This electronic detector circuit is to do audio detection. The circuit is for detecting one of those 3.6khz (approx) beepers from Radio Shack. The component used is a single IC (LM324 quad op amp) and a handful of parts.
Notes
The capacitor and resistor on the output of the peak detector are selected to give a reasonable decay time. I.e. so a single pulse doesn't stretch out and be miss-interpreted as an audio signal. I think I sample the output at 100ms intervals and signal a valid sound if three consecutive samples are true.
The only critical parts are the trimmer, capacitors and the 560 ohm resistor in the band pass filter. The diode is not critical: any small diode will do fine.
The trimmer is used to set the center frequency. I just run the beeper and adjust for the strongest output signal.
It uses a condenser mic, surplus. Probably any computer microphone will do. The 4.7K resistor is a typical load for those things
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