Tone Detector Circuit (updated 5/2008)

Purpose: This circuit detects a tone from an audio device such as an MP3 or CD player and can be used to trigger a prop. An example for its use would be a pair of talking heads where you need to trigger a prop, like a pneumatic pop-up, at a certain time. You'd record the audio for each head on separate tracks of a cd, then add short tones (the frequency of the tones would be out of the range of normal hearing) on one (or both) of the tracks to activate the prop.

I experimented with this circuit a couple of years ago but only recently got around to using it in a prop. So far, it has worked very well. Here are a few observations on using it:

  • I initially tried the circuit using low frequency tones (i.e. 20Hz). I found that for reliability, the length of the recorded tone had to be at least 0.5 seconds long. This was too long for the prop I was building so I decided to try tones above 15k. I was able to use 0.05 second long tones without any problem.
  • I generate the tones using the audio editing program Goldwave (Expression Evaluator/Waves/Sine, f=18,000Hz). Audacity, a free audio program, also has an easy to use tone generator.
  • I use the "mix" function in Goldwave to insert the tone into an existing audio track. The tone can be placed anywhere, on existing audio or on a dead spot, it really doesn't matter.
  • I'm using an MP3 player (uMP3 from Rogue Robotics) and have two sound files, one is a two minute long 18k tone for use in calibrating the circuit and the other is my program audio.
  • To calibrate the circuit, play the continuous tone and adjust the 5k pot until the led illuminates. The circuit has a fairly narrow bandwidth but using a multi-turn pot, the led will stay on for about two turns. Be sure to center the pot within this range.
  • Play the audio with the output split between the amplifier/speaker and the detector circuit.
  • My only concern right now is how stable the circuit will be with changing temperatures. With more use, I'm sure I'll learn the answer to this and will report back what I find.
  • Update from Halloween 2006: This circuit worked extremely well. There were absolutely no missed tones! It sat outside for several days with temperatures ranging from the low 40's to the upper 70's.
  • Update from Halloween 2007: Once again, the circuit worked very well. Set it and forget it, that's the kind of circuit I like!!

The output of the circuit goes low when a tone is detected, so it can be attached directly to a microcontroller (I'm using it with a BASIC Stamp2) or through a switching transistor to drive a relay or whatever. The output of the LM 567 will sink 100ma.

The LM567 can be set to a wide range of frequencies, so you can potentially use different tones for different purposes. The circuit below can be adjusted from about 9kHz to well over 20kHz. By substituting the capacitor C6 with a higher value, you can lower the frequency. The approximate formula is f=1/(R2 x C6) (capacitance is in farads). With a 10uf capacitor, you should be able to get down to around 20Hz.

Click here for a data sheet on the LM567. They are available from various suppliers, Digikey has them for under $2.



  Halloween Home last update 11/2013