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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