DTMF Control circuit


Posted on Feb 6, 2014

Control monitoring equipment at a TV repeater site. It had to accept commands sent as DTMF tones over the repeaters audio channel and be able to switch either AV signals or power supply feeds at currents up to 1 Amp. The switching requirements left no option but to use relays and the cheap and easily available BT53 ” style was chosen. As these are two pole relays but only one was called for, the other was


DTMF Control circuit
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put to use to drive a monitoring LED. The LED is illuminated when the relay is energised. It is simple to modify the the wiring to allow both poles to be used if the LED facility isn ’t needed. The design specification stated that the unit should co-exist with other MF controlled equipment, specifically, the single MF tone pair commands that GB3XG used and the sequence of four tones that GB3ZZ used. When I wrote the operating system for GB3ZZ over a decade ago, I deliberately left a gap in the valid command sequence "for future developments ”. In hindsight, this was a good idea as it left space for commands this unit could use. The range of MF commands available is *50# through to *59# although this can easily be changed by modifying the source code and re-assembling the PIC program. Unfortunately, GB3XG uses single tone commands so for example, keying *52# would be seen as four individual instructions that could have left the repeater performing unwanted actions. To overcome this problem, the audio feed to the repeater had to be disconnected as soon as the * ” was seen and then reconnected after the # ” was received. One of the relays is used for this purpose. The audio is looped through the board and interrupted for the duration of the command sequence. All the other relays have accessible changeover switch contacts (note: SK6 and SK7 have reversed pin-outs) and are available for general-purpose use. To make the unit as...




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