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Hi Mark
Nice project. I'm needing to do something similair but for a theatrical cue light system - several outstations with "standby" and "go" LED's on them. These can all be connected onto the same line by one XLR cable (standard in theatre audio). The idea is that a stage manager can send silent, visual cues. It's quite a common system but i'm looking at building my own.
The reason for using DTMF rather than digital switching (RS232 and the like) is that i'm keen for this to be able to run within a standard audio multicore. I've had problems sending digital data down these due to induced noise and audio should be a lot better (it's designed to have audio on adjacent cables within the multicore).
In my system, the outputs from the HF4514 will feed into a set of dipswitches where the outstation address is set. The first two outputs being for the first outstation, the second for the second, and so on. The dip switches will connect the outputs to the correct relays.

To cut to the quick, do you have any ideas on where to source the necessary IC's? I'm in the UK, not sure where you are? Nobody seems to stock them.
Many thanks
C