Computer - Radio Interface

As part of my interest in radio communications for emergency response, I wanted to put together a simplex repeater. For my initial experiments, I am currently using an old Sony Vaio PCG505TS laptop running F6DQM's Simplex V2.1.4. For the transceiver, I'm currently using my Wouxun KG-UVD1P hand-held. I needed to have an interface that provided galvanic between the radio and the transceiver to prevent ground loops.

While there are a number of good commercial interfaces available, why buy when you can build one much more cheaply. I mashed up a couple of designs I found on the Internet, along with jumper selections for right and left audio channels and DTR or RTS serial control line. The schematic is shown below.

I kept the cost as low as possible by using junk box parts wherever possible. The optoisolator and transformers were taken from a couple of 56K phone modems. The rest of the parts were also what I had on hand. The only part I had to buy was the enclosure, which I bought from RadioShack.

The following pictures are the interior and exterior of the completed interface.

I used a small prototyping perf board to mount most of the components. I used jumpers to select audio channels and control lines. I figured that these would be configured only once in awhile and left alone otherwise. Note that the unused audio input line is jumpered to ground so that that input wouldn't float.

The exterior of the interface is pretty basic. Just enough labelling so that the cables could be connected properly.

Below is a picture of the interface with the audio, control, and radio cables connected for use.

The cable that connects to the radio is made from a damaged Wouxun lapel microphone. I removed the wiring for the earpiece and replace it with a short PC-CDROM audio interconnection cable. If necessary, it can be unplugged and replaced with a cable for a different radio. The PTT button on the lapel microphone still works and can be used to key the transceiver for testing. I found that I had to place some clip-on ferrite beads on the radio cable. This was necessary because RF from the transceiver was apparently running down the cable, into the interface, and affecting the transistor in the optoisolator. I also added clip-on ferrite beads to the RS232 serial cable for good measure. The interface works well.