Power on reset circuit LM324
Electronic

When you use a computer, the registers in its circuit must be initialized, or reset, when power is turned ON. This is done by a function called "POWER ON reset." Many computerized products have a reset button, like the one found on the rear surface of a palm-top computer.

At power ON time, our reset circuit works just like a reset switch when is it pressed. Computers are also reset when the supply voltage drops, so they may not go out of control. In this project, the steady-state voltage to the circuit is 9 V.

When the supply voltage is changed from 6 V to 9 V, the output of the comparator U1A stays Low for a while to reset the circuit. Therefore, the LED display switches from "none" to "1" after a short time. The delay in display action corresponds to the reset period.

U1A is a comparator, while the LEDs develop the reference voltage for U1 A. (The voltage drop of the LEDs is approx. 1.5 to 2.0 V each, which is not significantly affected by the bias voltage.)

When power is turned ON or OFF, the LED display switches after the period over which C1 is charged or discharged.