This is an Infrared based Broken beam alarm to protect doors and entry passages. It gives a loud alarm when somebody crosses the Invisible Infrared barrier. It can protect the doors both day and night and is free from false triggering. The circuit is too simple and can be a good evening project for the hobbyists.
The circuit uses Infrared diodes to emit continuous IR rays which are not pulsed as in the case of remote handsets. The NPN Darlington Photo transistor is used as a light sensor. L14F1 is the ultra sensitive Darlington photo transistor with high gain. Its collector is connected to the positive rail through VR1 and the Emitter has a Green LED to indicate standby mode. The base of T1 is left free without connecting anywhere. This base junction is exposed through the window of T1 so that light photons will bias the base.
The degree of biasing depends on the intensity of photons.T2 is the alarm driver and its base is connected to the collector of T1.So that the base bias of T2 depends on the conduction of T1. When the Infrared rays illuminate the photo transistor, it conducts pulling the base of T1 to ground level so that it remains off. Red LED and Buzzer connected to the emitter of T2 remains off in this condition. When a person crosses the IR beam, T1 turns off and its collector voltage becomes high. T2 then conducts and activates Red LED and buzzer.
The circuit can be constructed on a small piece of Perf board. Assemble R1 and IR LEDs on a separate board. Fix IR LEDs on one side of the entry and the Phototransistor on the opposite side. Carefully align IR beam on the Phototransistor so that buzzer will be silent in the standby mode. Move hand across the beam, buzzer should sound. VR1 adjust the base bias of T2 to keep the buzzer off in the standby mode.