Mr Howdy And His Merry Wheelmen

September 27, 2010

DIY Hub Dynamo USB Charger Inside Handlebar

Filed under: DIY — howdy @ 19:47

Smart phones are quite power hungry, especially if you use features like GPS or continuous data streaming. After a couple of hours navigation or tracking a training ride, it’s time to start looking for a charger. If you mobile charger happens to be that of the USB type there are some options to get your phone juiced up en route, both commercial and DIY. We did an online search for words dynohub USB charger and among the DIY crowd basically two types of designs popped up.

One idea is to use a set of four NiMH cells to regulate the voltage from the hub generator and, in to the bargain, also provide intermediate power storage allowing removing of the battery pack and using it for charging or power source also off-board.

The other option is to use a low-voltage-drop 5-volt regulator to directly feed the USB power source. The following link provides excellent step by step instruction for building just such a device: and we are stealing it here with pride.

A slightly modified parts list and a schematic are repeated here for convenience.

  • Parallel Strip Veroboard
  • 5 Volt Regulator LDO (Low Dropout) LM2940 (CT)
  • Heatsink for LDO (optional)
  • C1             2200uF   16v
  • C4             .47uF      Tantalum bead
  • C5             22uF       Tantalum bead
  • Bridge Rectifier, 1.5A, 100V
  • Heat shrink tube 19mm
  • Terminal block (two pin)
  • USB-A connector
  • USB-A extension cable

We wanted to build the charger with the approach of fit and forget and what we came up with (unsurprisingly) was to use some tubing on the bike to hide the electronics. There is actually one commercial charger unit available that is installed  inside the steerer tube but we went for something slightly less arduous and decided to use the handlebar tube instead.

As can be seen in the pictures the layout of the veroboard is such that it keeps the board narrow and fairly long. We used a USB-A connector in one end and a terminal block in the other. This design means that the total length of the device will be quite long as the USB connector in the end of the extension cable adds to the total length quite a bit. We put the contraption inside a Salsa Woodchipper so the length was not an issue but for a shorter tube section a better option would be to use a terminal block in both ends. In most cases a terminal block is preferable to directly soldering wires to the board as it makes the connections more versatile and robust.

The device was placed inside the handlebar the USB connector inside first (unlike in the picture). We used a standard USB-A 1-meter extension cable. The cable was measured in such a way that one end reached the middle of the handlebar and on the other end the extra length was tucked away inside the tube. The USB cable, along with the input cable from the hub dynamo, were routed under the handlebar tape in a similar manner to a bar-end shifter or brake cable. The input power cable was directly connected to the hub generator making the device always-on.

The USB extension cable was actually pretty thick and it would’ve probably made more sense to use a charging-only extension cable with only two wires inside it if one been available. After the proper cable length was measured, a section of a 19mm heat shrink tube was pulled over the whole unit along with the USB cable and the device was plugged inside the tube. After wrapping the handlebar tape back on we simply put some electricians tape to cover the bar-end as the two cables coming from inside the tube and going under the handlebar tape didn’t allow any standard plug. When the job was finished, the only visible part of the setup was the USB connector in the end of the extension cable that was peeking from under the handlebar tape.

If the handlebars had already been crowded with existing cables from shifters and brakes it would certainly have been more convenient to use a thinner two-wire USB cable. Depending on the setup the two cables could also have been made to come out  from both ends of the bar.

After a reasonably long while (a couple of years), the charger finally got a worthy bar plug. Two custom bar plugs were crafted. One regular and another one with a slot to incorporate the cables coming from the charger and going under the handlebar tape. The plugs were milled in house, the diameter fitting tightly the inside of the handlebar (no room for bar tape).

Here are some pictures of the job. The picture of the plugs is straight from the mill without sanding or varnish. In the second picture the whole rig is visible with the finished plugs and a mount for the phone connected to the charger. The charger is still fully functional after a couple of years use and keeps the phone (low-end HTC) fully charged riding with a SAT NAV application on.

For some readers it might be of interest that we used this tool to generate the gcode for the plugs.


  1. Great write-up.

    I always love to see new variations and improvements.
    Keep it up, can’t wait to see more pictures.

    Comment by Arend — September 28, 2010 @ 05:45 | Reply

    • Thanks for your comment! And thanks for providing the excellent illustrated design in the first place!

      Comment by howdy — September 28, 2010 @ 18:14 | Reply

  2. Hey dude!! That’s excellent design!! Congratulation! I want to build it my self in the next month for my SON 28 dynamo!
    I Heard this dynamo may put up very high voltages at high speeds, will this take it without problem? I mean, 100V seams to be very high.
    Anyway, I was thinking of adding two 18650 lithium ion batteries to my project, so they would feed my gps data logger and tracker while not riding my bike.
    How would be the best way to add the circuitry to this project so the battery would act as a big buffer ?
    Thanks mate!

