The status corresponding to the presence or absence of articles at their reserved locations is detected by means of an optical switch comprising an emitter which sends infra-red rays toward a receiver. The article obstructs these rays and hence its presence is detected. The information obtained from...http://www.google.com.hk/patents/US4791411?utm_source=gb-gplus-share專利 US4791411 - Manual free-access vending machine
