LM 567 IC Touch-Tone Decoder
This touch tone decoder can be used to decode a DTMF (Dual Tone Multiple Frequency) signal from a telephone device or from other generator. Using the frequency values shown in the schematic diagram, this circuit decodes a standard touch tone codes. The components R1 and C1 for each 567 chip should be computed according to the formula: f=1/(1.1 *R1 *C1).
There are many single chip DTMF decoder available, but this circuits allows you to design your own frequency pairs, if you don’t want a standard frequency pairs coding. [Circuit’s schematic diagram source: National Semiconductor LM567 Datasheet]