    Comment by Eduardo Lindenmeyer — June 12, 2012 @ 14:23 | Reply

    • Hi,

      Thanks for your feedback! Much appreciated!

      I have run the circuit with Sturmey Archer X-FDD dynohub for a couple of years now with no issues. It’s hooked up as always-on so it has seen all the voltages I have thrown at it although most of the time with no load. I think the maximum input voltage for the LM2940 is 26V but I have no idea how well it can handle higher transient voltages or how much the voltage rating depends on the load. My maximum speeds have been around 50km/h so no experience on high speed descents.

      Sorry, can’t help you with the battery buffer scenario. I assume it would increase the complexity of the circuit considerably. One possibility, I guess, would be to keep the circuit as it is and just plug the output to a some sort of usb stick battery charger.


      Comment by howdy — June 12, 2012 @ 20:42 | Reply

      • Have you tried this circuit with any other dynohubs? I suspect there shouldn’t be a problem, but I’m tempted to rig some kind of zener diode circuit across the output from the bridge rectifier, just to be on the safe side :-). This thing opens up all kinds of potential uses–as much as I love my (Shimano) dynohub, it frustrates me that it’s useful for only a small portion of my ride. Now, charging things? That I can get behind!

        Comment by Andrew Marchant-Shapiro — August 19, 2012 @ 17:56

      • Hi Andrew,

        Thanks for you comment!

        I have a few SON hubs but can’t really remember if I have tried the circuit with them. You suggestion of adding a zener to protect the LM2940 from overvoltage sounds good. From my experience the regulator has so far been able to cope with the voltages from the hub but depending on one’s top speeds things might be different. It would be very interesting to actually _know_ the input voltage to the regulator under load with the circuit connected to a hub spinning fast as it could also be that the voltage in question would be capped well below the maximum 26V mentioned in the reference. I will have to do such a test if I build another one of these circuits.


        Comment by howdy — August 20, 2012 @ 08:55

  3. […]…de-handlebar/7 From this? Can you report back on it's functionality, i'm thinking of this over the forumslader. […]

    Pingback by Functional bikes. Not Porn not Anti - Page 112 - London Fixed-gear and Single-speed — February 22, 2014 @ 20:43 | Reply

    • The charger has been an install-and-forget device. It has continued to work without any issues. Also the usb connector hidden inside the heatshrink works well. It has protected the connector and kept it neatly hidden so that the whole setup is hardly visible against the black handlebar tube.
      With my (very) low-end android (HTC Wildfire) the charger is able to keep the phone fully charged when using it as a satnav but the more modern handsets might require more current and I guess it’s possible that some might not initiate charging at all with the current that is available from the device.

      Comment by howdy — February 28, 2014 @ 21:10 | Reply

  4. […] […]

    Pingback by Lighweight Touring Bike - London Fixed-gear and Single-speed — February 28, 2014 @ 00:57 | Reply

  5. hi.. i had no knowledge about circuit design
    but i wish to build a dynamo handphone charger based on yours
    can i ask what is the function of capacitor in this thing
    and why you use two type of capacitor
    that is tantalum and electrolye
    what is the different ??

    Comment by wincent — July 12, 2014 @ 01:26 | Reply

  6. Hi Wincent,

    I am not an expert on circuits myself but my guess is that there are two small ones that can be connected close to the regulator pins for stabilization and the larger one is sort of filter capacitance that is helpful when the load increases. I found a link with more discussion on the subject:


    Comment by howdy — July 19, 2014 @ 06:43 | Reply

  7. Nice, i have a question, what if i get those “usb wall chargers”, its says: input ac 100-240v, output dc 5.0v == 500mA

    Doesnt it do the same thing? convert AC to DC to charge the phone, im thinking, all i need to do is set the wire from the dynamo to the wall charger and connect an usb cable to whatever device.

    Comment by Russo — August 21, 2015 @ 17:10 | Reply

    • yup, it works, i just tested with an “usb wall charger” i connected the usb wall charger to the dynamo and then my phone into the charger, perfect.

      Comment by Russo — August 23, 2015 @ 01:58 | Reply

      • Wow, that’s unexpected. The usb wall charger is obviously not picky about the input voltage level as a general bike dynamo can hardly reach half of that. If your experiment applies to most usb wall chargers it would be a neat way to mobile power. Thanks!

        Comment by howdy — August 24, 2015 @ 16:35

  8. Reblogged this on my arse is killing me.

    Comment by myarseiskillingme — June 14, 2016 @ 22:23 | Reply

  9. Hi, can u explain which drill-marks have you done?? Because in other versions of this project the drill-marks are very different and I was wondering if this affects to the performance of the circuit.

    Comment by Pablo — September 7, 2016 @ 16:02 | Reply

  10. Sorry, I just cant figure what you mean by drill-marks?
    BTW, We are sort of moving, so I wouldn’t mind if you leave your comment here:

    Comment by howdy — September 10, 2016 @ 06:00 | Reply

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