BISTABLE CIRCUIT APPLICATIONS

We will just look in detail at two applications of the bistable circuit and leave you to devise some other circuits for yourselves.

FIRE ALARM

To start with the buzzer is off so the output from NAND gate A is 0. The thermistor is cold so its resistance is high and the inputs to NAND gate A are both 1.

When the thermistor is warmed its resistance falls and the lower input to gate A falls to 0. Gate A now switches on - its output becomes 1 and the buzzer sounds.
Cooling the thermistor again will not switch the buzzer off.
Can you work out why?


To switch it off again you have to press the reset switch R.

BURGLAR ALARM

To start with the LDR is in darkness and the buzzer is off so the output from NAND gate A is 0. The LDR is in darkness so its resistance is high and the inputs to NAND gate A are both 1.

When light from the burglar's torch is shone on the LDR its resistance falls and the lower input to gate A falls to 0. Gate A now switches on - its output becomes 1 and the buzzer sounds.
Switching off the light will not switch the buzzer off.
Can you work out why?

To switch it off again you have to press the reset switch R.