Abstract: A proximity detecting apparatus for generating a beam of infrared light rays in the form of successive radiation pulses occuring at a predetermined frequency for reflection off objects moving within the proximity of the apparatus. A photo-detector device positioned adjacent to the radiation beam senses the reflected rays and responsively produces an output signal. Other light radiation and electrical signal noise sensed by the detector are filtered from the output signal according to the pulse width and frequency of pulse radiation generated by the beam. A detected signal is established as valid only if the signal and the infrared source are in frequency synchronization and if an unbroken pulse train is detected for a predetermined length of time.