Garmin eTrex USB Power Supply

My old eTrex-H GPS had been sitting on the shelf for a few years so I decided to finally do something with it. My plan was to connect it to a server to use as a GPS time source. This is not a timing GPS so it's usefulness in this capacity is quite limited, but I figured it would be a fun project regardless.

However, I had a problem - I needed some way to power the GPS. When I purchased the GPS, I had opted not to purchase the expensive serial cable; instead I bought an aftermarket connector from pfranc and built my own from an old serial cable (total cost, under $5). Fortunately for me, this would prove to be a good decision as I could just add an additional power pin to the connector.

The eTrex GPS receivers require between 2.85 and 3.15 volts DC. Around 2.8 volts, the unit will switch over to use it's internal batteries, and supplying more than 3.15 volts could cause damage to the GPS. Since the GPS was going to be connected to a server full time it made sense to just draw power from the server rather than a separate power supply, so I chose the simplest option: USB. But a USB port supplies 5v, far above the maximum 3.15v, so it was obvious that I would need to add a voltage regulator to avoid frying the GPS.

Scrounging through my parts bin, I found what I needed: an LM317T adjustable voltage regulator, two 270 ohm and one 100 ohm resistors, and a 1.0uF capacitor. The resistor values won't ever be exact, which will slightly affect the output voltage. In my case, the measured output voltage was 2.96 volts - close enough.

I came up with the above circuit after reading over the LM317T datasheet.

Standard disclaimer: If you decide to build this circuit, do so at your own risk. If you damage your GPS or your computer, that's your responsibility.

Here is a pic of the completed regulator. After I took this picture, I slid a piece of heat-shrink tubing over the circuit board to protect it and prevent any short circuits.