# THE BISTABLE CIRCUIT

It would be very useful to have an electronic circuit that would:

Switch on an alarm if there was a fire and not switch it off until a reset switch was pressed.
Sound an alarm when someone tried to break into a car and keep it going until the owner came and pressed a switch in the car.
Make a truck move backwards and forwards between two light beans.
Switch on a warning signal when a train passes a certain point in the track and only switch it off when the train has passed another point.
Let a contestant in a quiz game press a switch to make a lamp go on but stop the other side lighting their lamp until the quiz master resets the whole system.

An electronic circuit that will do this is called a BISTABLE CIRCUIT.
This is a circuit that is stable in one of two states and will not change its state until the other switch is pressed.
The two inputs are made by two simple press Switches, labelled S and P and the output is Shown by the two LEDs. The switches are high when they are open and low when they are closed. When they are released they will spring open on their own, becoming high.

That may sound a little difficult so we will look at how one version of a bistable circuit works.
The version that we are going to use is made from two NAND gates connected together as shown in the circuit below.

Call the output of NAND gate X, C and the output of NAND gate Y, D.

To start with suppose that C = 1 and D = 0. That means that LED 1 is ON and LED 2 is OFF. R and S are open and so E and F are high (1).
We call this the SET STATE.
(Pressing or releasing S will not alter the output.)

Now suppose we press R. F becomes low (0) and so D becomes high. Input A to NAND gate X now becomes 1 and so, since E is already 1 the output from NAND gate X (C) goes low. LED 1 goes off and LED 2 comes on.
We call this the RESET STATE (pressing or releasing R will not alter the output).

 State E F C D 0 1 1 1 SET 1 1 1 0 1 0 0 1 RESET 1 1 0 1
The truth table for the bistable circuit is shown on the right.

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD