1. Introduction
This page describes an enhancement to a common bike light that'll enable a bike-mounted etrex to get its power from the bike light. Bike lights usually use C size batteries, which have much more capacity than the etrex AA size batteries. This dual use of the bike light greatly increases the etrex mean-time-between-power-plant-replacement (i.e. battery change). You should be able to get at least 30 hours of non-stop, normal mode (not battery save!) mountain biking navigation with this enhancement. And get this, when you do need some light, flipping the light on will automatically switch the etrex to use it's own batteries for power.
To make this enhancement, you'll need:
an etrex, of course, with a handlebar mount bracket.
A 3 volts bike light. It's important that the light is rated for 3v. A higher voltage will likely damage the etrex, a lower voltage will not feed the etrex. The fancy 6 or 12 volt lights will ruin your etrex GPS, so make sure you get this right. Mine looks like this picture at right. Its called the Smart Halogen, but I've read they have other models under the same brand that use 6v. The sure thing check: it takes two 1.5v batteries, C size.
A plug for the etrex. You can get one here (actually, you'll get two: one to build your data/power cable, and the spare one for this project).
A small 3 pin male/female connector. I used an electronics pin line socket with 'break' feature, mainly because that's what I had around. Other small connectors should also be fine.
Some soldering required here, to create the power cable and the connector.
2. Procedure
| The first step is to build a
small external power connector to your bike light. I used a 3 pin socket
used in soldering electronics chips. I just snipped three legs. It's small
and fits perfectly. The reason for the third leg is that it acts as a 'key'
to prevent connecting the male to the female with the wrong polarity. The
third cavity in the female is filled with solder or glue and marked white
with liquid paper. The pin itself is snipped. I started by drilling two small holes in the plastic shell and fed the connector legs through them as shown. Don't make the mistake I made - put the connector on the side closest to the etrex. On my bike the etrex is at the left side of the handlebar, but I failed to take that into account. No biggie, but a bit less elegant. |
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| Next, solder two wires from the connector and route them to the + and - terminals on both sides of the switch. Solder them to the terminals, insert the two C size light batteries and test that the socket gets around 3v (depending on the condition of the batteries). If everything is OK, glue the connector with epoxy or superglue on the inside of the shell. This will add insulation and strength to the connector. |  |
| Put the two halves back and test that when the light is off, you get around 3v on the connector. When the light is ON, the voltage is zero. If you squint a little, you might see that the leftmost pin socket is filled with solder. That's the 'key' I was refering to earlier. It'll prevent pluging the male connector with the wrong polarity (well, almost - you could if you really tried to). |  |
| Now, make a small power cable
with an etrex plug on one end and a male two pin on the other end. Mine,
shown here, is long because I got the connector on the wrong side of the
light, as mentioned above. Mark the key pin white with liquid paper.
Testing: |
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| Now all that's left is to put
everything together and test your cable. Mount the light and the etrex. Make sure they are both OFF.Connect them together with the cable. Testing: If the icon doesn't change, turn the etrex off, and recheck everything, especially the voltage. If the light batteries are drained to below 2 volts, the etrex won't use the external power. |
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