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I got my Easy Digi all wired up correctly, believe me, it took a while because of this issue. I've ruled out the FTDI controller I'm using because I can manually activate the optocoupler but supplying 5v to the input pins and still have the issue. Even when I switch off the power to the input pins, the HT stays transmitting. If I use a UHF frequency, it doesn't happen. Some sort of RF interference is keeping the output pins flowing.

My question is: Can I add a passive component somewhere on the output of the octocoupler to stop this? I was probing around when it was happening and when my meter leads touched the 2 output pins (the 2 sides of the diode), it would shut off.

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    $\begingroup$ I only play around with electronics but it sounds like RF coupling to me. Could you add some kind of shielding to encapsulate the parts ? A picture would help others respond I think. Are you using the UC Berkeley instructions ? The only good mode for digital FM that I've been able to find is MT63 which works well using acoustic coupling and there's an Android modem AndFLmsg. After making this work you want to try it. $\endgroup$
    – wbg
    Jun 1, 2022 at 15:39

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Add a 0.1uF or 0.01uF ceramic cap between the signal line and ground to short out any stray RF that is getting into the input and possibly output pins of your circuit. A low value series resistor of 100-470 ohms or so might also help along with the cap. Poke around with a cap and see where it helps.

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