[Colin] built a shield that has an audio input and can decode DTMF signals. The hardware is based around an MT8870 DTMF decoder chip ... There is a fifth pin that strobes after each new tone.

The device consists of a DTMF decoder attached to the headphone jack of a cheap handy talkie. When a DTMF tone is received, a NodeMCU connected to the decoder calls an IFTTT job to echo the key to ...