I designed this Wikipedia:DTMF decoder so that I could monitor the output of our local 440Mhz repeater for DTMF tones, and taken actions based on them. The plan was to be able to trigger weather reports from the weather station on the houseboat. However, due to the repeater not being able to pass DTMF tones, the project never really got past the "design and build it" stage.
The decoder listens for audio on the audio input line. Decoding is handled by a Holtek HT9170. The output goes into an AT89C2051 processor which formats the RS-232 serial output. In addition, there are LEDs that indicate DTMF detection and heartbeat status, and 4 open-drain FETs that can be used to control things, such as the Push-To-Talk line (PTT) on the transceiver. There are also inputs for squelch detection (the controller can report the opening and closing of the squelch line).
In the final version, a Linux box was to monitor the DTMF decoder for open squelch and a certain DTMF sequence. After the squelch closed, the Linux box would use Festival to convert the current weather report to speech, key the radio via the PTT line, then play the weather report out the sound card. This was completely implemented and tested with a radio acting as a repeater.
- DTMF Decoder Schematic (27 KB)
- (no image yet)
- (eventual link to Subversion repository)