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.