#536 Power OK Indicator

Testing a simple Schmitt Trigger power OK indicator using a 74HC14.

Build

Notes

A practical “power OK” indicator is a signal that ideally:

provides a clear “ON” signal a short time after power is available (to allow for startup fluctuations etc)
immediately asserts an “OFF” signal once power is removed

The circuit demonstrated here is based on Experiment #125 from ARRL’s Hands-On Radio Experiments Volume 3.

Features of the circuit:

the time constant of the R2=1MΩ, C1=1µF RC circuit provide the startup delay. These values are very large so that the effect is quite obvious.
The hysteresis of a Schmitt Trigger is used here to provide a clear de-bounced startup signal
The switch in the circuit is used to simulate power on/off.
The LED and current-limiting resistor simulate a load

Is this circuit suitable for real use as-is? No - it is just a demonstration of some core principles and has a number of simplifications that wouldn’t match a practical application:

power loss may not cause the the positive power rail to be grounded. In this situation, there’s no path for rapid discharge.
the 74HC14 always has power in this circuit, whereas in reality it would probably also lose power. A power reservoir for the 74HC14 would allow the the inverter to assert a positive low signal for some time after main power is lost.

Construction

Breadboard

Schematic

Behaviour

The on-off cycle is captured in the following scope trace where

CH1 (Yellow) - switch output
CH2 (Blue) - inverter input
CH3 (Red) - output

scope