I would not rule out the use of a Hall Effect sensor quite so quickly.
They are very easy to use, and do not require much in the way of interface
electronics.
The are available with a variety of outputs, including digital, which means
they have
All of the required signal conditioning and filtering built in. They may not
be used
For commercial models simply because they cost a $1.00 more than a reed
switch,
however this certainly is not a constraint when you are building one. To
make it weather
proof, you simply insert the wired sensor in a heat shrink tube filled with
silicon
caulking and shrink. Check out Digikey part US5881EUA-ND for an example.
They include
a pretty good application guide for use.

http://www.melexis.com/prodfiles/0004824_US5881_rev007.pdf

http://www.melexis.com/prodfiles/0003715_hallapps.pdf

They come in unipolar and bipolar flavors as well.

Researching more, I see that Infineon has one that looks perfect for your
application.
It is very low power, (typically 4 uA averaged supply current ) and is
designed for
Battery operation from 2.4 to 5.5 volts.

Infineon TLE4913

Digikey TLE4913INCT-ND	$0.92 qty 1

Regards,

Bob Bullock
Certified Microchip Consultant





-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
solarwind
Sent: Saturday, August 15, 2009 3:55 PM
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/
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
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