Abstract: A solid state relay is disclosed which is failsafe in the sense that it is prevented from failing in a closed circuit condition. The relay comprises a main switch adapted to be connected in series with a load and voltage source and responsive to relay opening and closing signals. A secondary switch is connected in parallel with the load and is switched a sense opposite that of the main switch by the relay opening and closing signals. If the main switch fails to open, the secondary switch is closed and the load is short circuited by it. Additionally, a fuse in series with the main switch is blown by the current through the secondary switch to latch the relay in the open circuit condition. Self-checking circuits are provided to latch the relay an open circuit condition if there is any circuit failure.

Abstract: This invention relates to presence sensing systems and more particularly to such systems which utilize beams of radiant energy, such as light beams, to detect the presence of a person or an object within a predetermined space. Additionally the invention relates to presence detecting systems combined with self-checking means to ensure that the detection system is operating properly.