555 Timer IC | Introduction

In this post, 555 Timer Integrated Circuit(IC) is introduced. The working of its internal circuit, it’s modes of operation and pinout are discussed.

Introduction:

555 timer IC is the most popular IC ever manufactured. It was introduced in 1972 by Signetics Corporation. It's a highly stable device for generation of accurate time delays and oscillation. Some of its major applications include precision timing, pulse generation, time delay generation, pulse width modulation(PWM).

Usually, 555 is operated in one of the two modes, astable or monostable:

·         Astable Mode: IC’s output oscillates continuously between the two stable states, HIGH and LOW, thus producing clock-like pulses at the output.

·         Monostable Mode/ One-Shot: corresponding to a trigger pulse as input, a pulse of a particular duration is generated. Usually used to create time delays.

Pinout:

This IC has 8 pins.The following figure shows the pin configuration of 555 Timer IC:

Internal circuit:

Internally 555 IC is made up of many transistors. But trying to understand it's working on transistor level can be difficult so we will look at the block diagram representation of the internal circuit, which is as shown in the figure below.

Here we have three resistors of value 5KOhm each connected in series between VCC and Ground such that they set voltage references of 2*VCC/3 and VCC/3 for the input of op-amps. Outputs of these op-amps are connected to the RS flip-flop and output is taken from the flip-flop with an output stage in between.

Pin Functions:

·         Pin1- Ground: Connects 555 IC to ground reference voltage.

·         Pin 2- Trigger: Connected internally to the negative input terminal of op-amp2. If the voltage at this pin drops below VCC/3, the internal flip-flop gets 'set' and switches the output at pin 3 from 'LOW' to 'HIGH.'

·         Pin 3- Output: pin from which the output of 555 is taken.

·         Pin 4-Reset: Apply Negative pulse to this pin, to disable or reset the IC. When not used for reset purposes, it should be connected to VCC to avoid false triggering.

·         pin 5- Control: An external voltage applied to this pin can be used to modulate the output voltage. When not in use, connect this pin to ground with a 0.01uF capacitor in between to eliminate any possible noise.

·         pin 6- Threshold: Connected internally to the positive input terminal of op-amp1. If the voltage at this pin rises above 2*VCC/3, the internal flip-flop gets 'reset' and switches the output at pin 3 from 'HIGH' to 'LOW'.

·         Pin 7- Discharge: connected directly to the Collector of an internal NPN transistor 'Q1', which is used to 'discharge' the timing capacitor to ground, when the output at pin 3 is 'LOW'.

·         Pin 8- VCC: Connects IC to the supply voltage with respect to ground.

Working:

Threshold and Trigger pins are generally used to give input to our IC. So, the working of 555 IC can be best explained by considering different voltages at these pins. Three cases are possible for the input voltages and are discussed below. Let pin 5 is connected to 0.01uF capacitor and then to ground, pin 4 and pin 8 are connected to VCC. Let's start the analysis:

Case1: Threshold voltage > 2*VCC/3 and Trigger voltage > VCC/3.

Then the output of op-amp1 is HIGH and that of op-amp2 is LOW. Input R and S to the RS flip-flop are logic '1' and '0', so the output of the flip-flop and also 555 IC is 'LOW'. Also, Q1 transistor's base is at logic 'HIGH', there is a short circuit(very low resistance as Q1 is in saturation) between pin 7 and ground.

Case2: Threshold voltage < 2*VCC/3 and Trigger voltage < VCC/3.

Then the output of op-amp1 is LOW and that of op-amp2 is HIGH. Input R and S to the RS flip-flop are logic '0' and '1', so the output of the flip-flop and also 555 IC is 'HIGH'. Also, Q1 transistor's base is at logic 'LOW', there is an open circuit(very high resistance as Q1 is in the cut-off state) between pin 7 and ground.

Case 3: Threshold voltage < 2*VCC/3 and Trigger voltage > VCC/3.

Then the output of op-amp1 is LOW and that of op-amp2 is also LOW. Input R and S to the RS flip-flop are logic '0' and '0', so the flip-flop goes into the memory state, i.e. it remains same as it's previous state.

These three cases are utilized by the 555 circuits to operate in various modes. We will look at how 555 works in the Astable and Monostable mode in my next posts.

Conclusion:

I hope you find this tutorial useful. comment your opinions and topics you want me to discuss in future.

Thanks for reading!