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.