Counting people entering and leaving a zone

摘要

A counting device can be used to switch the lighting on when a person enters the room and switches the lighting off when the room is empty. The device has a passive infra-red sensor which emits a pulse whose polarity is dependent on the direction of movement of a person into or out of the room. The pulse is passed to a counter which is incremented or decremented accordingly. The value of the count is representative of the number of persons present in the room. When the count indicates the room is empty the lighting is extinguished. The sensor may comprise two pyroelectric detectors A and B wired in opposed parallel manner and having spaced fields of view through a slit X. Movement of a person in direction Z produces a positive pulse followed by a negative pulse, whiist movement in the opposite direction produces a negative then a positive pulse. Further circuitry detects the polarity of the first pulse of the pair.