the capacitor, C, which is also connected to the input of the NAND gate U1 via the stabilizing resistor, R2 decreases until the lower threshold value of U1 is reached at which point U1 changes state and the output of U1 now becomes HIGH. This causes NAND gate U2 to also change state as its input has now changed from logic “0” to logic “1” resulting in the output of NAND gate U2 becoming LOW, logic level “0”.

Capacitor C is now reverse biased and discharges itself through the input of NAND gate U1. Capacitor, C charges up again in the opposite direction determined by the time constant of both R2and C as before until it reaches the upper threshold value of gate U1. This causes U1 to change state and the cycle repeats itself over again.

Then, the time constant for a NAND gate Astable Multivibrator is given as T = 2.2RC in seconds with the output frequency given as f = 1/T.

For example: if the resistor R2 = 10kΩ and the capacitor C = 45nF, the oscillation frequency of the circuit would be given as:

Then the output frequency is calculated as being 1kHz, which equates to a time constant of 1mS so the output waveform would look like:

3.10 Monostablemultivibrator Circuit

Monostable Multivibrators or “one-shot” pulse generators are generally used to convert short sharp pulses into wider ones for timing applications. Monostable multivibrators generate a single output pulse, either “HIGH” or “LOW”, when a suitable external trigger signal or pulse T is applied.

This trigger pulse signal initiates a timing cycle which causes the output of the monostable to change state at the start of the timing cycle, ( t1 ) and remain in this second state until the end of the timing period, ( t2 ) which is determined by the time constant of the timing capacitor, CT and the resistor, RT.

The monostable multivibrator now stays in this second timing state until the end of the RC time constant and automatically resets or returns itself back to its original (stable) state. Then, a monostable circuit has only one stable state. A more common name for this type of circuit is simply a “Flip-Flop” as it can be made from two cross-coupled NAND gates (or NOR gates) as we have seen previously. Consider the circuit below.

Simple NAND Gate Monostable Circuit

Suppose that initially the trigger input T is held HIGH at logic level “1” by the resistor R1 so that the output from the first NAND gate U1 is LOW at logic level “0”, (NAND gate principals). The timing resistor, RT is connected to a voltage level equal to logic level “0”, which will cause the capacitor, CT to be discharged. The output of U1 is LOW, timing capacitor CT is completely discharged therefore junction V1 is also equal to “0” resulting in the output from the second NAND gate U2, which is connected as an inverting NOT gate will therefore be HIGH.

3.11 Bistable Circuit:

The Bistable Multivibrators circuit is basically a SR flip-flop that we look at in the previous tutorials with the addition of an inverter or NOT gate to provide the necessary switching function. As with flip-flops, both states of a bistable multivibrator are stable, and the circuit will remain in either state indefinitely. This type of multivibrator circuit passes from one state to the other “only” when a suitable external trigger pulse T is applied and to go through a full “SET-RESET” cycle twotriggering pulses are required. This type of circuit is also known as a “Bistable Latch“, “Toggle Latch” or simply “T-latch“.

NAND Gate Bistable Multivibrator

The simplest way to make a Bistable Latch is to connect together a pair of Schmitt NAND gates to form a SR latch as shown above. The two NAND gates, U2 and U3 form the bistable which is triggered by the input NAND gate, U1. This U1 NAND gate can be omitted and replaced by a single toggle switch to make a switch debounce circuit as seen previously in the SR Flip-flop tutorial.

When the input pulse goes “LOW” the bistable latches into its “SET” state, with its output at logic level “1”, until the input goes “HIGH” causing the bistable to latch into its “RESET” state, with its output at logic level “0”. The output of a bistable multivibrator will stay in this “RESET” state until another input pulse is applied and the whole sequence will start again.

Then a Bistable Latch or “Toggle Latch” is a two-state device in which both states either positive or negative, (logic “1” or logic “0”) are stable.

Bistable Multivibrators have many applications such as frequency dividers, counters or as a storage device in computer memories but they are best used in circuits such as Latches and Counters.

4. 555 Timer 4.1 Introduction

Simple Monostable or Astable multivibrators can now be easily made using standard commonly available waveform generator IC’s specially design to create timing and oscillator circuits. Relaxation oscillators can be constructed simply by connecting a few passive components to their input pins with the most commonly used waveform generator type IC being the classic 555 timer.

