Super light Sensitive Intruder Alarm Circuit

Intruder alarms are popular devices used in high-security areas as well as civilian houses to detect and alarm the presence of any intruders. There are different kinds of intruder detection alarms, some detect movements by using a laser, some use pressure variations etc.  Our intruder alarm works using an LDR (light dependent resistor). Also known as photoresistor its resistance varies with the intensity of light falling on it. Its resistance decreases with increase in light.

Our intruder alarm needs to be placed opposite to a light source since it detects movements using the changes in the intensity of light falling on it. When a shadow falls on the LDR its resistance increases and triggers the alarm circuit.


Here is the circuit diagram of an ultra-sensitive intruder alarm. The shadow of an intruder passing few meters nearby the circuit is enough to trigger the alarm.

Here IC2 uA 741 is wired as a sensitive comparator whose set point is set by R6 &R7. The voltage divide by LDR and R9 is given at non-inverting pin of IC2. At standby mode, these two voltages are set equal by adjusting R9. Now the output (pin6) of the comparator will be high. Transistor Q1 will be off. The voltage at trigger pin of IC1 will be positive and there will be no alarm. When there is an intruder near the LDR the shadow causes its resistance to increase. Now the voltages at the inputs of the comparator will be different and the output of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a monostable multivibrator. The output of IC1 will be amplified by Q2 (SL 100) to produce an alarm.

Intruder Alarm Circuit Diagram with Parts List

Intruder Alarm Circuit
Intruder Alarm Circuit Diagram


  • To set up the alarm, power up the circuit and adjust R9 so that LED  D1 goes off.
  • The LDR can be housed in a dark tube to increase sensitivity.
  • The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with  R9 for fine adjustment.

Our website has more Security Circuits that you may like to read:

1. Metal Detector Circuit

2. Burglar Alarm Circuit

3. InfraRed Motion Detector

4. Fire Alarm Circuit

5. Door Handle Alarm



  1. i have done dis circuit using with IR SENSOR MODULE AND 555 TIMER. what happens if we connect the IR sensor module directly to the buzzer.instead of connecting it to 555 timer of reset pin. plzz help me its urgent..

  2. i have connected the circuit .but mah sensor is working properly but i am nt getting the sound frm speaker..

  3. Excuse me, i connected the circuit and the sensor are working, but the buzzer continue to make sound when shadow are not detected and when shadow are detected the buzzer will not make any sound. Any help are appreciated.

  4. Seetharaman, I have connected the component on vero board and is working perfectly well. Few days later, buzzer continue to sound even when no shadow is on the LDR. I change the LDR but the buzzer kept sounding without sensing shadow. Pls what could be the posible problem?

  5. DamnAssKush

    i tried this circuit and it worked perfectly,

    thx for givin me good info.

  6. Bukar g dapchi

    I have connected the component on bread board correctly and the led will light on,the ldr is working because i have test it on ammeter.but when shadow fall on the ldr the buzzer will not sound and sometimes when already there is shadow on the ldr and i connect it the buzze will sound,suppose to if i remove the darkness also the buzzer to stop the sound what is the problem? Please urgent rescue.

  7. i have tried the circuit on bread board but it’s not working

  8. Hi, I want to use this circuit in my bag and i want the speaker to be loud so can i use a 12v 7amp battery and a 12v car horn??
    If not please suggest a circuit

  9. The buzzer is on even when there is no shadow on the LDR . pleaseeeeeeee help me………….

  10. The buzzer is on even when there is no shadow on the LDR . What could be the reason?

  11. Assassin thunder

    Will any one help me,the circuit was working very good for 2 days after making then after which buzzer is not working what will be the fault i have checked the buzzer ,i have also check the ic it is working because it is triggring .

  12. Hi
    Am a beginner to this world. Learning to do a circuit for my home.
    This is a nice site for implementation.

    Just a suggestion.
    PLEASE MENTION ALL THE COMPONENT LIST (ONE-BY-ONE) like how many register(ohms), capacitor, transistor etc.


  13. Hi, Was just wondering, the uA741 spec says it has a 15V supply. Will it still work with the 9v supply shown in the above diagram? Thanks.

    • Seetharaman

      Hi Rat1 741 can operate from plus minus 2 volts on wards up to plus minus 20 volts. In the above circuit the ref Pin No2 voltage is held at half the supply voltage (9 volt) that is plus minus 4.5 volts.

