Lin,
Your description is a bit vague, but depending on exactly what you want, a
bicycle wheel sensor might work. It is a magnetically operated reed
switch: a small magnet is attached to a spoke and trips the reed switch
once a revolution. The speedometer then figures out how fast you are going
because you have told it the circumference of the wheel. A cheap one won't
cost very much, or you can probably get the switch in "raw" form from
digikey, mouser, or perhaps someone who sells burglar alarms (they are used
to detect open windows.) The stronger the magnet, the sooner it will sense
the presence of the object.
Hal
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