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Electronics Circuit::
 

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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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