The 555 Timer is a very versatile low cost timing IC that can produce a very accurate timing periods with good stability of around 1% and which has a variable timing period from between a few micro-seconds to many hours with the timing period being controlled by a single RC network connected to a single positive supply of between 4.5 and 16 volts.

The NE555 timer and its successors, ICM7555, CMOS LM1455, DUAL NE556 etc, are covered in the555 Oscillator tutorial and other good electronics based websites, so are only included here for reference purposes as a clock pulse generator. The 555 connected as an Astable oscillator is given below.

NE555 Astable Multivibrator.

Here the 555 timer is connected as a basic Astable Multivibrator producing a continuous output waveform. Pins 2 and 6 are connected together so that it will re-trigger itself on each timing cycle, thereby functioning as an Astable oscillator. Capacitor, C1 charges up through resistor, R1 and resistor, R2 but discharges only through resistor, R2 as the other side of R2 is connected to the discharge terminal, pin 7. Then the timing period of t1 and t2 is given as:

• t1 = 0.693 (R1 + R2) C1

• t2 = 0.693 (R2) C1

• T = t1 + t2

The voltage across the capacitor, C1 ranges from between 1/3 Vcc to approximately 2/3 Vcc depending upon the RC timing period. This type of circuit is very stable as it operates from a single supply rail resulting in an oscillation frequency which is independent of the supply voltage Vcc.

In the next tutorial about Sequential Logic Circuits, we will look another type of clock controlled flop-flop called a Data Latch. Data latches are very useful sequential circuits which can be made from any standard gated SR flip-flop and used for frequency division to produce various ripple counters, frequency dividers and latches.

4.2 Inputs of 555/556:Trigger input: when < 1/3 Vs ('active low') this

makes the output high (+Vs). It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M .

Threshold input: when > 2/3 Vs ('active high') this makes the output low (0V)*. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high input impedance > 10M . * providing the trigger input is > 1/3 Vs, otherwise the trigger input will override the threshold input and hold the output high (+Vs).

Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k .

Control input: this can be used to adjust the threshold voltage which is set internally to be 2/3 Vs. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.

The discharge pin is not an input, but it is listed here for convenience. It is connected

to 0V when the timer output is low and is used to discharge the timing capacitor in astable and monostable circuits.

4.3 Output of 555/556:The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducers directly, including LEDs (with a resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series), relay coils (with diode protection) and some motors (with diode protection). The output voltage does not quite reach 0V and +Vs, especially if a large current is flowing.

To switch larger currents you can connect a transistor.

The ability to both sink and source current means that two devices can be connected to the output so that one is on when the output is low and the other is on when the output is high. The top diagram shows two LEDs connected in this way. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately.

4.4 Loudspeakers

A loudspeaker (minimum resistance 64 ) may be connected to the output of a 555 or 556 astable circuit but a capacitor (about 100µF) must be connected in series. The output is equivalent to a steady DC of about ½Vs combined with a square wave AC (audio) signal. The capacitor blocks the DC, but allows the AC to pass as explained in capacitor coupling.

Piezo transducers may be connected directly to the output and do not require a capacitor in series.

4.5 Duty Cycle of 555 Timer IC

The duty cycle in a 555 integrated circuit (IC) is the percentage of time that the output is high for each cycle of the square wave. For example, if the total cycle time is 1 s and the output is high for the first 0.4 s of each cycle, the duty cycle is 40%.

With an astable circuit, the duty cycle must always be greater than 50%. In other words, the duration for which the output is high must always be more than the duration during which the output is low.

The explanation for this is pretty simple: For the duty cycle to be 50%, the capacitor would have to charge and discharge through the same resistance. The only way to accomplish that would be to omit R1 altogether, so that the capacitor charged and discharged through R2 only.

But the problem with that is that you would end up connecting pin 7 directly to Vcc. With no resistance between pin 7 and the voltage source, the current flowing through pin 7 would exceed the maximum that can be handled by the circuitry inside the 555, and the chip would be damaged.

There's a clever way around this limitation: Place a diode across R2. This diode bypasses R2 when the capacitor is charged. That way, the capacitor charges through R1 and discharges through R2.

