The modification shown below will allow the RadioShack DigiTraveler GPS for PDA (part number 20-1601) which is being cleared out at many RS stores to be powered by 7 - 20? volts through the "red" wire on the included PDA cables and will lock the GPS into the "on" mode.
I used a TO-220 style 5v regulator, but since the GPS draws 75 mA max by the specs, you could also use the smaller TO-92 package 78L05.
This modification works because the "red" wire (pin 3 of the RJ-12 connector) is the serial input of the GPS and apparently is only used to sense if a computer or PDA is connected to it. (Since the RS-232 serial standard calls for a logic 0 to be +12v and a logic 1 to be -12v, then no matter what state the pin is in, it will not be 0v.) When we use this pin to supply, say, 12 V then it kicks the GPS into the "on" mode. Then we just tap off of that pin to feed that +12 V to our regulator and get it down to 5v to make the electronics happy.
If you have a TinyTrack 3, you can solder over "JP7" on the TinyTrak 3 board to pass the TinyTrak's incoming power on to Pin 4 of its GPS connector. If you then solder the "red" wire of the PDA cable to this pin 4, the "black" wire of the PDA cable to pin 5, and the "brown" wire to pin 2, you now have your TT3 powering this GPS and getting the serial data back to the TT3 with only a 3-wire cable!
After performing this modification, the device will still fit back into its original case with no other changes.
Yes, I know that the solder job on pin 3 of the RJ-12 connector is ugly. I promise I don't normally solder like that, but this was the 4th incarnation of this mod on that particular pin and I didn't want to clean it up. :-) You may also notice that I lowered the power LED located between the two metal boxes. Good eye.
Many thanks to Tom, OE1KTS of http://www.grundlgasse.at/thomas/digitraveler/ for inspiration and source data.
The procedure should be simple enough to just look at the picture.
This text and the picture below are copyright 2003 by Joseph Cotton KJ5O, kj5o at lightningflash.net
