A HeNe laser is situated so that it shines on the center
of the bob at θ = 0. A photodiode across from the laser
detects the light and converts it to voltage. This voltage
is used to determine time. As the bob swings back and
forth, the light is broken by the leading edge of the bob
and comes back when the trailing edge passes by. The
time that the light is broken is called shadow time ts.
The average velocity over that range can be calculated
by