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Timers
Photogates
A photogate includes a light source and a light detector. In the normal
state of the photogate, light from the source arrives at the detector,
where it is absorbed to produce an electric current. This current is used
to generate the signal to a timer that says, in essence, "nothing is in the
gate." When an object intercepts the light beam from the source, the
current generated at the detector drops and this reduces the signal to the
timer.
Updated 6/17/99
by Peter N. Saeta
.