Let there be … music!

April 13th, 2008

I like to listen to music. Since I think riding a bicycle with ear plugs in is a sure sign of poor judgment, we have a set of small speakers that we can use with our MP3 player. Unfortunately, the speakers go through batteries pretty quickly. Somebody on the Tandem@hobbes mailing list mentioned that it was possible to build a converter that can power a portable GPS receiver from a dynamo hub. Dynamo hubs are small AC generators built into the front wheel of the bicycle, usually used to power a headlight. We had just added dynamo hubs to our bikes, so I figured we could adapt the idea to power the speakers.

First, I have to offer a big thanks to Dan Lyke, who put together a quick write up on the process, with clear instructions on what to buy and how to tie it all together. All he asked in return was that I document the process. Done. Dan’s instructions were so clear that I’ve borrowed from them liberally for this article. The best resource for dynamo hubs in general is Peter Jon White. The headlights we use, Lumotec IQ Fly N Plus, have two pairs of connectors on the back. These are run in parallel to the main light and are meant to power tail lights. They provide a very convenient place to tap into the power for the speakers. They use .10 inch spade connectors.

While this article is about powering speakers, the same device can be rigged with a USB connector to power a portable GPS receiver. The basic idea is this:

  1. The dyno hub puts out AC.
  2. That has to be converted to DC
  3. The DC power has to be regulated so that we don’t fry the speakers. The speakers call for 4.5V DC, but 5V regulators are common and the speakers won’t mind the extra juice, so I used a common 5V regulator.

Here is a circuit diagram.

  • This is the basic circuit diagram. I used a premade rectifier.
Don’t worry, it is even easier than it looks.

  • AC in, DC out.

The first component is a bridge rectifier. This converts the AC to DC. You can either build one out of four diodes, or buy one that’s usually a square piece with 4 leads (mine was round). If you are up for building one, you probably won’t need this article. Hub generators don’t put out a lot of power, so a simple 50V model is fine. Wikipedia explains bridge rectifiers. Rectifiers have 2 connectors for the AC and two for the DC. You need to pay attention to the polarity on the DC side when you connect the other elements.

  • I picked this size at random. Some where there is a formula to tell me what the "best" size is, but device works, so this is close enough.

The second component is an electrolytic capacitor. I’m a little vague on this part, but Dan said to put it in, so I did. Since Dan advised that size was not important, I bought one at random – it works. Every reference to capacitors I’ve seen reminds you that it is *critical* that you get the polarity right (+ to + and – to -), otherwise it may explode. So, solder the positive lead on the capacitor (usually the negative one is the one that is labeled) to the positive output from the rectifier, the negative lead to the negative output from the rectifier. I used a “radial capacitor” rather than an “axial capacitor” because it fit my physical layout better. The difference is simply a question of whether the two connectors are on one side of the capacitor (radial) or on opposite ends (axial).

  • The common 7805 voltage regulator.
The third component is the voltage regulator, an LM7805. Unregulated power goes in one side (Pin #1) and 5V max comes out the other side (Pin #3). There is another pin (#2) that is a common ground. With the tab facing away from you and down, the leftmost pin is pin 1. Solder the “+” output from the bridge rectifier to this pin, solder the “-” lead from the bridge rectifier to the middle pin.

The last component is whatever connector fits your device.

  • The diagram on the back of the speakers tells me what voltage I need and what the polarity should be.
  • These speakers work well for only being 5 inches long.
  • The spade connectors go into the back of the light. The other connector plugs into the speakers.
My speakers use a 3.5mm connector. Solder the positive connector (following the diagram on the back of the speakers) to the regulated power from the 7805, Pin #3. Solder the negative connector to the common ground on the 7805, Pin #2.

I covered all the connections with heat shrink tube. This protects the connectors and hides my ugly solder work. Check the continuity over everything before you shrink the tube. Remember that the heat shrink tube has to be in place, but slid of of the way, before you make any connection, otherwise you will not be able to slide it into place.

  • Step one, solder the rectifier (the small black puck) to the capacitor (the blue cylinder). They are not connected except at the electrical connection; the capacitor is just snug against the ...
  • This shows how the voltage regulator, on the right, will link the DC supply to the wires that will connect to the speakers.
  • More heat shrink tube to make things look good.
  • One last piece of heat shrink to hold it all together. I don't know how durable this will turn out to be.

That’s it! It took me about 90 minutes to put it all together. In addition to the specific components above, I used a soldering iron, a multi-meter to check continuity, and some small (20 gauge or so) wire that I had from the original dyno hub installation.

The bike powers the speakers as long as I am moving about 4 mph. On really steep, long climbs, that isn’t always possible. This causes the speakers to stutter as they cut in and out. Sheldon Brown added a simple NiCad battery in line to provide power when not moving on one of his setups. I may do something like this in the future.

The total parts cost (excluding the bike, etc) was under $8.

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