APRS Modem

Had a whim to put together an APRS modem to use while on route to a SOTA activation, after watching the progress of another amateur on the way to a peak.  After a quick search of the net, I found this design that was based on a design from WB8WGA. The original design information was originally published in QEX March/April 2005.

The design here was well documented with PCB artwork and several software versions.  A temperature sensor is incorporated into the design.  It is based on a PIC 16F88, which I had a couple of in the parts draws.  It only took about half an hour to make a pcb using the press ‘n’ peel system.

A bit rough, but it will do, a few touchups needed

A bit rough, but it will do, a few touchups needed

I probably could have left the iron on a little longer, as in a couple of places, the transfer was not complete, but a touch up here and there with a pen fixed it.

I dont know how old the press ‘n’ peel was but probably more than 10 years!

A recycled fruit juice container cut in half long ways makes for a convenient etching tray.  Ferric Chloride was used… yuck….

Etching tray using fruit juice container cut in half

Etching tray using fruit juice container cut in half

When the etching is complete, you can take the lid off and pour the etchant back into the container it came from.  To speed up the etching process, there is an aluminium sheet under the tray with a 20 ohm resistor clamped to it.  The resistor goes to a 12 V supply to slowly warm up the etchant.  It has been so hot the last few days, this was probably not necessary.

Etched board

Etched board

The board came out ok.   If I had more time it would have been good to lay it out with SMD parts but this was to be a quick project, so it is to be loaded with leaded parts as originally documented.

Most of the parts were in the junk box, so it was pretty easy to put together in a small amount of time.   One thing that is always a challenge is a box to put projects in to.

Leaded part loaded

Leaded part loaded

I recently purchased a cheapo metal bender from ebay.  I was a bit disappointed when it arrived as it looked like all it would only bend very thin metal.  It was only cheap, so what was I expecting, next time will think before an impulse buy like this.  Anyway, it was worth making an enclosure with the bender as an experiment.

Modem in enclosure

Modem in enclosure

I used 1mm thick aluminium sheet for the enclosure as 1.6mm was too much for the bender!  It came out OK in the end.  To finish the enclosure, it was sprayed with etch primer and then some flat black paint.

To program the PIC 16F88, I dusted off an old Silicon Chip PIC programmer and struggled for a bit to remember how it worked, but had a programmed chip in a short while.

The modem was then interfaced to a cheapy 2m/70cm hand held and a PC terminal connected to configure it.

Amazingly, after power up, the PC displayed the sign on message from the modem and it was configured with the appropriate parameters.  An antenna was then connected to the hand held tuned to the APRS net, 145.175MHz and after some time, packets were decoded.  I have a small GPS receiver that will probably fit inside a DB connector backshell, this is the next thing to do.


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