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Project Notes

#537 Schmitt Switch Debouncer

Testing a simple Schmitt Trigger debounce circuit using a 74HC14.



The hysteresis of a Schmitt Trigger used in conjunction with a suitable RC filter provides effective switch debouncing:

  • the RC time constant for charge/discharge filters most of the switch bounce
  • Schmitt Trigger hysteresis can help with ensuring positive on and off transitions

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


I’ve put double-inversion in the circuit; the result is that the LED is normally on, and turns off while the pushbutton is depressed.

Time constants are purposely large:

  • on, a function of R2=10kΩ, C1=1µF: 10ms
  • off, a function of R1=100kΩ, C1=1µF: 100ms

R2 needs to be much less than R1, else pressing the button will not pull down the voltage sufficient to switch the inverter.





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

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


Credits and References

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 just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - 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
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.