Automation ECE engineering projects Security Based Automation Sensor based

Intruder detector Burglar alarm that auto-activates at Night

Hello Engineers. I hope you ‘re doing good. In the place like godowns, construction sites, telecom sites, small factories, Lawns, etc, high security is required and also present. But at some of those places, working hours are only during day time. So, during the night when security guards aren’t much active, we need to automate the security. A system is needed to alert the nearby security and staff which is called the Intruder detector Burglar alarm system. An intruder detector or burglar alarm circuit is needed to automatically detect the intruders & alerting the authorized security guards or officials. By making very loud sirens or sounds. These systems are put at the entry and are activated during the no-entry time to detect intruders activity. So let’s move further and let’s start with our topic ” Intruder detector burglar alarm circuit”. Or automatic intruder detector burglar alarm which auto activates when it’s dark.

This circuit is made on one of my website’s visitor’s request.

 Chima Hillary
  I want a circuit, an intruder alarm circuit that only activate at night time when any intruder enters my Telecom site at night. 


Hello Chima, Thanks for visiting my web site, and I will design and upload the circuit within the next 2-3 days and will let you know. Thank you.

So, basically Chima Hillary is one of my readers. He wants an intruder detector alarm circuit which is powered on all day but is only auto-activated when it’s dark. And generates a loud alarm only when it detects someone to enter their telecom site’s gate at night.

What is an Intruder Detector Burglar Alarm?

An intrusion detection alarm or burglar alarm is an alarm circuit which is consisting of some electronics components & sensors. Used to detect any human activity or entry gate security violations. The alarm makes use of different types of sensory elements like motion sensor or laser security alarm, for intrusion detection. After sensing, the sensor gets activated and thus sends a signal to the circuit which then activates the alarm system.

Also, kinds of circuits can be turned-on either manually by any authorized-person or made to get auto-activated after working hours. Below is the circuit for this project followed by the components list and explanation.

Circuit Diagram for Automatic Intruder detector Burglar alarm

intruder detector burglar alarm

Components needed

  • PIR sensor 1pc
  • LDR – 2pc
  • R1 820K -1pc ( you can also use 750K according to your sensitivity adjustments, to be tested practically with the both)
  • Q1,Q2 TIP122 -1pc
  • D1,D2,D3,D4 – 1N4007 – 4pcs
  • RELAY 12V – 2pcs
  • DC Barrel jack female connector ( buy from amazon|this-one-is-non-profit-link)
  • 12V 2 – 4.5AH SLA Battery ( available online cheap on amazon @ $11 only )
  • 12V 2 A broadband DC adapter
  • C1 330n – 1pc
  • C2 100n – 1pc
  • 220 V ac electric GONG BELL 9 inch (check the price online if it’s not in local shops)
  • PCB
  • Heat sink for Q1 and LM7805

Basic Principle

The circuit, as well as the materials list, are given above. The whole circuit is mainly powered by a 12v DC adapter during day time and 12v battery during the dark. This circuit has been made with custom features for one of the readers. This circuit has additional independency feature as it’s not needed to power-ON or Off. The circuit is a loud sound alarm burglar alarm with enhanced functionality that it automatically turns ON on detecting dark. After that starts serving its motion detection and human detection work. On detecting day time, it auto-shuts down and then the battery starts charging via DC adaptor.


Here, 2 LDRs (Light Dependent Resistor) are used to detect dark, and PIR(Passive infrared sensor) is used to detect motion. As we know, LDRs have some working conditions and principles. With the increasing intensity of the light beam falling over them, LDR’s resistance is decreased and vice-versa. When resistance is decreased, of course, the conductivity is increased.

The 12v DC adaptor is always ON and powering LDR, Relay 1 as well as charging the battery. During the daylight, both of the LDR’s resistance is dropped very low i.e. As low as from 1M ohm to 100ohms. So now, R1 is greater, and this makes the Q1’s base to get shorted with the ground (low resistance barrier). Now, RELAY is also off due to Q1 TIP122 is off and relay keeps COM connected to NC. On the other hand, it also keeps charging the 12v battery in cycle mode.

With the decrease in Daylight, till evening when it starts to be dark, the LDR’s resistance reaches its peak.

Now, there is enough barrier between the base and the ground to turn the transistor ON. The transistor Q1 when turned ON, also turns the relay ON. Now, the relay’s COM is connected to NO by relay’s moving rod. This then makes battery power to supply the LM7805 regulator. The output of LM7805 powers the VCC of PIR.