When a diode is used in this way, you have complete control over the duration of both the charge and discharge time. If R1 and R2 have the same value, the capacitor takes the same amount of time to charge as it does to discharge, so the duty cycle will be 50%. If R2 is smaller than R1, the duty cycle is less than 50% because the capacitor discharges faster than it charges.

If you use this approach, you must adjust the formulas for calculating the time intervals as follows:

T = 0.7 (R1 + R2) C1

Thigh = 0.7 R1 C1

Tlow = 0.7 R2 C1

4.6 Monostable:

Schematic of a 555 in monostable mode

The relationships of the trigger signal, the voltage on C and the pulse width in monostable mode

In the monostable mode, the 555 timer acts as a "one-shot" pulse generator. The pulse begins when the 555 timer receives a signal at the trigger input that falls below a third of the voltage supply. The width of the output pulse is determined by the time constant of an RC network, which consists of a capacitor (C) and a resistor (R). The output pulse ends when the voltage on the capacitor equals 2/3 of the supply voltage. The output pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C.[5]

The output pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by

where t is in seconds, R is in ohms (resistance) and C is in farads (capacitance).

While using the timer IC in monostable mode, the main disadvantage is that the time span between any two triggering pulses must be greater than the RC time constant.

4.7 Bistable:

Schematic of a 555 in bistable mode

In bistable (also called Schmitt trigger) mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via pull-up resistors while the threshold input (pin 6) is simply floating. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground (low state). No timing capacitors are required in a bistable configuration. Pin 5 (control voltage) is connected to ground via a small-value capacitor (usually 0.01 to 0.1 uF); pin 7 (discharge) is left floating.

4.8 Astable:

Standard 555 astable circuit

In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:

[8]

The high time from each pulse is given by:

and the low time from each pulse is given by:

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

The power capability of R1 must be greater than .

Particularly with bipolar 555s, low values of must be avoided so that

the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above. The first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0V to 2/3 of VCC from power-up, but only from 1/3 of VCC to 2/3 of VCC on subsequent cycles.

To achieve a duty cycle of less than 50% a small diode (that is fast enough for the application) can be placed in parallel with R2, with the cathode on the capacitor side. This bypasses R2 during the high part of the cycle so that the high interval depends approximately only on R1 and C. The presence of the diode is a voltage drop that slows charging on the capacitor so that the high time is longer than the expected and often-cited ln(2)*R1C = 0.693 R1C. The low time will be the same as without the diode as shown above. With a diode, the high time is

where Vdiode is when the diode has a current of 1/2 of Vcc/R1 which can be determined from its datasheet or by testing. As an extreme example, when Vcc= 5 and Vdiode= 0.7, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 and Vdiode= 0.3, the high time = 0.725 R1C which is closer to the expected 0.693 R1C. The equation reduces to the expected 0.693 R1C if Vdiode= 0.

The operation of RESET in this mode is not well defined, some manufacturers' parts will hold the output state to what it was when RESET is taken low, others will send the output either high or low.

4.9 Power on reset or Trigger:

In VLSI devices, the power-on reset (PoR) is an electronic device incorporated into the integrated circuit that detects the power applied to the chip and generates a reset impulse that goes to the entire circuit placing it into a known state.

A simple PoR uses the charging of a capacitor, in series with a resistor, to measure a time period during which the rest of the circuit is held in a reset state. A Schmitt trigger may be used to deassert the reset signal cleanly, once the rising voltage of the RC network passes the threshold voltage of the Schmitt trigger. The resistor and capacitor values should be determined so that the charging of the RC network takes long enough that the supply voltage will have stabilised by the time the threshold is reached.

One of the issues with using RC network to generate PoR pulse is the sensitivity of the R and C values to the power-supply ramp characteristics. When the power supply ramp is rapid, the R and C values can be calculated so that the time to reach the switching threshold of the schmitt trigger is enough to apply a long enough reset pulse. When the power supply ramp itself is slow, the RC network tends to get charged up along with the power-supply ramp up. So when the input schmitt stage is all powered up and ready, the input voltage from the RC network would already have crossed the schmitt trigger point. This means that there might not be a reset pulse supplied to the core of the VLSI.

