Antitheft device using IR light barrier - has adjacent source and photodetector located in common housing and mutually offset for alarm operation

摘要

The device uses an IR source (11) and a photodetector (12) to provide a light barrier which causes the alarm to be operated when it is broken. The source (11) and the photodetector (12) are located side-by-side within a common housing (18), with the photodetector (12) angled so that it receives the beam provided by the source (11) after its reflection from a spaced fixed point. The photodetector (12) uses a photodiode (21), with its receiving face lying in a plane (20) which is angled to the axis of the optical focussing lens (19), with the photodiode (21) able to slide along this plane (20) depending on the distance to the fixed point from which the beam is reflected.