Just my 2c:

- The problem with the reed switch you describe is unlikely to happen in 
real life, and can be easily remedied in software. Just check for minimum 
time b/w pulses, or implement an even more clever algorithm. I have no idea 
why Olin thinks it is not a good choice (it's certainly good enough for 
commercial speedos).

- If you go with the coil option, you need an amplifier that will take the 
pulse from the coil and get it up to 5 volts. Put a low pass filter (in the 
simplest form, a cap) on the input of the amplifier.

Vitaliy


solarwind wrote:
On Sat, Aug 15, 2009 at 7:55 PM, Olin Lathrop<olin_piclist@embedinc.com> 
wrote:
> A reed switch is not a good choice. This is usually done with a permanent
> magnet between two spokes of the back wheel as far radially outward as
> possible and a pickup coil on the frame. A little analog electronics and
> you get a pulse into the PIC once per wheel rotation.

Good idea, but the analog electronics part is wayyyyy too complicated
for me. But I'm going to go for it as I can already see an advantage
with the analog solution: if I were to use a reed switch, it may
rapidly switch on and off if the magnet stalls near the switch.
However, with a coil, it is possible to filter that out.

This approach also raises a few more questions:

* What kind of coil would need to be used? Thickness? Resistance?

* Would a coil pulled out of an analog clock work? (Was formerly used
in the clock to move the seconds hand.)

* How much coil is needed and how will it be configured physically?
Will it be wrapped around a pencil, for example?

* I'll look at the signal produced by the coil through an
oscilloscope, but what other electronics is needed to isolate the
wanted signal? Or can a PIC alone and it's A to D converter tackle the
task?

I'll have to do some reading on analog filter design.

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