#536 Power OK Indicator
Testing a simple Schmitt Trigger power OK indicator using a 74HC14.
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.
The on-off cycle is captured in the following scope trace where
- CH1 (Yellow) - switch output
- CH2 (Blue) - inverter input
- CH3 (Red) - output