555 timer ic, its equivalent circuit and its applications
- 1. 555 TIMER IC Assigned by Sir Touqeer Presented by K-14EL-04
- 2. 555 timer ic ?????? What is this? ■ It is an ic an integrated circuit. ■ It is used in electronic circuits and it covers a wide variety of applications. ■ It works as an square wave oscillator ,a timer , pulse generator.
- 3. Why so named as 555? ■ It is hypothesized that it is so called 555 because of three 5 kilo ohms reistors used inside it. ■ But what the inventor stated is “It was just a random number an arbitrary number”.
- 4. Market available versions…… 555 , 556, 558. ■ The 555 timer comes as 8 pin DIP (Dual In-line Package) device. ■ There is also a 556 dual version of 555 timer which consists of two complete 555 timers in 14 DIP ■ 558 quadruple timer which is consisting of four 555 timer in one IC and is available as a 16 pin DIP in the market.
- 5. Who invented it? Hans Camenzind! ■ The IC was designed in 1971 by Hans Camenzind. ■ Born in Switzerland. ■ under contract to Signetics, an American company. ■ later acquired by Dutch company Philips Semiconductors
- 6. What’s inside the 555 ic?
- 7. A look inside 555 says… ■ A standard 555 ic has 23 transistors ■ It has 15 resistors built-in. ■ And 2 diodes. ■ A total of 40 discrete elements it has!
- 8. 555 seems to be complex! Then How to study it? ■ Complex has been transformed into simpler one. ■ A simpler version has two comparators ■ Three 5 kilo ohm resistors. ■ A set-reset flip flop ■ A pnp transistor
- 11. Pin functions ■ Ground: This pin is used to provide a zero voltage rail to the Integrated circuit to divide the supply potential between the three resistors shown in the diagram. ■ Trigger: As we can see that the voltage at the non-inverting end of the comparator is Vin/3, so if the trigger input is used to set the output of the F/F to ‘high’ state by applying a voltage equal to or less than Vin/3 or any negative pulse, as the voltage at the non- inverting end of the comparator is Vin/3. ■ Output: It is the output pin of the IC, connected to the Q’ (Q-bar) of the F/F with an inverter in between as show in the figure.
- 12. Pin functions ■ Reset: This pin is used to reset the output of the F/F regardless of the initial condition of the F/F and also it is an active low Pin so it connected to ‘high’ state to avoid any noise interference, unless a reset operation is required. So most of the time it is connected to the Supply voltage as shown in the figure.
- 13. Pin functions ■ Control Voltage: An external voltage applied to this terminal changes the threshold as well as trigger voltage. Thus by imposing a voltage on this pin or by connecting a pot between this pin and ground, the pulse width of the output wave can be varied. when not used, the control pin should be bypassed to ground with a 0.01uF capacitor to prevent any noise problem. ■ Threshold: The pin is connected to the non-inverting input of the first comparator. The output of the comparator will be high when the threshold voltage will be more than (2/3) Vin thus resetting the output (Q) of the F/F from ‘high’ to ‘low’.
- 14. Pin functions ■ Discharge: This pin is used to discharge the timing capacitors (capacitors involved in the external circuit to make the IC behave as a square wave generator) to ground when the output of Pin 3 is switched to ‘low’. ■ Supply: This pin is used to provide the IC with the supply voltage for the functioning and carrying of the different operations to be fulfilled with the 555 timer.
- 15. 555 ic operating modes. It’s a multivibrator! ■ A multivibrator is a circuit which rapidly switches between two states. ■ It has three operating modes which correspond to three configuration of multivibrator. ■ Astable mode of operation: It is also known as self triggering or free running mode. It has no stable state. It has two quasi stable states that automatically changes from one to another. It changes from high to low state and low to high state without any trigger input after pre determine time. This mode is used to generate square wave oscillations, clock pulse, PWM wave etc.
- 16. Mono-stale mode. ■ it is also known as single shot mode. It has one stable state and one quasi stable state. It jumps into quasi stable state from stable state when trigger input is applied and comes back to stable state after pre determine time automatically. It is used in generating pulses, time delay etc.
- 17. Bi-stable mode: It is also known as flip-flop mode. It has both stable states. Two different trigger inputs are applied to change the state from high to low and low to high. It is used in automatic switching applications, to generate pulse of variable time etc Once on remains on till triggered to another level.
- 18. Astable mode of operation. First make these points clear! ■ The threshold comparator has the inverting terminal always at 2/3 of Vcc because of the voltage divider. ■ The trigger comparator has its non– inverting input always at 1/3 of Vcc because of the voltage divider. ■ The output is taken from Q of the FF. ■ The transistor is NPN and its base is connected to Q’.
- 20. At the instant When the switch is just on! 1. C is at 0 volts completely uncharged. 2. This means the trigger (LC) provides a high output and sets the flip flop. 3. Whereas the threshold comparator provides a low output making rest zero. 4. As the flip flop is set the Q out put is high and Q’ is low. 5. Since Q’ is low the transistor is off.
- 21. When the capacitor charges upto 1/3 of Vcc. 1. Now the trigger comparator gives a low output means S is at 0. 2. But the threshold is still gives low output means R is still at 0. 3. And this the condition for the hold mode of SR FF. 4. So the output remain at the previous state that is Q’ at low and Q at high.
- 22. When the capacitor charges up to 2/3 of Vcc. 1. Now the threshold comparator provides high output resetting the FF i-e R=1. 2. But the trigger comparator is at low output that is S =0. 3. This the reset condition for the FF and Q is low Q’ is high. 4. Initially, the output of the 555 was high now it is low and at the same spot the transistor is ON. 5. And the capacitor starts discharging through RB
- 23. When the capacitor discharges below 2/3 of Vcc. 1.The threshold comparator gives low output that is R=0. 2. But the trigger comparator is still at the high output that is S=0. 3.Again the hold mode but this time for short because the comparator is discharging fast only through RB. 4. So the output is low still as previous.
- 24. When the capacitor discharges below 1/3 of Vcc. 1. At this spot the trigger comparator gives a high output that is S=1. 2. But the threshold comparator is still at low output that is R=0. 3. This is again the set condition for the FF. 4. The output is at high and Q’ is at low. 5. The output of 555 switches to high and the transistor is OFF. 6. Since the transistor is OFF the capacitor again starts charging from 1/3 of Vcc to 2/3 of Vcc. 7. In this the cycle repeats itself again and again and we have a square wave output.
- 25. IN SHORT! ■ The capacitor charges and discharges between 1/3 of Vcc and 2/3 of Vcc. ■ Consequently the output goes High when the capacitor is charges from 2/3 of Vcc to 1/3 of Vcc. ■ The output goes low when the capacitor is charged from 1/3 of Vcc to 2/3 of Vcc. ■ When the capacitor is in process of charging or discharging the FF is in HOLD mode.
- 26. Mono-stable mode of operation.
- 31. Bi-stable mode of operation