PDA / GPS bike mounted USB charger

December 27th, 2008 by Arend Leave a reply »

An update on this post is found here and a step by step guide is posted here: (still a work in progress)

I love to go on holiday by bike. And as a tech nerd I can’t be bothered by plain paper-maps!
Having a GPS-mapping application like OziExplorer on your bike makes navigation very easy.

The problem however is with the rapid draining of the battery-powered GPS / PDA.
Especially mobile phones / PDA’s have a very short (battery) lifespan, often no more then a couple of hours when the display is always on and a GPS-application running. Normally I just dragged a lot of AA aka penlite batteries with me and powered the phone with a simple battery holder like this one:

Of course this isn’t the best solution and I was looking for better alternatives.
Then I stumbled upon this genius, hub-dynamo driven “Bike mounted USB Charger” on an English bicycle forum.
The idea is quite simple. You take a HUB-Dynamo (Shimano DH-3R30) (I ordered one from ebay for less then 40 euro).
The hub-dynamo generates an alternating current that needs to be rectified and the voltage output should be regulated to 5 volts for USB powered devices.

The hub-dynamo:

These are the parts you need, available at any electronic (parts) store.
D1 – D4     1N5818
C1             2200uF   16v
C2, C3       470uF     63v   Low ESR
C4             .47uF      Tantalum bead
C5             22uF       Tantalum bead
R1             47K         .25W

Solder the parts according to this schematic:

And there you go, you have the best energy- efficient charger, all done with green energy!

This is my effort. It still needs some good housing and connectors but it works perfectly:

For more detailed information visit the bike forum at http://yacf.co.uk/forum/index.php
Many thanks go to Simon Galgut for providing the electronic schematic.


  1. T says:

    Brilliant work. I’m curious how it performs for you. Have you thought about adding batteries?

  2. Arend says:

    It works quite well. It even charges the onboard battery when driving at a reasonable speed (20 km/h). But I haven’t tested it with my new HTC Desire yet. This phone will consume a lot more energy and is known to even drain the battery when connected to a regular car charger (500 mA) instead of the HTC OEM version rated at 1 Amp. Adding batteries might be needed now.

  3. Matthijs says:

    Could you mail me a pic of the bottom side of the printboard? And could you tell me to which pins of the USB socket I have to connect the + and ground?

  4. Phil Waud says:

    Thats really simple!

    If I connect 4 AA rechargeables between the +ve and -ve before C4, will I be able to charge them from the hub and run my iPhone 3GS?

    Sorry to ask, but Ive been trawling sites for hours and my head is spinning!


  5. Arend says:

    I’m working on a step by step guide in this post:http://www.arenddeboer.com/diy-hub-dynamo-usb-charger/

  6. BoVis says:

    Dag Arend,

    leuk ontwerp ! Maar ik kom er net achter dat ik een LM 2940CT 12 V ic heb. Ze bestaan in verschillende uitvoeringen, en ik moet dus de 5V hebben ..


  7. BoVis says:

    Hij doet t !
    thanx !

  8. BoVis says:

    For a garmin Nuvi 1000mA is required. Is there a simple modification possible ?

    garmin nuvi GPS:

  9. Arend says:

    The hub-dynamo is the limiting factor, a 3Watt hub can only generate 500mA.

  10. Cage says:

    Could you mail me a pic of the bottom side of the printboard? And could you tell me to which pins of the USB socket I have to connect the + and ground?

    • Arend says:

      Totally missed your reaction, do you still need it ? In the first paragraph I have a link to a step by step guide for building this. It shows you how to connect things up.

  11. Adam says:

    I have a Garmin eTrex 20 that has a pretty decent battery life (I get about 23 hours with the backlight off on a pair of cheap AA batteries) It will run off of USB power, but once that power is removed, it will automatically shut off versus switching over to the on-board power. Would adding a decent sized capacitor in series with the USB load help alleviate the on-off associated with stop and go biking? My EE knowledge is rusty at best… I just enough background to get me into trouble.

Leave a Reply