Was playing with some cheap radios and decided that the keypad DTMF tones could be used for remote control.
I had some old source that detected touch tones using a AVR microcontroller and decided to convert it for Arduino.
This resulted in a library that detect audio tones played into any Arduino pin.
The audio is first clipped to a 1-bit signal (played loudly into a digital pin) then sampled at 10kHz.
256 1-bit samples (25.6mS) can then be analyzed for the content of one or more audio tones.
The analyzer uses a crude 1-bit DFT but works surprisingly well at detecting multible tones such as DTMF or guitar plucks.
The result can be presented as either a DTMF symbol or approximate signal strength for any frequency analyzed (up to 5kHz).
Here is the library for now (will move to github). [Updated broken link 05/22-2018]