Control apparatus

A control apparatus responsive to manual stimulus comprising:

• a photosensitive detector;
• a transmitter having a light source for projecting a beam of light along a path normally directed away from said detector;
• a receiver for receiving an output from said detector and for generating an enable pulse having a duration determined by a length of time for which the beam is reflected towards the detector by a reflective article;
• conversion means for varying an output signal level as a function of the duration of the enable pulse, wherein the conversion means comprises first means for increasing the signal level in response to and during a first enable pulse generated when a reflective article is placed in said beam a first time, and for decreasing the signal level in response to and during a second enable pulse occurring after the first enable pulse, said second enable pulse being generated when a reflective article is placed in said beam a second time;
• second means for decreasing the signal level during the first enable pulse when the signal level reaches a maximum value, and for increasing the signal level during the second enable pulse when the signal level reaches a mimimum value, and a bidirectional counter having:
• a clock input responsive to a first clock signal when the enable pulse is asserted on an enable input;
• and
• a direction input responsive to direction control means for configuring the counter to count up in response to the first enable pulse, and for configuring the counter to count down in response to the second enable pulse;
• the first means comprises a bistable circuit for switching the direction input between a first logic state and a second logic state in response to the leading edge transition of the enable pulse;
• and
• the second means includes means for triggering the bistable circuit to switch the direction input between first and second states in response to either a leading edge transition of the enable pulse or a leading edge transition on an overflow output from the counter.