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I wanted to make a gift for someone that uses a cheap DTMF phone as a controller.

I thought I could just take it apart and rewire the keypad's ribbon cable but it turns out that the whole thing is glued shut and accessing the keypad with a dremel would mangle the case.

Does anyone know if there's an easy way to capture the output of the phone cable that's attached to it? Ideally I'd just connect that into a UC and read off what numbers the user was pressing but I can't find a spec for it anywhere.

Thank you!

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  • \$\begingroup\$ You can do this with a raspberry pi and lots of elbow grease. google.com/… \$\endgroup\$
    – Voltage Spike ♦
    Commented Dec 19, 2023 at 21:45

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I think this will be far more trouble than it's worth. The phone will be designed to work on a 50 V system (but it might work at > 24 V) and then you'd need a filter and DTMF decoder chip (which are available). Only at that stage can you start to consider interfacing with your microcontroller.

Does the person like chocolates?

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  • \$\begingroup\$ haaaaa very reasonable. yeah 50V is a lot of juice to step up from USB power or a battery. I'll just dremel it and hook into the keypad -- will be a bit ugly but so it goes. thank you! \$\endgroup\$
    – user358829
    Commented Dec 19, 2023 at 20:27
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    \$\begingroup\$ Actually, you're confusing the open-circuit voltage of a POTS line with the actual operating (off-hook) voltage that the phone requires, which is probably on the order of 6-8 V at about 60-80 mA. It may even work just fine at the 5 V available from USB. Just couple the audio into a DTMF decoder chip using either a load resistor and a capacitor or a small transformer, and you're good to go. \$\endgroup\$
    – Dave Tweed
    Commented Dec 19, 2023 at 22:19

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