Foxhunt Transmitter: Replacing The Relay with a Transistor

One of the first mods I wanted to make to my initial Baofeng Foxhunt Transmitter was to replace the clunky 5v relay with a more efficient transistor to switch the PTT.

Not having a lot of electronics engineering experience, I did a little research into some ‘transistor as switch’ circuits and a general purpose NPN transistor — the 2N2222A — to test my set up.

Putting the multimeter to the Baofeng connector revealed about 3.2V on the PTT circuit which runs to ground when the PTT is open. Using another basic circuit example, I set about wiring up the Base to a 4.7k resistor to the output pin on my Arduino that sets the PTT. I ran the Collector to the PTT wire (3.2v) and the PTT-/Spkr- and Emitter to ground. When the Arduino sets the trigger pin High the transistor switches on and sinks the 3.2v from Collector to Emitter to ground, triggering the PTT.

In testing, the function of switching this way works well, though my wiring has again introduced alot of noise on the audio output from the DFPlayer Mini. To fight this, I introduced a 1:1 audio isolation transformer, but continued to run into strange issues with the ptt sticking. Initially, I added a 10k pulldown on the transistor Base as reccomended in one example circuit, but problems continued.

Eventually, I traced the source of many of my issues to a connection on my hacked Baofeng cable which was intermittently shorting one or more of the connections! With this corrected, audio now sounds great with the isolation transformer, pull down transistor does not seem to affect use, but probably a good idea to leave it in. Looking up some specs for other radios, I found that the Vertex handhelds should have 2.2k resistor to ground for their PTT, so I added one into the circuit and it seems to be fine with Baofeng UV-5R and Explorer QRZ-1 (a rebranded TYT UV-88).

I will eventually test with a few other handhelds as well to try to make this circuit as multipurpose as possible, but so far so good.