By timing the interval between successive spokes, the speed of the drum can
be found, and hence the required speed feedback is obtained. An alternative to this
optical sensor would be a 5 V. proximity switch.
So far so good, now we come to the problem of controling the speed
of the main motor. The circuit uses a thyristor – a device which can be made to pass
current into the motor for part of the positive half of the A.C. wave. The earlier in
the half wave it is switched on, the more current it will pass and the faster the motor
will go (see figure 4).
Thus in order to control the speed a pulse has to be sent to the thyristor to switch it
on at the required point in the positive half cycle. The circuit of figure 5
generates a reference pulse for the processor shortly after each positive-going zero