DIY Hub Dynamo USB Charger Inside Handlebar


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. The Schematic of the Charger

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.Parts for the Charger

  • 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


The ChargerWe 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 Charger inside HandlebarThe 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.Picture of Plugs

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 Charging a Phoneafter a couple of years use and keeps the phone (low-end HTC) fully charged riding with a SAT NAV application on.

Update: September 2016 - still going strong

After n years the charger is still fully functional, along with the cable and connector. Lately it has mostly serviced charging a small auxiliary battery pack that, in turn, is charging a mobile. The external battery also conveniently acts as a buffer against something unexpected, shielding the more expensive phone. This setup also works regardless of the current draw from the device.




Hi Howdy,
I've done all the things correctly (I think) but the voltage regulator won't give me 5 volts. The first two pins of the voltage regulator give me around 15 volts (depends on how quickly the wheel spins) and the rest of the circuit the same. But when I mesure the last two pins of the voltage regulator, the multimeter says 0. I don't know very much about electronics so I can't find the problem there because the circuit is the same as yours. Maybe it's a problem of the voltage regulator not working correctly?
Appreciate your answer


The LM2940 voltage regulator should have 3 pins arranged like this.

LM2940 Connections




I don't have a sample with me right now but the above (googled) picture should display their meanings and, from your description, it sounds like you have it connected it the right way.

In general, if any pin gives you very close to 0 then there's a fair possibility that it is grounded so you might want to check the connection there. LM2940 ought to have an internal short circuit protection and might still be ok even if it was connected to the ground.


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