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How can I use a LM2907 and a microphone to distinguish between two given frequencies; either 5400 Hz or 1900 Hz? Answered

I want to use a LM2907 and a microphone to pick up a frequency (also be able to distinguish between the two frequencies and background noise). I see before me that there will be two outputs. 5V or 0V for "picking up frequency" or "Just background noise" and another output telling whether the microphone picks up the 5400 Hz or the 1900 Hz by 0V or 5V, respectively (the order is not that important when I recon I just can put on an inverter). 
What I need to know is the values for my C´s and R´s in the circuit. Can anybody help? I think the Minimum Component Tachometer-circuit will be a good starting point. 
The outputs are used for inputs for logic gates.

3 Replies

Janniche (author)2012-05-02

Thanks for both of your answers! I will have a look at the one from iceng. I will not try and go into microcontrollers because we are doing out project without.

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steveastrouk (author)2012-05-02

A.'s solution is classic method. Another cheat I've used is to make sure the dual tones are a multiple of DTMF frequencies, and then underclocked a decoder chip.

These days, its possible to do the decode on a microcontroller running a Goertzel filter algorithm - there's a really really good Instructable on the art here. , written by LargeMouthBass.


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