IR Based Security Alarm Circuit
IR based security alarm circuit can
detect any movement and trigger the alarm. This circuit is very useful
in homes, banks, shops, restricted areas where an alert alarm is needed
on any movement. This circuit is based on IR sensor where an IR beam is
continuously falling on a photodiode, and whenever this Infrared beam
breaks, by any kind of movement, alarm is triggered.
IR sensor consist an IR LED and photodiode, in
which IR LED emits IR radiation and photodiode detects the radiation.
Photodiode conducts current in reverse direction, whenever light falls
on it, and voltage across it changes, this voltage change is sensed by
voltage comparator (like LM358) and generates output accordingly. [Also check: IR Sensor Circuit]
In this IR based security alarm circuit, we have
placed IR LED in front of photodiode, so that IR light can directly
falls on photodiode. Whenever someone moves through this beam, IR rays
stops falling on photodiode and Buzzer start beeping. Buzzer
automatically stops after some time, as buzzer is connected to 555 timer in monostable mode.
This kind of Alarm can also be build Laser light, (like Laser Security Alarm Circuit)
but the benefit of using IR sensor is that IR light in invisible while
Laser is visible. Although both are useful and have different scope.
Components
- IR pair (IR LED and Photodiode)
- 555 timer IC
- IC LM358
- Resistor 100, 10k, 100k, 330, 220 ohm
- Capacitor 10uF
- Variable resistor – 10k
- Buzzer
Circuit Diagram and Explanation
We have used op-amp LM358 in this circuit, LM358
has two voltage comparators inside it, and we have used one comparator
here. Non-inverting end (PIN3) of voltage comparator is connected to the
Photodiode and inverting end (PIN 2) of voltage comparator is connected
to a variable resistor of 10k. Output of the voltage comparator (PIN1)
is fed to the Trigger pin of 555 timer. 555 Timer is configured in
monostable mode here.
While the IR radiation is falling on Photodiode,
the voltage at the non-inverting end (+) of voltage comparator is higher
than the inverting end (-) and the output of the comparator is HIGH.
And as the comparator output is connected to the trigger PIN of the 555
timer, so the when the Trigger pin 2 is high, the 555 output is low. So
during the period when the IR rays falls on Photodiode, the 555 timer
output remains LOW. To understand the working of IR sensor with
comparator LM358 go through this article: IR Sensor Module Circuit
Now when there is some movement, the falling of IR
rays on Photodiode gets interrupted and the voltage at inverting end
(Threshold voltage) of voltage comparator becomes higher than
non-inverting end, for some moment. This makes the output of comparator
LOW and also makes the Trigger PIN 2 of 555 timer LOW. This will trigger
the 555 timer, and 555 timer’s OUTPUT goes HIGH and Buzzer beeps for
short duration. Duration of the beep can be increased by changing the
value of resistor R1 or capacitor C1 (RC network in monostable mode of
555 timer IC). Note that 555 IC gets triggered when Trigger PIN 2 gets LOW.
Notes:
- Normally the range of IR LED is 2 meter, but it can be increased by using Lens.
- AC alarm can be used in place of Buzzer, by using Relay.
- IR LED and Photodiode must be properly aligned so that IR rays can directly falls on photodiode.
- The sensitivity of the sensor can be changed by variable resistor RV1.
Video:
