IR Based Security Alarm

Published  October 31, 2015   49
J Jayant
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.

IR Working

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.



  • 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

Infrared Security Alarm Circuit Diagram

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.



  • 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.

Have any question realated to this Article?

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Submitted by KILTON ROY on Fri, 02/26/2016 - 23:44


sir uin rv u have used pot or not i see there is a one pot there if yes plese say

Submitted by Alex on Tue, 03/29/2016 - 07:56


I am working on this project with a friend and we cannot get it to work properly. We believe the photo diode isn't picking up the IR signal. Any feed back as soon as possible would b greatly appreciated.

Submitted by Alex on Wed, 03/30/2016 - 05:50


Now we got the circuit to properly to work with a LED in place of the buzzer. However, when the buzzer is in the normal place and movement moves across the photo diode; the buzzer turns on and stays on. Any ideas?

Submitted by lenin on Sun, 08/28/2016 - 01:04


Sir when i connect this circuit i am not getting result...the buzzer is countineously in on state....could u please help meeee

Submitted by Akash on Fri, 09/02/2016 - 13:03


Maker's of the circuit please help us in finding a way about it.... buzzer is buzzing continuously...instead we placed a LED..same result.... Yours help will be remember forever...

Submitted by Jasper on Fri, 11/11/2016 - 01:03


What does the variable resistor do in this circuit? i have tried to figure this out many times but I can't figure out the purpose. please help

Submitted by chiranjeevi on Tue, 11/29/2016 - 21:43


thank you. its working for me. I want t extend this project. Instead of using a buzzer, I want this circuit to send a sms alert. So how can I proceed.

Submitted by Chandrasekhar on Sat, 12/09/2017 - 13:34


sir , instead of buzzer , I wanna insert relay , can I increase the time period of trigger state ??

Submitted by seeker on Tue, 01/23/2018 - 14:56


I just noticed that the IR led has the value of 10K different from what is in the circuit which states 100 ohms only, apparently the photo diode is connected to 100 ohms instead of 10k.. ur schematic and actual photo shot of the circuit didnt match..

that's maybe why i can't make the circuit work.
kindly answer my query.. cheers

Submitted by hemant on Wed, 02/28/2018 - 18:25


will the beeping gets higher when the obstacle comes close?
if no what should be done to vary the beeping as he object moves closer to the reciever?

Submitted by Mohammed Abdul… on Wed, 08/29/2018 - 18:00


What should I do if I want the circuit to be effective if I have a distance of 20 meters ? Can I separate the IR LED from the receiver section ? Yes Sir.

Follow the circuit diagram for making connections and selecting component values. I am not sure where you spotted a 330 ohm resistor only a 220 resistor is used for speaker.

Also you can use the forums for technical help and it is much active than comment section