A position detector includes a source of radiant energy, such as infrared light. A sensor is spaced from the source. The source and the sensor can be carried spaced apart from one another by a housing. Control circuits, carried by the housing, are coupled to the source and the sensor. Pulsed radiant energy, emitted by the source is incident on the sensor only when transmitted by a solid optical medium which has a predetermined orientation relative to the housing. When the medium has the predetermined orientation, the sensor receives transmitted radiant energy from the source. When the medium moves from the predetermined orientation, the sensor ceases to receive the transmitted radiant energy from the source, and the control circuits can generate an alarm indication.