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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

About LEAP#482 ElectretAudioSensor
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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.