DTMF Decoder Project


This circuit decodes touchtones and allow you to control external devices. It will give 20 distinct outputs based on how the touchtones are received. The upper bank of outputs is activated by using the "*" key and the lower bank is activated by using the "#" key. You have 1-0 "*" and 1-0 "#". To activate one of these outputs, press the desired number then "*" or "#". The "*" key might be considered as an ON and the "#" key might be considered on OFF. This would yield 10 pairs of commands or each output could be used independently to yield 20 outputs.

This circuit uses the SSI202 decoder chip as the dtmf detector and decoder. The 74175 is a latch to hold the selected number until the "*" or "#" is pressed. The 7485 is a comparator to detect the "*", "#" or a numeric key. The 74154 is a 4 to 16 line bcd decoder that takes the 4 bit data and expands it into individual output pins.