Ham radio audio connection problem

Discussion in 'The Projects Forum' started by RyanDW, Oct 11, 2013.

  1. RyanDW

    Thread Starter New Member

    Oct 11, 2013
    I have a couple of Baofeng UV-5R radios that have 2 connectors on the side for audio in and out - as well as data and push-to-talk (PTT).

    Here's the pin-out for the connectors:

    What I WANT to do is tie the audio (mono) from 2 radios back to back to each other - speaker out of 1 to the mic in on the other. The reason is that I'm cheap and I don't want to spend a bunch of money on a cross-band repeat capable radio. Besides, I already have a few of these cheapie radios, and I like to tinker ... and this was a fun project ... until tonight.

    I picked up the parts today, but when I complete the circuit (nevermind audio hum) the PTT is activated (negatives are tied together internally apparently) so both radios go into transmit. See the web link above to see that the negative to negative on the same radio activates the PTT.

    I tried listening to the speaker (-) via the mic (-) while using the speker (+) but the audio is very low so there must be some high impedance between those 2 negatives.

    I was thinking I might be able to use an audio opto-coupler to make the connections, and it might be the way to get rid of the hum too. I don't have any diodes to try filtering the negatives. I think I'd need to use 2 optos if that's the way to do it, so that the connections are covered in both directions.

    Thoughts? Ideas?

    If an opto is the way to fix it, can someone please refer me to a schematic for this scenario? I don't mind using an external power supply if necessary.

    Thank you all!

  2. cornishlad


    Jul 31, 2013
    Don't know anything about this radio but there is a diagram here http://www.uv3r.com/uv5r.html look for the schmatics link. It looks to me that the internal speaker and mic and PTT control are disconnected when the double jack is inserted. Probably you knew that.
    Did you realise that the speaker feed will probably need to be attenuated before feeding it to the mic input. Perhaps by 40 or 50 db.
    As the mic seems to be connected internally to a chip it COULD be an electret that needs the bias voltage. This could be arranged in the dedicated speaker mic as there is +v present. But of course it could be the speaker itself, used as a mic. If it is an electret you will have to fix the bias resistor in your lead hack.

    The internal mike does have a ground connection but the internal speaker does not. It could be fed from a bridge-mode output from the chip. In which case the grounds will have to be isolated in some way.
    Last edited: Oct 12, 2013