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.
Photodiode Alarm Circuit Diagram
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.
IR Transmitter Circuit
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.
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