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Project Notes

#482 Electret Sound-level Trigger

A simple circuit using a TL072 op-amp and 74LS14 inverter for getting a clear digital trigger signal based on the sound level detected by an electret microphone.



The purpose of this circuit is to get a clear digital signal (as close to rail-rail as possible) based on the sound level picked up by an electret microphone. It is not intended to amplify the audio input, but to provide a trigger once a variable threshold is reached.

I put this circuit together in four basic stages:

  • the electret input is amplified with a high-gain inverting amplifier
  • an op-amp configured as a variable comparator turns the analog input to a binary output
  • an RC filter with a time constant of araound 47ms provides some “de-bouncing” and extends the minimum duration of any triggered input
  • a series of inverters pull the signal to as-near rail-rail as possible, and allow tapping the output with the desired polarity (active-low or active high)






Here’s a scope trace of a sound triggering the output.

  • CH1 (yellow) is the interter output
  • CH2 (blue) op-amp output
  • CH3 (red) electret output (note the 200mV scale)


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.