Entry
Alarm
This is an Infrared based Brocken 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 Phototransistor is used as a light sensor. L14F1
is the ultrasensitive Darlington phototransistor 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 phototransistor, 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 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.