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Project Notes

#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

Build

Credits and References

About LEAP#536 SchmittCMOS/TTL
Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.