Light barrier having multiplexer for actuating transceiver pairs and multiple channel operation

In combination with a system utilizing a plurality of individual transmitters of light, a like plurality of individual receivers of light, a like plurality of light conducting paths whereby each transmitter is connected to its corresponding receiver by a corresponding path so that the light emitted by any transmitter can only impinge upon the corresponding receiver, the light emitted being in the form of pulses, the transmitters being selectively actuated and deactuated in selected format by a multiplexer, the corresponding receivers being synchronized to respond in the same format by a demultiplexer whereby when the system functions properly each receiver will receive and respond only to the light pulse emitted by the corresponding transmitter, an evaluation circuit comprising:

• first means for initiating a measuring operation in which each transmitter-receiver pair is individually actuated in a selected sequence;
• a first channel for producing a first signal when each receiver has properly received a light pulse emitted by its corresponding transmitter, the first signal suddenly changing value whenever a receiver fails to receive a light pulse emitted by its corresponding transmitter;
• second means for initiating a testing operation in which each transmitter-receiver pair is individually actuated in the same sequence but in such manner that each receiver will not receive any light pulse emitted by its corresponding transmitter;
• a second channel for producing a second signal when each receiver has not received any light pulse, the second signal suddenly changing value whenever a receiver receives a light pulse;
• and
• a third channel for monitoring the response of said first and second channels, the third channel increasing the response sensitivity of the first channel when the first signal changes value and increasing the response sensitivity of the second channel when the second signal changes value.