  14. Kabir Lawal Ibrahim Matazu

    I got a circuit from you,Iast three years ago and I be happy

  15. Manaswini Patel

    What does k1 written below d buzzer mean?
    If i go to the shop to buy the buzzer ,what should i ask for? Also, please suggest a few alternatives for the buzzer K1 incase it’s not available in the market. Please help. Urgent!!

  16. which LDR is used for super sensitive intruder alarm?

  17. i would like to know the block diagram of the circuit and the value of the speaker used,type of the capacitors and resistors.will be grateful for your reply

  18. this alam sys is helpful but the actual distance
    and cost of the cct is not shown in the block
    i wants the operating distance of cct.

  19. seetharaman

    Hi Ashnuka you can use any low power general purpose pnp transistor

  20. What are the replacements for BC157 other than mentioned below?I’ve searching it and it’s equivalents but no luck.Can i use any pnp?

  21. i want video for making sensitive intruder alarm

  22. please…Seetharaman, i need to know how to get the intruder alarm circuit block diagram and its discriptions…please help me sir.

  23. Hi Seetharaman,thanks for your help.But i found ua741 and SL100 in here.Thank You again.

  24. Seetharaman

    Hi Ashnuka 741 is a standard opamp and it is freely available. you can use TL071 also. for SL100, you can use 2N1711, 2N2219, 8050, BD137, CL100, BEL100N etc.

  25. what are the equalant components for ua741 and SL100?

  26. Christiane

    . .i’m an EcE student here I got my best project here in circuits today .. .and my professor are so happy that I have my project . remember the transistor based intercom . ..very simple but it was so great ..thank you

  27. Christiane

    . .hi i’m me . .can you help me find what to replace sL100 and bc157 . . out of the market again …. can you give at least 10 replacement to each transistor …I don’t have much money to spent searching the net . .I’l come back tomorrow . .its about our project please help me. .i’m out ..I will be so thank full if i can find answers tomorrow,,

  28. hi seetharaman! I just wanna ask something. I’ve tried you circuit but i have a big problem, cause when I connect the power supply to the circuit, the alarm was already triggered and the the potentiometer really does not vary the sensitivity of the LED… please help me to trouble shoot my circuit.. THANK YOU!

  29. hi guys, what if i want to add-on a timer that act as switch to automatically ON the circuit when we needed? what should we put in there and how? hope u guys can help me.. tnx

  30. hello seetharaman thanks 4 helping me.. i make this circuit and it works..tnx 4 the help..^^..take care alwys

  31. helo mr. seetharaman..i’ll follow your suggest to me..thank you very much for helping me…^^

  32. ah ok..tnx its ok that I replace the MPS2222A to the SL 100 and also the Mps4126 to BC 157?? tc alwys MR. seetharaman..tnx^^

  33. Seetharaman

    Hi Popsy SK100 is a medium power PNP transistor in T05 casing. BC 157 plastic low power PNP transistor and not an equivalent for SK100 2N2905 is OK.

  34. helo Mr. Seetharaman,sir i dont get it..the Sk100?or did you mean the BC 157? thx^^

  35. Seetharaman

    Hi Popsy MPS2222A can be used for SL100. MPS4126 can be used for SK100 provided the operating voltage is less than 20Volts.

      • seetharaman

        your LDR may be faulty use 10mm LDR it will work all right.

  36. seetheraman can i also replace the Mps2222A in SL 100 and the MPS4126 in SL100??they are general purpose right?tnx^^

  37. Seetharaman

    Hi Popsy you can use 2N1711 for SL100 and 2N2905 for BC157

  38. hey guys,is there a replacement or a equivalent in the 2 transistors,the BC157 and SL 100..can you help me guys..tnx

  39. seetharaman

    Hi Batutuy The circuit is correct. When the light falls on the LDR it resistance will be low, hence IC output at Pin no6 will go high. Q1 emitter will go high LED will be in off ststus. When the shadow falls on LDR, its resistance will go high and Q1 will conduct and LED will glow with a negative going pulse to IC1 to produce alarm.

    • Hello sir,i have some problem in my circuit. when i supply 9V to circuit LDR and LED both properly worked but buzzer doesn’t worked when shadow of intruder fall on LDR…
      Plz,try to my problem shoot….


  41. the I think that the LDR and R9 must be at the inverting pin because the non inverting pin is fixed

  42. wat is this k1 written below d buzzer n if i go to the shop to buy the buzzer ,what should i ask for?

  43. we are technical student please we want the real diagram of an amplifier we will be glad if our request is granted thanks

  44. Home Security Advice

    Great idea!
    The only thing I’d be concerned about with a super sensitive alarm is it false triggering all of the time. When used in the right application, its a great circuit.