Interfacing bike dynamo with arduino

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello all,

I have a bike dynamo that puts out 6VAC, 0.5A, 3W. I'd like to connect it to my Arduino Nano so that a Processing sketch of mine can react to the speed of pedalling.

I know that I'll have to rectify the dynamo's output to DC, limit the output voltage to 5V, and reduce the output current to 40mA so that I don't fry the Arduino. What I'm not so sure about is how to go about building a circuit to do this. If anybody with more knowledge/experience is reading, could you please check my working!?!

Hypothesis 1: Given that R=V/I, V=6, and I=0.04, R should equal 150. So a fairly beefy 5W 150 Ohm resistor in series with the dynamo's output should safely reduce its output current to 40mA.

Hypothesis 2: To reduce the output voltage from 6 to 5V, we need a voltage divider. Given that P=VI, the power output of our circuit is now 6*0.04=0.24W. With this reduced power, we only need small (1/2W) resistors for the voltage divider. The divider should look like this:

5V out
|
|
6V in ---------[100 Ohm]---------+
|
|
to GND via 500 Ohm resistor

Afterthought 1: after the voltage divider the current will be tiny - I'm thinking this doesn't matter one jot because I'm interested in measuring voltage.

Afterthought 2: can I hook up the GND end of my voltage divider straight to the Arduino's GND? Can it sink enough current to do this?

Hypothesis 3: After all this questionable electronic tomfoolery, I should be getting 0-5VAC out of the dynamo, with very low current (and therefore power). Only thing left is to rectify the AC voltage to DC.

Datasheets for rectifiers are scary. I need help here! I need fairly smooth rectification here (minimal ripple) because I'm using the 0-5V as a control voltage of sorts. At a guess I'd say I need a bridge rectifier and some smoothing caps, but I'm a bit out of my depth.

Can anybody shed some light on how to take this further?

Thanks in advance,

Chris

2

THere are far more useful means of measuring the speed of pedalling than measuring the out put voltage of the generator. Bicycle generators output is probably far from linear in the first place.

Make life easy on yourself and use a reed switch attached to the chainstay and a small magnet attached to the chain ring like bike computers use and get exact results rather than a ball park / semi meaningless figure by trying to measure the AC from the generator.

3

Use Richard Crowley's way, because the frequency is liniar at the speed

4

Wow, thanks all. Richard, your technique makes good sense to me, since I already have 4 dynamos here. Time to test! This is going to turn into a bike-controlled 4-player Pong game, BTW!