This circuit can do the
job of a Gatekeeper and alerts you if someone passes through the gate.
The alarm can be an AC bell or a Lamp. The alarm turns on for 1 minute
and stops if the light barrier is restored again. Infrared rays are used
as the light barrier to activate the alarm system based on a
Phototransistor. The high gain NPN Darlington phototransistor L14F1 conducts when its face is illuminated with IR rays.
This brings its collector to ground potential. IC1 is used as a
simple voltage comparator with a potential divider R2 and R3 connected
to its inverting input. So that half supply voltage (6 volts) is
available to its inverting input. Its non inverting input is connected
to the collector of the phototransistor. Normally the output of IC1 will
be low since T1 is conducting.
When the IR beam breaks, the collector of T1 becomes high and the
voltage at the non inverting input of IC1 increases above the voltage at
the inverting input and output becomes high. This triggers the relay
driver T2 and relay turns on. Capacitor C1 gives a short delay at the
non inverting input of IC1 to prevent false triggering. Capacitor C2
keeps the base of T2 high for a short time even if the IR rays restore.
- IR rays should be aligned exactly to the Phototransistor so as to keep the alarm off.
- Normal range of the circuit is 2 meters. This can be increased to 5 meters if a convergent lens is used in front of the IR LEDs.
- A buzzer can be used in the place of the relay, if AC alarm is not required.
- Instead of IR LEDs, a Laser pointer can be used to increase the range up to 25 meters.
- If Laser is used, take all precautions to prevent direct viewing.
- Enclose to the Phototransistor in a small case with an opening in front. This prevents the entry of ambient light.
- Fix the IR LEDs on one gate pillar and the Photo transistor on the opposite pillar with exact alignment