Infrared anti-theft alarm circuit 3a

It can be used as an alarm in invisible places, as
a finish photo timer during competitions, or as an electric toy, etc.
    The circuit consists of two parts : transmitter and receiver. In the transmitter circuit, the transistors VTi and VT2 form a multivibrator
, and the light-emitting diode VL is the infrared emission light source. In the receiver circuit, the photodiode LD is the light-receiving element.
Its maximum detection distance can reach 30m. The light-emitting diodes on
    the transmitter and the photosensitive diodes on the receiver are respectively arranged at the focus of the lens embedded in the respective box hole
. The lens can be made of ordinary glass with a diameter of 48mm and a magnification of 10 to 20 diopters.