[arm-allstar] Baofeng COS without opening the radio

wb4bxo at mindspring.com wb4bxo at mindspring.com
Sat Mar 2 20:45:20 EST 2019

I have put together a little circuit that allows connection to a Baofeng UV-5R HT providing COS without having to open and modify the radio itself. This works great for AllStar, packet, APRS, or most anything that requires a soundcard connection to the radio complete with COS. I have even used this circuit without the sound card to connect to an RC-210 repeater controller.

I built the circuit based on the schematic of the UV-5R but I believe most of the Chinese radios will work with it. I have tried it on several different Chinese radios including the Anytone AT-D868UV DMR radio and they all worked fine. However, even though these radios are mimicking the Kenwood two prong speaker/mic connector, this circuit will not work on my Kenwood TH-F6. This is due to the fact that the Chinese radios do not block the DC of the audio amp from reaching the headset, which is why you hear a pop when the squelch opens and closes on these radios, and is what this circuit counts on to turn on Q1 and provide COS. The Kenwood doesn’t work (or have the pop) because it has a blocking capacitor and an MOS switch in the audio line that prevents the DC from coming out the jack. But I’m much happier using a $35 radio for this purpose anyway.

I’ve hand etched a couple of boards for it so far and may send it to a board house to make some prettier boards, especially if others are interested in it (I’ve already quoted them and its just too pricey for small quantities, which is why I decided to hand make the few I needed). The boards I have made have some extra pads on the end to allow soldering directly to the audio jacks of one of the EBay sound FOBs, just makes it a bit cleaner than having to cable that side of it, though it still requires the two jumpers to the GPIO lines on the sound chip. Also note that there is an extra connection at Pin 4 on the sound card side that should allow using with an RA-35 or similar that already has the open collector driver for PTT. I have not tried this on an RA-35 but feel confident it would work, my target was primarily the CM108 open board sound FOB you can get on EBay and Amazon.

Also note there is no activity detection nor time out timer in this circuit, so I leave the TOT of the attached radio set to something appropriate in case things get hung up in transmit. And as with other interfaces, make sure to keep the RF from the attached HT from getting back into the RPi.

Schematics, some pictures, and this doc can be found here, hope it helps someone.

Steve – WB4BXO

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