4.10 Edge-triggering:

edge-triggering circuit

If the trigger input is still less than 1/3 Vs at the end of the time period the output will remain high until the trigger is greater than 1/3 Vs. This situation can occur if the input signal is from an on-off switch or sensor.

The monostable can be made edge triggered, responding only to changes of an input signal, by connecting the trigger signal through a capacitor to the trigger input. The capacitor passes sudden changes (AC) but blocks a constant (DC) signal. For further information please see the page on capacitance. The circuit is 'negative edge triggered' because it responds to a sudden fall in the input signal.

The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high (+Vs).

4.11 555/556 Inverting Buffer (Schmitt trigger) or NOT gate:

555 inverting buffer circuit

(a NOT gate)

NOT gate symbol

The buffer circuit's input has a very high impedance (about 1M ) so it requires only a few µA, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).

It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:

• Input low (< 1/3 Vs) makes output high, +Vs• Input high (> 2/3 Vs) makes output low, 0V

When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger.If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least 1/3 Vs to make

the output switch back.

4.12 Features

• Direct replacement for SE555/NE555

• Timing from microseconds through hours

• Operates in both astable and monostable modes

• Adjustable duty cycle

• Output can source or sink 200mA

• Output and supply TTL compatible

• Temperature and stability better than 0.005%

• Normally on and off output

4.13 Applications• Precision timing

• Pulse generation

• Sequential timing

• Time delay generation

• Pulse width modulation

• Linear ramp generator

5. Diodes:

A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams such as Figure. The term “diode” is customarily reserved for small signal devices, I ≤ 1 A. The termrectifier is used for power devices, I > 1 A.

Semiconductor diode schematic symbol: Arrows indicate the direction of electron current flow.

When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage.

Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.

When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: “closed” when forward-biased and “open” when reverse-biased.

Oddly enough, the direction of the diode symbol's “arrowhead” points against the direction of electron flow. This is because the diode symbol was invented by engineers, who predominantly use conventional flow notation in their schematics, showing current as a flow of charge from the positive (+) side of the voltage source to the negative (-). This convention holds true for all semiconductor symbols possessing “arrowheads:” the arrow points in the permitted direction of conventional flow, and against the permitted direction of electron flow.

Diode behavior is analogous to the behavior of a hydraulic device called a check valve.

5.1 Thermionic and gaseous state diodes:

Thermionic diodes are thermionic-valve devices (also called as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by the vacuum within a glass envelope. Early examples were fairly alike in appearance to incandescent light bulbs.

In the thermionic valve diodes, a current through heater filament indirectly heats cathode, another internal electrode treated with the mixture of barium and strontium oxides, which are the oxides of alkaline earth metals; these substances are taken because they have a small work function. (Some valves make use of direct heating, in which a tungsten filament acts as heater and cathode both.) The heat causes thermionic emission of the electrons into vacuum. In forward operation, a surrounding metal electrode known as anode is positively charged so that it

electrostatically attracts emitted electrons. But the electrons are not easily released from unheated anode surface when the voltage polarity is reversed. Thus, any reverse flow is negligible.

For much of 20th century, thermionic valve diodes were taken in use in analog signal applications, and as rectifiers in many power supplies. Nowadays, valve diodes are only used in niche applications like rectifiers in electric guitar and high-end audio amplifiers also a specialized high- voltage equipment.

A thermionic diode is a thermionic-valve device (also known as a vacuum tube, tube, or valve), consisting of a sealed evacuated glass envelope containing two electrodes: a cathode heated by a filament, and a plate (anode). Early examples were fairly similar in appearance to incandescent light bulbs.

In operation, a separate current through the filament (heater), a high resistance wire made of nichrome, heats the cathode red hot (800–1000 °C), causing it to release electrons into the vacuum, a process called thermionic emission. The cathode is coated with oxides of alkaline earth metals such as barium and strontium oxides, which have a low work function, to increase the number of electrons emitted. (Some valves use direct heating, in which a tungsten filament acts as both heater and cathode.) The alternating voltage to be rectified is applied between the cathode and the concentric plate electrode. When the plate has a positive voltage with respect to the cathode, itelectrostatically attracts the electrons from the cathode, so a current of electrons flows through the tube from cathode to plate. However when the polarity is reversed and the plate has a negative voltage, no current flows, because the cathode electrons are not attracted to it. The unheated plate does not emit any electrons itself. So electrons can only flow through the tube in one direction, from cathode to plate.

