A photodetection system and related circuitry designed to sense the intrusion of an opaque object in a light plane. Upon sensing the intrusion, the system quickly initiates a machine-stopping sequence prior to the opaque object coming into contact with the machine while that machine is operating. The system comprises a SYSTEM microcontroller and a WATCHDOG microcontroller, both of which operate an output control device of the light curtain switch of the invention, wherein the output control device comprises a plurality of electromechanical relays. Both microcontrollers also perform system hardware and software integrity checks. Furthermore, the WATCHDOG microcontroller evaluates the performance of the SYSTEM microcontroller by transmitting a false intrusion signal to a photosource, determining whether the false signal has been detected, and then verifying that the SYSTEM microcontroller has initiated the machine-stopping sequence. The machine-stopping sequence may be overridden by the WATCHDOG microcontroller, or it may be initiated if the false signal is not detected. The combination of electronic, mechanical, and optical modules of the present invention provides improved reliability and accuracy. In addition, the SYSTEM microcontroller and the WATCHDOG microcontroller are designed so as to reduce the system's sensitivity to spurious signals.