Dynohubs: power from bikers

Lots of people are into mechanical generation of electrical power these days, so here’s a nice tip from the guys at Uncommon Projects: dynohubs.  I had a note in my inbox for months that just said “dynohubs, Tarikh.”  So I wrote, and he offered a lovely explanation, and a link:

Dynohubs are cool. They are sort of stepper motors in reverse,
actually sealed in a bicycle hub. They generate AC current which can be
easily rectified and smoothed to DC and (mine anyway) generates nearly 400mA
without significant speed. There’s some minor friction from the magnets but
they’re much better than the old style “bottle” hubs (which are
significantly cheaper).

Dynohub in Uncommon Action!

Perhaps you were asking me because I’ve been meaning to make an instructible
for months about how to power or charge your iphone/device from your bike?

They’re not necessarily cheap. A quick google search for the Shimano 3N71 that Tarikh mentions comes up with prices ranging from $99 – $150.  3W at 6V, that’s half an amp. Not a bad start.

Surplus Center

Mike Olson likes to move heavy things.  So when he says something like this:

Hands down the best site for finding cheap powerful motors is:


You can’t search by torque, but they usually have an excellent seclection of powerful gearheads and linear actuators.

I had great results with one of these guys a while back. Probably bigger than what you need, but produces huge torque with very little current. The only problem is that it’s very heavy.

I believe him.  So if you’re looking for motors that can lift heavy stuff, follow Mike’s lead.

Using an Accelerometer to Sense Which Way Is Up

ITP just got some nifty flat panel mounts that can rotate 360 degrees. They’re very easy to move, it takes only one hand. When I saw them, I thought, “what good is a rotating mount if the content on the screen can’t rotate too?” So I came up with a little system to sense the screen’s rotation. Here’s how to turn those screens into a very big iPhone. Thanks to Michael Dory for his help in coding this and Dan O’Sullivan for the final clue.

The screens have a mac mini mounted on the back to display digital content. I added an Arduino with an accelerometer mounted on it to sense the angle of the screen’s rotation, then sent that data into Processing.  This example doesn’t do much, but the code can be re-used for any Processing application that needs to know the screen’s rotation.

Rory Nugent modified my existing code and made it much better.  I’ve incorporated his changes here, thanks Rory.

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XBee to USB modules

Droids.it has a breakout board for XBee radios, and a USB-to-Xbee board available.

The USB board is nice, because in addition to having a built-in 5V-to-3.3V regulator and indicator LEDs, it’s got all the pins of the XBee broken out on the side of the board. Makes development of XBee projects easier.

The breakout board also has its own regulator on board, making it easy to interface with a 5V project. It’s also got the serial pins broken out to a single row, making it easy to mount on the side of a breadboard.

The prices on both are reasonable for the European market. As the US dollar continues to sink, they’re a bit pricier for US customers, but still reasonable, for now.

Thanks to Rob Faludi and Jeff LeBlanc for testing help, and to Luigi Carnevale for supplying sample boards.

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Startup Checks

When you’re making a microcontroller circuit that drives a high current load like a motor or an incandescent light, it’s not uncommon to make a mistake and create a circuit that draws too much electrical energy on startup and causes the microcontroller to reset itself constantly. To debug this, it helps to put a routine in your startup() function so you can see when the microcontroller has started up. For example, I often blink an LED three times in the startup. If the LED blinks three times then stops, I know the microcontroller has successfully run the startup and gone into its main loop. If it blinks constantly, I know the microcontroller is continually resetting, so there’s a problem.

Hans Steiner recently showed me his trick for checking for the startup routine on the Arduino: he writes to the microcontroller’s EEPROM, or permanent memory, and reads back the result. Every time the Arduino resets, it’ll increment the EEPROM value. Since this value is stored even when the Arduino is not powered, you’re going to get a new number each time it’s run.

Thanks to Hans for the code.

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