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Audio Detector Circuit

[Editors note: I grabbed this off our list server. This was too good to pass up. The Trinity Firefighting Contest uses a small piezo speaker to generate a tone that emulates a fire alarm. The following email answers a requst for a circuit that will detect that sound.]

Editors Second Note: Larry was kind enough to provide a better writeup of this article. Check out his article in the May 2000 Issue

----- Original Message -----
From: Larry Barello <larry@barello.net>
To: SRS Mailing List <srs@seattlerobotics.org>
Sent: Monday, March 06, 2000 12:21 PM
Subject: SRS: Audio Detector for FireFighting

Someone asked, a couple weeks ago, for a circuit to do audio detection.
This is what I used.  Here is one that uses a single IC (LM324 quad op amp)
and a handful of parts.  This circuit is for detecting one of those 3.6khz
(approx) beepers from Radio Shack.

AudioDetector.gif (28194 bytes)

A few notes:

1. 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.

2. 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.

3. The trimmer is used to set the center frequency.  I just run the beeper
and adjust for the strongest output signal.

4. It uses a condenser mic, surplus.  Probably any computer microphone will
do.  The 4.7K resistor is a typical load for those things.

Larry Barello