Make:

How do they "measure the speed of rotation" ?

Is there a magnet involved and a hal effect sensor ?

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I should imagine it would be using an accelerometer.

When the sensor is at the top of the wheel, it is pointing one way, thusly reading +g

Rotate the wheel 180deg and the sensor will be the other way up, reading -g

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Sounds good !

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An accelerometer would be too complicated--you are forgetting radial acceleration. Even finding the points of max and min a would be a pain, let alone taking into accounts real changes in speed.

A magnet, or better yet a simple brush, would be much easier to manage.

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Would the small three pin device with the text:

41A
648

be the key to an answer what is used here ?

Could it be a bipolar hall effect transistor....

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Hessel,

Which image are you looking at? I can't make out enough detail to read the text on the components :-/

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Just had a look on ladyada.net, and it most probably is a Hall effect sensor:

http://ladyada.net/make/spokepov/makespokepov.html

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@Mr Dan

I found this photo with a slightly higher resolution:
http://bp0.blogger.com/_i9rXZ8BZKVY/R8yT_cFZHMI/AAAAAAAAAHQ/HqyJP_pBY2Q/s1600-h/IMG_6117.jpg

I also expect it is a Hall Effect sensor. I was wondering if it would be possible to apply this on a Car, where there is no place to put the magnet close to the sensor.

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Hi guys, it is indeed a hall effect sensor (unipolar). The disadvantage of this is that the sensor uses more battery than the LED itself. So far I haven't found one with a decent current consumption, if anybody knows one, please post it...
The other components are a PIC, LED, resistor, two batteries.

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Allegro A3213 and A3214
Micropower, Ultra-Sensitive Hall-Effect Switches look good !

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