Three lines are used as Inputs. Two input signals come from the level switches
which indicate when the drum is full and empty. As shown in figure 2, these switches
are connected to +5 V. through 10 Kohm pull up resistors.
The third input is used by the processor to determine the speed of the drum for
feedback purposes. A reflective optical switch points at the metal spokes of the
pulley which turns the drum. When a spoke goes past the switch light is reflected
and a ‘1’ is generated on the P.I.O. input line, which is usually at ‘0’. The circuit
requires a transistor amplifier to step up the output from the receiver to TTL level for