Sunday, February 13, 2011

Constructing a programming interface

A little while ago, I was given a pair of EF Johnson UHF mobile radios. These put out 35 Watts of power, which is nice, but like most radios not specifically designed for amateur use (and several that are, as I understand it) you can't set the operating frequency directly from the radio. Instead, one must use a special programming cable to store a number of frequencies (in this case, about 100, in banks of 16) in the radio. And these cables aren't free - aftermarket versions run about $40 a pop. And as my motto with ham stuff has lately been "why buy a thing when you can build it?", I decided to make one of my own.

There are a couple of schematics out there for this cable, all based around a MAX232 IC (integrated circuit). I used one designed by Kyle Yoksh, K0KN.

Basically, what the MAX232 does is convert RS-232 signal voltage levels to TTL signal levels. So a lot of the work has already been done in the IC. All I had to do was solder the IC to a circuit board, attach some capacitors, attach a female DB9, and attach an appropriate connector to interface with the radio.

I was able to obtain all the necessary parts from Electronics Supply Company here in town. I probably paid a little more than I would have if I'd bought from, say, Mouser Electronics, but I like supporting local businesses.

Soldering the entire circuit together was a bit of a learning experience, which I won't recount here as it's extremely boring in the telling. I know that if I built this circuit again, I'd do it slightly differently, but I'm still pleased with the way I did it.

Remember how I said "appropriate connector" above? This whole "appropriate connector" business is easier said than done. It's an 8-pin connector that looks like an RJ45, but it's more narrow. I wasn't able to find one online, so I decided to adapt a bit of Cat 5 that I had laying around instead. This also had the advantage of not requiring me to find a crimper for that weird connector. So, instead of using the Johnson connector, I'm filing down (sloooowly) the RJ45 on the end of the aforementioned Cat 5 stub. Once I've done that, I just need to secure the board and try it out.

For a project box, I decided to use an old Altoids tin that I had laying around. I drilled a hole in it for the Cat 5 stub, but for the DB9, I ended up poking a hole in it with my needle nose pliers and then just working at it with a metal file and clippers for a while. I made the holes for the screws to secure the DB9 to the box using a sheet metal screw I had laying around. A bit of pressure and it poked right through the tin.

OK, now for some pictures. :)

Here's the project board inside the Altoids tin. As you can see, it's really not that complicated of a circuit - a few capacitors, a voltage regulator, and the appropriate connectors on either side, and that's it.

Close-up of the project board.

And this is what the thing will look like once it's finished.

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