As with all Bell Labs projects, the DTMF spec was carefully crafted. The audio tones for the low-group and high-group were carefully chosen to avoid harmonics that might interfere with decoding ...

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 ...