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Project Notes

#037 ElectretTrigger - triggers a tone sequence when mic input over threshold

Use an Arduino to monitor the amplified signal from an electret microphone. When a threshold is exceeded, plays a ditty on the piezo speaker.

Here’s a quick video of the circuit in action:



The electret microphone is biased to about 1V, and the AC component of the signal tapped via C1.

The microphone signal is then fed to the LM324 setup as a subtracting amplifier:

  • positive input is set to Vcc/2 = 2.5V
  • microphone signal fed to negative input

    Vo = G1 * Vmic + G2 * Vcc G1 = -R2/R1 = -330kΩ/2.2kΩ = -150 G2 = (R1 + R2)/R1 * R4/(R3 + R4) = (2.2kΩ + 330kΩ)/2.2kΩ * 22kΩ/(22kΩ + 22kΩ) = 75.5

When no microphone signal, we are reading Vcc/2 at the output.

But it is really sensitive to changes in the microphone signal. Basically every 1mV results in a 150mV change in the output.

The amplified signal is read by the Arduino, and when the signal exceeds a threshold, the Arduino triggers a tune played on the piezo speaker.

Other amplifier configurations are possible. For example the ElectretADC demo uses a non-inverting amplifier configuration.


The Breadboard

The Schematic

The Build

NB: for simplicity, the circuit is powered from the Arduino 5V pin. That works fine, although it is equally possible to use a separate power supply (but ensuring there is a common ground connection).

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.