Bi-Stable LED Circuit – B1P19

In project 6, we built a circuit that alternatively toggled two LEDs and we called this an astable multivibrator circuit as it was a free running circuit and didn’t have a stable state. We’re now going to build a circuit that can toggle two LEDs with the help of switches. This is called a bi-stable multivibrator circuit as it has two stable states – one state being when LED 1 is ON, the other when LED 2 is ON.

Bi-Stable Multivibrator - schematic

This is what the circuit diagram looks like and again, we’ve worked with all the components so let’s move to the simulation.

Bi-Stable Multivibrator - simulation

We have two LEDs that are being controlled by two NPN transistors. This means that the base has to be positive compared to the emitter for them to switch ON. The circuit is symmetric in nature, but because no two transistors are exactly alike, one will switch ON before the other. In this case, Q1 has switched ON, causing LED 1 to switch ON as well.

If we press switch S1, then it will force the base of Q1 to be at ground potential and this will switch Q1 OFF. Current can flow through R1, R5, R7 and this will switch ON Q2, turning ON LED 2. If we press S1 again, there will not be any change to the circuit as Q1 is already OFF and Q2 does not depend on S1. This is the first circuit state.

Now, if we press S2, it will force Q2 to switch OFF and current can flow through R2, R6, R8 causing Q1 to switch ON. This is the second circuit state. Pressing S2 again will not cause any change in the state.

This circuit forms the basis of one of the main components used in digital electronics, which is called a flip-flop or Set-Reset circuit. If we consider LED 1, pressing S1 will cause it to switch OFF or RESET itself, while pressing S2 will cause it to switch ON or SET itself. The converse relationship exists for LED 2.

Bi-Stable Multivibrator - layout

Let’s use the breadboard layout to build and test it. Flip-flops are used to create digital memory elements and this makes a bistable circuit very useful.

Bi-Stable Multivibrator - demo