This Photodiode based
Alarm can be used to give a warning alarm when someone passes through a
protected area. The circuit is kept standby through a laser beam or IR
beam focused on to the Photodiode. When the beam path breaks, alarm will
be triggered. The circuit uses a PN Photodiode in the reverse bias mode
to detect light intensity. In the presence of Laser / IR rays, the
Photodiode conducts and provides base bias to T1.
The NPN transistor T1 conducts and takes
the reset pin 4 of IC1 to ground potential. IC1 is wired as an Astable
oscillator using the components R3, VR1 and C3. The Astable operates
only when its resent pin becomes high. When the Laser / IR beam breaks,
current thorough the Photodiode ceases and T1 turns off. The collector
voltage of T1 then goes high and enables IC1. The output pulses from IC1
drives the speaker and alarm tone will be generated.
A simple IR transmitter circuit is given which uses Continuous IR
rays. The transmitter can emit IR rays up to 5 meters if the IR LEDs are enclosed in black tubes.