PIR sensor is then turned ON after getting +5V Vcc in. Everything is silent until the PIR detects any motion activity. Once the PIR detects the motion activity, it gives a TTL output to transistor Q2. Q2, on getting turned ON, makes the relay activated. An electric GONG bell is also connected to the relay’s NO. Now the relay makes the circuit between 220v supply and the electric gong bell complete. A loud sound up to several meters.


  • Use a 12v 2A DC adaptor connected to the DC barrel jack. Always keep it ON.
  • Keep circuitry away and safe from water.
  • Arrange the PIR sensor’s position so that it is only detecting that who has entered inside from gate. Or site ( telecom site for example) entrance.
  • Position the LDR’s in such a way that they get clear view of light and dark. I suggest put circuitry in a waterproof arrangement and make LDRs to face sky.
  • Just always keep 12v 2a DC adaptor ON and it’s ready.

So guys, this was all about this circuit. I hope you liked it, if you have any questions regarding this, you can ask me in the comment section. Thank you.

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Aabhishek Sharma is a Computer Science Engineer and a Digital Marketing Trainer and Consultant. Aabhishek Sharma also loves working on Electronics Projects and working with microcontrollers and Designing Home automation Gadgets.

16 Replies to “Intruder detector Burglar alarm that auto-activates at Night

      1. Hello Aabishek It have so nice to view the blog thank you for such innovative ….So can I get a simple circuit project for home security in terms of intercom a wired one ..thank you

        1. Hello Brian, Thanks for visiting and sure, I’ll try to provide you what you want. Can you please elaborate more on what features exactly you want? Thanks for visiting and keep in touch.

  1. Awesome article, loved even though i don’t have nay mastery in electronics, i got the basic functioning of LDR and PIR and their applications in Automatic Intruder detector Burglar alarm.
    Thank you

  2. Interesting. Very good design. I will try to construct this. I have following comments:

    1. On material list, since Q1 and Q2 are both TIP122, quantity should be 2 pcs.

    2. Where can I procure the LDRs and PIR?

    3. After studying your design, can we add some sort of a “search light” (high intensity light) parallel to the gong circuit to view the intruder and at the sametime surprise him/her. Send them away running. Thinking of a camera to capture the intruder, but that is more complicated and not needed at this time.

    4. I suggest to put a switch to “silence the bell” or “reset switch “ just in case the security guard finds it as a false alarm (example a dog triggers the PIR) Gong is hurting the ears.

    5. The system will automatically function for a while, what happens if the battery deteriorated and charge wont last till following morning? No one will know esp if it was installed on a remote site. What is the back up power? Just asking.

    Sorry for the many questiond and comments. But truly, i like your design and will try to build it. Thank you very much. BTW. I am a DIYer from the Philippines.

    1. Hello, Augusto Thanks for visiting and also the appreciation for my work.Answers to questions:-
      1. Yes Q1 and Q2 are both TIP122 and yes quantity should be 2pcs i.e 1pcs each, so that’s what I meant.
      2. You can get LDR and PIR sensor from any local electronics components retail shops or anywhere online like
      3. Yes you can add the Bright light like any 220v AC tube light or AC LED light to the relay in parallel with the gong BELL so as long as the relay is activated, the Light will also glow.
      4. Yes if an authorized person has any work on the telecom site at the night then to stop or turn off the bell, you can add a switch between battery’s positive and the relay’s COM pin and keep it ON always and can be turned off if authorized person has to enter at the night.
      5. No, the system is fully automated, as the 12v wifi DC adapter is all time turned ON and in the day time, the battery keeps charging. During the night time, the relay is changed and now, from NO pin, the PIR and control circuit gets activated to work upon the night.
      Power Backup is just Good enough as the gong bell only and the relay only goes ON when the PIR detects any motion. The only thing that consumes battery all night is the PIR sensor that doesn’t consume much. So, a 4.5 ah battery is very much good enough and more enough.

      Everything in this circuit, from system control and detection to the power management and battery charging, is automated and self-independent. Just you need to keep that 12v wifi adapter all time turned on and employ PIR and LDR in the proper direction and suitable scope of detection direction.
      I have made this circuit custom as per requirements for Chima(one of our readers).
      Thank you for visiting and I hope all your doubts are clear now.

  3. If the power from the charging adaptor fails at night, the relay will oscillate.
    If there is no mains power the main bell will not work either so the battery is pointless.
    Alarms are supposed to use batteries to power the sounder as well as the circuit.

    1. Hm, So you can provide the adapter as well as the main bell, an inverter/ups connection.
      Yes, there are some alarms that are battery-powered too, but as I said this one was custom made and as per requirements of one of my reader and also as per components feasibility by him and his requirement of having it all day kept ON and it can auto activates during the night. In addition, he wanted something to be heard around 200-300metres. So All these things are as per custom requirements.

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