STUDY OF IC 555 (TIMER IC)

• Contains built in timer circuit hence widely known as timer IC.
• Can produce precise time delays.
• IC 555 is an eight pin package.
• Has three basic modes of operation:-

o Astable (no trigger required).
o Monostable (one trigger required).
o Bistable (two triggers are required).

• Special features:-

o Low-cost timer IC.
o Precise time delays are possible with a wide range of frequencies that can be set.
o Three basic useful operating modes MONOSTABLE, ASTABLE AND BISTABLE.
o Complementary outputs from R-S Flip-Flop namely Q and Q’ (Q-bar).
o Time delay produced at the output is independent of voltage fluctuations. It depends on external resistance and capacitance.
o Compatible with both Analog and Digital circuits.

• IC Specifications:-

(TA = 25°C, VCC = 5 V to 15 V, unless otherwise specified)
o Supply Voltage :- 3V to 16V
o Maximum load current :- 200mA
o Supply Current :-

 (VCC = 5 V, RL = ∞; Test Condition); 3mA (Typical); 6mA (Max).
 (VCC = 15 V, RL = ∞; Test Condition); 10mA (Typical); 15mA (Max).

o Timing Error, Monostable

 Initial Accuracy:- 1 %
 Drift with Temperature: - ( RA = 1 k to 100kΩ, C = 0.1μF; Test Condition); 50ppm/°C
 Accuracy over Temperature:- 1.5 %
 Drift with Supply:- 0.1 % V

o Timing Error, Astable

 Initial Accuracy:- 2.25
 Drift with Temperature: - (RA, RB =1 k to 100kΩ, C = 0.1μF; Test Condition); 150ppm/°C.
 Accuracy over Temperature: - 3.0%.
 Drift with Supply: - 0.30 % /V.

o Threshold Voltage: - 0.667 x VCC.
o Trigger Voltage: - 1.67 x VCC.
o Trigger Current: - 0.5μA (Typical); 0.9μA (Max).
o Reset Voltage: - 0.4V (Min); 0.5V (Typical); 1V (Max).
o Reset Current: - 0.1mA (Typical); 0.4mA (Max).
o Threshold Current: - 0.1μA (Typical); 0.25μA (Max).
o Control Voltage Level: - (VCC = 15V, Test Condition); 9V (Min); 10V (Typical); 11V (Max).

• Pin Configuration:-

1. Pin 1. – Ground, The ground pin connects the 555 timer to the negative (0v) supply rail.

2. Pin 2. – Trigger, The negative input to the lower comparator. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from the “LOW” to  the“HIGH” state.

3. Pin 3. – Output, The output pin can drive any TTL circuit and is capable of loading up to 200mA of current at an output voltage equal to 1.5V.

4. Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low pin.

5. Pin 5. – Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.

6. Pin 6. – Threshold, The positive input to the upper comparator. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.

7. Pin 7. – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.

8. Pin 8. – Supply +Vcc, This is the power supply pin.

• Internal Block Diagram:-

1. Voltage divider: - IC 555 contains a resistor voltage divider between  the supply voltage and ground with three equal 5 kilo-ohm resistors. Hence, the name IC 555. The main function of Voltage divider is to produce two reference voltages two third of the supply voltage and one-third of the supply voltage.

2. Operational Amplifier: - A comparator circuit which operates input voltage with some reference voltage. The output of the comparator is either high or low.

  The threshold voltage is greater than or equal to two-third of the supply voltage. (Input to the non-inverting terminal).
 Trigger voltage less than or equal to one-third of the supply voltage. (Input to the inverting terminal).

3. R-S Flip-Flop:- ‘S’ pin of the flip-flop is driven by the upper comparator containing threshold as the input.
‘R’ pin of the flip-flop is driven by the lower comparator containing trigger as the input.
‘Q’ is conducted to internal NPN transistor known as the discharge transistor because it controls the charge and discharge time of the externally connected capacitor.
Working:-
‘Q’ drives the discharge transistor when ‘Q’ is high, as transistor goes to saturation leading to its conduction which sets the path for the capacitor to discharge.
When ‘Q’ is low the discharge transistor foes in the cut off state leading to an open switch system which helps the capacitor to get charged.
“Q’ (Q-bar)” is taken as the common use output of the IC.
PIN 4 the reset pin of the IC makes the IC’s timer to its reset position when the Q-bar is set low.

Truth Table:-

• Summary:-

1. Voltage divider produces two reference voltage, two third of the supply voltage and one-third of the supply voltage, for the conduction of respective comparators.

2. When the threshold input is greater than two third of the supply voltage, the upper comparator output goes high and sets the flip-flop making the transistor on leading to the discharge of the capacitor and leading to a low pulse at the IC’s output.

