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Figure 4 - An electret microphone is both a microphone and high gain amplifier
Figure 4 - An electret microphone is both a microphone and high gain amplifier

Electret microphones are extremely common in just about any electronic device that needs to hear sound. The small round can microphones are extremely sensitive to any sound, and they include a built in electronic audio preamplifier right in the tiny metal can. What's interesting about these microphones is that they are powered by 3 to 12 volts, and use the same pin for power as for output, so there are only two connections to the can. Figure 4 shows several electret microphone that I salvaged from various audio appliances. You can see that there are two pins or solder points on the bottom of the can. Looking carefully at the pins, you will be able to see that one pin is also connected to the outer can by a small metal strip. This will always be the negative terminal.



Figure 5 - The sound activation circuit on a solderless breadboard
Figure 5 - The sound activation circuit on a solderless breadboard

The sound activated triggering system is shown in Figure 5 built on a solderless breadboard for initial prototyping and testing. There are not many wires or components to this circuit, so it is very easy to create. You can actually omit the LED and resistor R1 if you want because it is only there to help set the level of the sound activation as VR1 is adjusted. When you are adjusting VR1, the LED will turn off right at the point where the comparator is set to its maximum response, so this helps when setting the maximum sensitivity of the system.

When you are testing the circuit, either have the output from the 74121 one-shot feeding the relay board or another LED through a 1K resistor so you can see or hear the response from the system as you adjust VR1 and tap on the microphone with your finger. You will know the circuit is working when the relay closes or when the LED flashes for 1 second in response to some sound. The circuit will run fine from 3 volts to 6 volts and will respond to sounds as subtitle as a finger snap when adjusted properly. If you do not see any activation from sounds when adjusting VR1 through the entire range, check the polarity of the microphone, as this does matter.

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