decode a chain of the telephone DTMF signals

This circuit allows you to decode a chain of the telephone DTMF signals. It serves so much to know that number has marked the telephone like thus also a VHF one or to look around in the hidden messages that some television transmitters insert in their band of audio.

The heart of the circuit is a DTMF decoder IC with capability to decode 16 tones. When a valid DTMF signal is applied to the input of the circuit, a proper 4-bit code will appear on Q1~Q4 output pins and STD pin will generate an interrupt signal for informing a micro-controller. You can consider the true-table of the circuit operation:

KEY	DATA
	Q0	Q1	Q2	Q3
1	0	0	0	1
2	0	0	1	0
3	0	0	1	1
4	0	1	0	0
5	0	1	0	1
6	0	1	1	0
7	0	1	1	1
8	1	0	0	0
9	1	0	0	1
0	1	0	1	0
*	1	0	1	1
#	1	1	0	0

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