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baofeng aprs ptt controller

I’ve been fiddling with some aprs ptt circuits for baofeng hand-helds for a while now. My original goal was to hook up a raspberry pi with a gps module in my car. I’d run direwolf on the pi and connect it to a cheap baofeng radio.

My first attempt was basically this circuit.

Baofeng radios trigger the ptt by shorting the sleeves of the two audio connectors. I isolated the audio lines with two audio transformers and set up the pi to trigger the transistor with a GPIO pin. The transistor would short the two sleeves triggering the ptt. I also added a volume control pot to the audio in line so I could crank the volume all the way up on the radio and set the incoming audio to a normal level. That way I never need to remember exactly where to set the radio volume, I can just crank it to full volume every time.

I tested the circuit and it was triggering the ptt correctly, so I 3d printed an enclosure for it using an openscad model that I found online.

I connected everything together and tried it out. The ptt was consistently triggering from direwolf, but I wasn’t being picked up on aprs.fi. After messing around with it for a while, I finally tried listening to the signal from another radio and thats when I found out that the raspberry pi’s audio output was very low, even with the volume turned all the way up. I’m not sure if it was a problem with the circuit or the transformers or the radio, but the audio was just too low to be audible over the radio.

Next, I rebuilt the circuit with an amplifier in line with the pc audio out. I used a 2n3904 buffering an LM386 that is powered directly from the raspberry pi.

Now the audio is intelligible and I am being picked up on aprs.fi. There is a problem though. Sometimes the ptt gets stuck on. I don’t know if it is RF getting in and messing with the pi or triggering the transistor or what, but it will work for a while and the the ptt gets stuck on, so at this point, I don’t trust it. I guess there will be another version in the future.

2 Comments

  1. Joe Joe

    What does it do with different length cables?

    What does it do if you wrap the enclosure in tin-foil?

    Trying to think of ways to confirm it’s RF that is the issue and not some other (programmatic) issue. 🙂

    I like this post and am eager to see the next revision!

  2. dewaine dewaine

    why is it that is so hard to set up aprs or packet. my son is trying but seems to be getting nowhere fast. thanks va1dwg.

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