In a mercury-arc valve, an arc forms between a refractory conductive anode and a pool of liquid mercury acting as cathode. Such units were made with ratings up to hundreds of kilowatts, and were important in the development of HVDC power transmission. Some types of smaller thermionic rectifiers sometimes had mercury vapor fill to reduce their forward voltage drop and to increase current rating over thermionic hard-vacuum devices.

Throughout the vacuum tube era, valve diodes were used in analog signal applications and as rectifiers in DC power supplies in consumer electronics such as radios, televisions, and sound systems. They were replaced in power supplies beginning in the 1940s by selenium rectifiers and then by semiconductor diodes by the 1960s. Today they are still used in a few high power applications where their ability to withstand transients and their robustness gives them an advantage over semiconductor devices. The recent (2012) resurgence of interest among audiophiles and recording studios in old valve audio gear such as guitar amplifiers and home audio systems has provided a market for the legacy consumer diode valves.

5.2 I-V Characteristic Curves of Semiconductors:

Semiconductor devices such as diodes, transistors and thyristors are all constructed using semiconductor PN junctions connected together and as such their I-V characteristics curves will reflect the operation of these PN junctions. Then these devices will have non-linear I-V characteristics, as opposed to resistors which have a linear relationship between the current and voltage.

So for example, the primary function of a semiconductor diode is rectification of AC to DC. When a diode is forward biased (the higher potential is connected to its Anode), it will pass current. When the diode is reverse biased (the higher potential is connected to its Cathode), the current is blocked. Then a PN junction needs a bias voltage of a certain polarity and amplitude for current to flow. This bias voltage also controls the resistance of the junction and therefore the flow of current through it. Consider the diode circuit below. When the diode is forward biased, anode positive with respect to the cathode, a forward or positive current passes through the diode and operates in the top right quadrant of its I-V characteristics curves as shown. Starting at the zero intersection, the curve increases gradually into the forward quadrant but the forward current and voltage are extremely small.

When the forward voltage exceeds the diodes P-N junctions internal barrier voltage, which for silicon is about 0.7 volts, avalanche occurs and the forward current increases rapidly for a very small increase in voltage producing a non-linear curve. The “knee” point on the forward curve.

Fig

Likewise, when the diode is reversed biased, cathode positive with respect to the anode, the diode blocks current except for an extremely small leakage current, and operates in the lower left quadrant of its I-V characteristic curves. The diode continues to block current flow through it until the reverse voltage across the diode becomes greater than its breakdown voltage point resulting in a sudden increase in reverse current producing a fairly straight line downward curve as the voltage losses control. This reverse breakdown voltage point is used to good effect with zener diodes.

Then we can see that the I-V Characteristic Curves for a silicon diode are non-linear and very different to that of the previous resistors linear I-V curves as their electrical characteristics are different. Current-Voltage characteristics curves can be used to plot the operation of any electrical or electronic component from resistors, to amplifiers, to semiconductors and solar cells.

The current-voltage characteristics of an electronic component tells us much about its operation and can be a very useful tool in determining the operating characteristics of a particular device or component by showing its possible combinations of current and voltage, and as a graphical aid can help visually understand better what is happening within a circuit.

6. Relay

A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric

power systems these functions are performed by digital instruments still called "protective relays".

Relay-lamp

A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.

When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If

the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing.

When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to semiconductor circuit components. Some automotive relays include a diode inside the relay case. Alternatively, a contact protection network consisting of a capacitor and resistor in series (snubber circuit) may absorb the surge. If the coil is designed to be energized with alternating current (AC), a small copper "shading ring" can be crimped to the end of the solenoid, creating a small out-of-phase current which increases the minimum pull on the armature during the AC cycle

6.1 Types Of Relays

1. Latching RelayLatching relays are also called impulse relays. They work in the bistable mode, and thus have two relaxing states. They are also called keep relays or stay relays because as soon as the current towards this relay is switched off, the relay continues the process that it was doing in the last state. This can be achieved only with a solenoid which is operating in a ratchet and cam mechanism. It can also be done by an over-centre spring mechanism or a permanent magnet mechanism in which, when the coil is kept in the relaxed point, the over-centre spring holds the armature and the contacts in the right spot. This can also be done with the help of a remanent core.In the ratchet and cam method, power consumption occurs only for a particular time. Hence it is more advantageous than the others.

