Set-Reset Circuit (555 Timer) – B2P3

We’re now going to build a circuit that behaves like an SR flip flop but has only one output.

Set reset circuit

This is what the truth table for the SR flipflop looks like and we have 2 inputs and 2 outputs. For this project, we are only concerned with the Q output. Let’s use two tactile switches to simulate the two inputs and let’s connect an LED to simulate the output.

Set reset circuit

This is what the circuit diagram looks like. Switch S1 acts as the SET switch. R3 is used as a pull-up resistor and this causes the supply voltage to appear at the trigger pin. Since the voltage at the trigger pin is greater than 1/3rd of the supply voltage, it causes the output to stay LOW. When S1 is pressed, it takes the voltage at pin 2 to ground and this causes the internal comparator to change its state, which causes the output to go HIGH.

We will use the reset pin, that is pin 4 to reset the timer. R2 acts as a pull-up resistor which normally keeps the timer out of reset. When we press and release S2, it resets the 555 timer and the output is LOW again.

Set reset circuit

Pin 5 is the control voltage pin and if we look at the internal structure then we can see that it is connected to the voltage divider at the 2/3rd supply voltage point. The voltage at this point is fed to the comparators and any fluctuation in this voltage, due to a noisy power supply, for instance, can cause the 555 timer to erroneously change state. An external 10nF capacitor can be added to pin 5 to try and filter out any noise. Alternatively, we can also inject an external voltage to this pin if we want to change the comparator threshold voltages.

Set reset circuit

Here’s what the assembled PCB looks like. Like before, ensure that the IC base and 555 timer are oriented correctly. Pressing the SET switch will also force the output to the 1 or ON state while pressing the RESET switch will always force the output to the 0 or OFF state. We will use this set-reset concept in future circuits, so let us move on to the next project.