My mobile rig, a Kenwood TM-D710E, needed a GPS receiver for APRS use. The GPS-710 wasn’t readily available and, besides, is quite expensive. I therefore decided to build my own.
I found a Royaltek RGM-3600 GPS receiver on eBay for less than Â£20. This is a very good SiRF III based receiver. The magnetic case made a nice fridge magnet as only the bare module was needed. Although the listing didn’t mention it, this turned out to be the RS-232 version of the module. If it had been the TTL version, I would have needed an additional MAX232 level converter. The module pinout is available here.
Bob WB4APR (yes, that Bob) has very helpfully provided the pinout of the RJ45 (okay, 8P8C) connector on the TM-D710 display head. It shows that pin 6 is ground and pin 7 is +10V. This is a handy place to leech a few tens of milliwatts for the GPS receiver.
I decided that the easiest way to get access to the +10V pin is by repurposing a RJ45 Y adaptor. Apparently these splitters are sometimes used in ISDN installations. I bought mine from Maplin for Â£5.99.
The male end of the adaptor isn’t ideal for soldering but a little bit of work with a craft knife exposes the contacts enough to solder wires into them. Because the result is a bit fragile, I decided to cover the solder joints with epoxy. There are better adaptors that would be easier to tap into.
The GPS requires a 5V power supply. I wanted to make sure the system draws as little power as possible, so I used a Recom R78 series switching DC-DC converter from Rapid. The R78 is a small self-contained device that’s pin compatible with the 78xx linear regulators so it’s trivially easy to use. I decided to add an optional input capacitor and blocking diode for extra protection. The whole assembly was then secured onto the Y adaptor with epoxy to minimise mechanical stress.
All internals of the GPS receiver. The 2.5mm stereo jack is wired according to Kenwood’s instructions (page APRS-2).
I made two short (~10cm) cables to connect the GPS to the display head, a 2.5mm stereo and a RJ45. Both are wired straight through. I’m planning to replace the jack lead with one with 90 degrees angled jacks for neatness. The box is attached to the back of the display head with velcro. The fact that the receiver antenna sits vertically does not seem to matter.
The +10V is supplied even when the radio is off. In the off state the rig seems to draw about 40 mA of current from the battery. Some of this is for the standby state of radio and some, probably most, is for the GPS. Overall that’s a negligible amount so I can just leave the GPS on all the time. This is good as the module seems to default to 19200 baud if left unpowered for a long time. The Kenwood only supports baud rates up to 9600, so I had to change the baud rate using the serial terminal on my computer and a suitable interface cable.