On Sat, Aug 15, 2009 at 6:55 PM, solarwind<x.solarwind.x@gmail.com> wrote:
> I've gotten (more so) into bicycle riding recently and have been
> looking for a speedometer. The ones sold at stores are expensive for
> such a simple device, so I'm going to make one (which is more fun,
> anyway). And now that I know how to build a PCB, this project should
> be more interesting.
>
> The most complex part about this is going to be the detection of RPM.
>
> One option is to use a reed switch mounted near the spokes and a
> magnet mounted on the spoke itself. A few questions about this though:
>
> * Will a reed switch be fast enough (in terms of switching speed
> capability) for this purpose (bicycle tire RPM)?
>
> * Does a reed switch open and close cleanly or does it generate a lot
> of noise? Will denouncing be required?
>
> * Is a reed switch reliable? Will it fail to switch sometimes even if
> the magnet passes by the switch?
>
> What other options are available to detect RPM?
>
> Once the RPM is available, some simple math involving angular velocity
> and the diameter of the tire and such is enough to calculate the speed
> quite accurately. Other components of the project will include: a PIC
> microcontroller and an LCD display.
>
> The whole thing can be powered by batteries, small solar panel, or
> even a low power bicycle generator with a good regulator and
> capacitors.

I just thought of another idea:

Rip open an old mouse and extract the phototransistor and infrared
LED. Mount each on the two forks of the bicycle so that they face each
other. When the spoke gets in the way, the light to the
phototransistor gets cut off and can be detected by the
microcontroller.

Would this work?
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