2. Reed RelayThese types of relays have been given more importance in the contacts. In order to protect them from atmospheric protection they are safely kept inside a vacuum or inert gas. Though these types of relays have a very low switching current and voltage ratings, they are famous for their switching speeds.

3. Polarized RelayThis type of relay has been given more importance on its sensitivity. These relays have been used since the invention of telephones. They played very important roles in early telephone exchanges and also in detecting telegraphic distortion. The sensitivity of these relays are very easy to adjust as the armature of the relay is placed between the poles of a permanent magnet.

4. Buchholz RelayThis relay is actually used as a safety device. They are used for knowing the amount of gas present in large oil-filled transformers. They are designed in such a way that they produce a warning if it senses either the slow production of gas or fast production of gas in the transformer oil.

5. Overload protection RelayAs the name implies, these relays are used to prevent the electric motors from damage by over current and short circuits. For this the heating element is kept in series with the motor. Thus when over heat occurs the bi-metallic strip connected to the motor heats up and in turn releases a spring to operate the contacts of the relay.

6. Mercury Wetted RelayThis relay is almost similar to the reed relay explained earlier. The only difference is that instead of inert gases, the contacts are wetted with mercury. This makes them more position sensitive and also expensive. They have to be vertically mounted for any operation. They have very low contact resistance and so can be used for timing applications. Due to these factors, this relay is not used frequently.

7. Machine Tool RelayThis is one of the most famous industrial relay. They are mainly used for the controlling of all kinds of machines. They have a number of contacts with easily replaceable coils. This enabkes them to be easily converted from NO contact to NC contact. Many types of these relays can easily be setup in a control panel. Though they are very useful in industrial applications, the invention of PLC has made them farther away from industries.

8. Contacor Relay

This is one of the most heavy load relay ever used. They are mainly used in switching electric motors. They have a wide range of current ratings from a few amps to hundreds. The contacts of these relays are usually made with alloys containing a small percentage of silver. This is done so as to avoid the hazardous effects of arcing. These type of relays are mainly categorized in the rough use areas. So, they produce loud noises while operated and hence cannot be used in places where noise is a problem.

9. Solid State relaySSR relays, as its name implies are designed with the help of solid state components. As they do not have any moving objects in their design they are known for their high reliability.

10. Solid State Contactor RelayThese relays combine both the features of solid state relays and contactor relays. As a result they have a number of advantages. They have a very good heat sink and can be designed for the correct on-off cycles. They are mainly controlled with the help of PLC, micro-processors or microcontrollers.

7. Relay Applications

In general, the point of a relay is to use a small amount of power in the electromagnet -- coming, say, from a small dashboard switch or a low-power electronic circuit -- to move an armature that is able to switch a much larger amount of power. For example, you might want the electromagnet to energize using 5 volts and 50 milliamps (250 milliwatts), while the armature can support 120V AC at 2 amps (240 watts).

Relays are quite common in home appliances where there is an electronic control turning on something like a motor or a light. They are also common in cars, where the 12V supply voltage means that just about everything needs a large amount of current. In later model cars, manufacturers have started combining relay panels into the fuse box to make maintenance easier. For example, the six gray boxes in this photo of a Ford Windstar fuse box are all relays:

In places where a large amount of power needs to be switched, relays are often cascaded. In this case, a small relay switches the power needed to drive a much larger relay, and that second relay switches the power to drive the load.

Relays can also be used to implement Boolean logic. See How Boolean Logic Works for more information.

Advantages

The major advantage of a automatic light control system over stand-alone lighting controls or conventional manual switching is the ability to control individual lights or groups of lights from a single user interface device. This ability to control multiple light sources from a user device allows complex lighting scenes to be created. A room

may have multiple scenes pre-set, each one created for different activities in the room. A major benefit of lighting control systems is reduced energy consumption. Longer lamp life is also gained when dimming and switching off lights when not in use. Wireless lighting control systems provide additional benefits including reduced installation costs and increased flexibility over where switches and sensors may be placed.