Infrared anti-theft alarm circuit one a

Infrared alarms use infrared light-emitting tubes to emit invisible
light. When the light is blocked by the human body, an alarm is issued.
    The circuit is shown in the figure. It consists of an infrared light transmitting and receiving circuit (composed of an infrared light-emitting diode VL and a phototransistor
VTi), a switching circuit (composed of transistors VT2-VTs) and an actuator - relay KA.