Simple... make it read nema 0813 and add a GPS will tell you speed and
direction without the need of regressing back to mechanics...

STeve

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
solarwind
Sent: 15 August 2009 23:55
To: PICLIST
Subject: [PIC] Bicycle Speedometer Project

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.

-- [ solarwind ] -- http://solar-blogg.blogspot.com/
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