3. When the trigger input is less than one-third of the supply voltage, the lower comparator output goes high and sets the flip-flop making the transistor off leading to the charge of the capacitor and leading to high pulse at the IC’s output.

• Applications:-

1. ASTABLE MULTIVIBRATOR

• Circuit Diagram:-
  
• Can generate a variable output voltage that oscillates between high and low voltage levels. Hence, named Astable Multivibrator.
• In this, mode no external input is required to trigger the circuit. The capacitor voltage itself controls the two opposite comparators making the flip-flop set or reset alternatively.
• The output of this circuit is not symmetric, it is not a perfect square way as the charging and discharging time depends on different parameters.
  Charing time depends on the 470k resistor, 1k resistor, and the 10microFarad capacitor.
  Whereas, discharging time depends on the 470k resistor and 10microFarad capacitor.
  Hence, On time is never equal to Off time.
  T(on) = 0.693 * (Ra + Rb) * C.
  T(off) = 0.693 * (Ra + 2Rb) * C.
• Working:-
  The capacitor voltage is exponentially increasing, the output of IC 555 remains HIGH. But, when it exceeds two-third of the supply voltage the PIN-6 of the IC makes the output of the upper comparator HIGH making the output of the flip-flop go LOW. It sets the flip-flop making the Q-bar low causing a low output of IC making LED turn OFF. At the same time, Q is high which makes the discharge transistor ON, allowing the capacitor to discharge through it.
  Frequency of this circuit can be calculated by the formula: -
  F = 1.44 / (Ra + 2 * Rb) * C       Hz.
  Duty Cycle of the circuit is given by: -
  D % = (Ra + Rb) / (Ra + 2 * Rb) * 100 %

2. MONOSTABLE MULTIVIBRATOR

• Circuit Diagram:-
  
•  Can generate a variable output voltage that oscillates between high and low voltage levels, but when a triggered pulse is given. Hence, named Monostable Multivibrator.
• In this, mode one external input is required to trigger the circuit. The Push to ON switch sets and resets the flip-flops.
• The output of this circuit is dependent on our switching speed, it may or may not produce a perfect square way as the charging and discharging time depends on different parameters and the frequency at which we switch.
  T = 1.1 * R * C.
• PIN-2 is kept at supply voltage in order to make trigger above one-third of it.
• When the switch is made ON the PIN-2 is connected to ground potential making the time delay start.
• PIN-4 is connected to supply voltage in order to make the reset PIN inactive as we don't want to use the facility in this circuitry.
• PIN-3 is the output. Hence, LED is connected as an indicator.
• A 0.22microFarad capacitor is used to keep control of the voltage fixed at two-third of the supply voltage and filter the external unwanted noise.
• The delay can be changed by varying the value of 470k potentiometer or the 230microFarad capacitor.
• Thus, the value of RC time constant decides the width of the pulse at the output.
• Working:-
  The capacitor voltage is exponentially increasing, the output of IC 555 remains HIGH. But, when it exceeds two-third of the supply voltage the PIN-6 of the IC makes the output of the upper comparator HIGH making the output of the flip-flop go LOW. It sets the flip-flop making the Q-bar low causing a low output of IC making LED turn OFF, indicating that the time delay is over. At the same time, Q is high which makes the discharge transistor ON, allowing the capacitor to discharge through it.

3. BISTABLE MULTIVIBRATOR
4. PWM (Pulse Width Modulation)

• Circuit diagram:-
  
• In this case, the reference voltage two-third Vcc remains unchanged.
• PWM is an analog technic of pulse modulation where the pulse width of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal.
• The width is decided by the control voltage of the IC PIN-5.
• When the modulating voltage is maximum the width is maximum and at minimum (maximum negative amplitude) the pulse width is minimum.

5. PPM (Pulse Position Modulation)

• Circuit Diagram: -
  
• The input modulating signal must be converted into pulse width modulation before getting the PPM waveform.
• PPM is an analog technique of pulse position modulation where the position of the pulse is varied in accordance with the amplitude of the modulating signal.
• The PWM signal is used to trigger the monostable circuitry, causing variations in the position of the output waveform.

6. FSK (Frequency Shift Key Generator)

• Circuit Diagram: -
  
• FSK sends binary data in the form of frequency.
• IC 555 uses Astable Multivibrator mode to produce FSK generator with the input signal as 0 or 1.
• The frequency of square wave generated by IC 555 depends on the transistor switch.
• Working: -When the input is 0 the transistor remains OFF. Hence, Ra is in the circuit generating a lower frequency.
  When the input is 1 the transistor becomes ON. Hence, Rc is in the circuit which acts as a shunt to Ra leading to the generation of a higher frequency waveform.
• ~Jay Mehta
  

  

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