Abstract: A device and method for detecting the presence or absence of an object which has repetitive motion are disclosed comprising, a receiver for receiving a signal from the object, and circuitry for determining the presence or absence of the object to be detected, wherein the circuitry records the signal from the receiver as a pattern of data during at least part of the repetitive motion of the object, compares the data with a previously recorded data pattern and, produces an output signal based on the comparison. The signal may be received during discrete time intervals, and may be light which can be transmitted with varying intensity. The circuitry may determine the value of signal received at a receiver and produce a binary value. The signal received at the receiver can be reflected from the object to be detected.