John Hale | Cycling | Modifying the Garmin Quick-release Mount

23/07/07 - Modifying the Garmin quick-release mount for long rides

I accept no resposibility for the results of this tutorial; it should work fine, but if it's put together incorrectly it could break your GPS device.

1 - Intro

The aim of this tutorial is to explain how I modified my Garmin quick-release bike mount (for the Forerunner x05) so that it can be used as a charging cradle. This allows the GPS unit to be used on long cycles/triathlons such as double centuries, ironman where the inbuilt battery is insufficient.

Parts List:

Garmin quick-release bike mount
Spare Garmin charging cradle, or suitable spring connector (does anyone know of one?)
A length of USB lead with a male plug (one of these ) on the end. Check the length by running it from your handlebars to wherever you want the battery pack (mine will go on the stem). The other end of the cable should be trimmed back (by about 1cm) to get at the individual wires. Strip the end off the red and black wires but cut off the other two wires, as they are not needed.

Optional Parts (battery pack):

Mintyboost kit (or similar parts)
Enclosure - either an Altoids tin or a plastic box ( Rapid order code 30-1932 )

Tools:

Small screwdrivers
Glue (hot glue + glue-gun are desirable, otherwise something viscous that sets slightly rubbery is best)
Stanley knife
Soldering iron + solder
Wire cutters
Wire stripper
Multimeter to test it

2 - Disassembling the charger cradle

Screw locations:

There are three screws on the underside of the cradle that must be removed. These are hidden under the foam that is stuck to the base, but the picture shows their locations.

Once the screws have been removed, pull the cradle apart.

Inside:

Once you've taken apart the cradle, you should be able to remove the PCB and the large metal weight in the base.

Removing the connector:

Once you have the four main parts of the cradle - the two parts of the casing, the PCB and the metal weight - set aside all but the top half of the casing. Attached to the underside of this piece is the connector that we want. It is simply pressed onto two plastic posts, so using a flat-bladed screwdriver, gently prise it off.

-- This part may well be a standard data connector (the spacing between the pins is 1/8"), and if someone knows what it is, please could they contact me here --

3 - Fitting the connector into the bike mount

Cutting the hole for the conenctor:

Now that we have our connector, its pretty much just a matter of hacking a hole in the mount into which it will fit. Get your Forerunner and place it into the mount (but don't clip it in fully), mark off where the contacts will need to be on the mount. Now, using a sharp Stanley knife, cut through the ribs of the mount to create a hole just big enough to fit the connector. It's best to start too small and keep checking against the connector as you go. You'll also need to cut away the middle ribs in front of the connector (ass you can see in the picture), otherwise the contacts will not fit.

After carefully making the hole, you should be able to slot in the connector so that it lies flush with the top of the mount (the plastic body of the connector should be flush, not the sprung contacts).

You may also need to trim down the sprung connections of the connector, as otherwise when they are pressed down the the GPS, they may hit the bottom of the mount.

Drilling the hole for the USB cable:

Now you must drill a small hole for the USB cable. This hole should come into the very corner of the cavity (in the mount) where the connector will sit. It can be either on the left or the right - your choice.

Connecting it up:

With hole drilled, take your USB cable (half of) and feed it through the hole. You now need to solder the red wire (positive) of the USB lead to the far left pin of the connector, and the black wire (ground) to the far right pin. This is shown by the + and - symbols in the diagram. Cut off the other two leads if you haven't already (unless you desperately want to use the mount for data transmission, in which case you'll need to solder them on too - but I don't know which way around they go).

Now you need to glue the connector in place - I used hot glue, as it is easy to work with and forms a good waterproof bond. Make sure you use plenty of glue to keep water away from the solder joins you've just made.

A bit of glue around where the cable exits the mount is also a good idea.

Now TEST IT. Plug the cradle into a USB port on your computer and check (using a multimeter) that the left hand pin is at about +5V and the right hand pin is 0V.

4 - Battery Pack

For the USB battery pack, you can use any commercially available battery pack that will provide a steady 5V USB output. Otherwise, you can make your own.

I built the Mintyboost kit from Adafruit. Instead of using an Altoids gum tin, I used a plastic enclosure from Rapid ( order code 30-1932 ). If you use this case, you will have to clip off the PCB mounting posts.

Alternatively, you can build yourself a similar device - it's just a boost circuit based around a Maxim MAX756 DC-DC convertor.

Once you've built a battery pack, check that it produces a steady 5V output.

5 - In Use

I've yet to do my double century, but when I do, I'll let you know how well this works.

I've read that the mintyboost kit with a decent pair of Lithiums can power the Garmin Edge 305 for 17hrs, so it's likely to do that (or more) for the Forerunner too.

--- --- UPDATE --- ---

I used the battery pack successfully with my GPS on the double century. I had the GPS turned on for about 13.5 hours and it was still displaying three (out of 4) battery bars.

This was using the battery pack reasonably early on, so the battery pack was used up by the end. Also, the batteries I used were some cheap Tesco ones that I'm sure wouldn't come close to a pair of Lithiums (for example).

My estimate would be a life of about 17hours with a good pair of batteries, or perhaps more if I didn't have the device set to record data every second, and if I wasn't wearing my HRM.

6 - Links

ladyada.net - webpage of the Mintyboost and many other cool projects.
Garmin - who make the Forerunner
Where I bought the quick-release kit from
Rapidonline - great electronics supplier
MBlog - Motionbased Blog

There are many other useful links on my Links page.

John Hale - cycling stuff - modifying the quick-release mount