Entry Alarm Circuit

This is an Infrared based Broken beam alarm to protect doors and entry passages. It gives a loud alarm when somebody crosses the Invisible Infrared barrier. It can protect the doors both day and night and is free from false triggering. The circuit is too simple and can be a good evening project for the hobbyists.

The circuit uses Infrared diodes to emit continuous IR rays which are not pulsed as in the case of remote handsets. The NPN Darlington Photo transistor is used as a light sensor. L14F1 is the ultra sensitive Darlington photo transistor with high gain. Its collector is connected to the positive rail through VR1 and the Emitter has a Green LED to indicate standby mode. The base of T1 is left free without connecting anywhere. This base junction is exposed through the window of T1 so that light photons will bias the base.

The degree of biasing depends on the intensity of photons.T2 is the alarm driver and its base is connected to the collector of T1.So that the base bias of T2 depends on the conduction of T1. When the Infrared rays illuminate the photo transistor, it conducts pulling the base of T1 to ground level so that it remains off. Red LED and Buzzer connected to the emitter of T2 remains off in this condition. When a person crosses the IR beam, T1 turns off and its collector voltage becomes high. T2 then conducts and activates Red LED and buzzer.

Entry Alarm Circuit diagram

The circuit can be constructed on a small piece of Perf board. Assemble R1 and IR LEDs on a separate board. Fix IR LEDs on one side of the entry and the Phototransistor on the opposite side. Carefully align IR beam on the Phototransistor so that buzzer will be silent in the standby mode. Move hand across the beam, buzzer should sound. VR1 adjust the base bias of T2 to keep the buzzer off in the standby mode.

L14F1 Phototransitor

L14F1 and BC547 Pins


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  • sadif

    In morning during sunlight it cant work
    Plz help


    SIR CAN I USE L14G2 – Phototransistor (Equivalent – L14F1) , OR TALE ME SOME OTHER PHOTOTRANSISTOR

  • kathir

    in entry alarm circuit red led always glows….can you give a solution to fix it..

  • aivie

    sir, how do the variable resistor in this project look in exact, coz when i went to buy for it, they are giving a bunch of resistors tnx

  • Hotshock

    What phototransistor can be substituted for the L14F1 which, I am told, is no longer manufactured and not available.

    • Jim Keith

      Check out this photodarlington that is available on ebay:

      Or use the Honeywell SDP8105-001
      (DigiKey 480-3577-ND)
      I am currently working on a datasheet submission for this useful device.

  • Avinash


    Thanks a lot for the circuit. I want to build one and fix this in our corridor. Can these components be used for a distance around 2 meters? Please let me know. Thanks!

  • jess

    i have been working with electrons for 5 years now but i was wondering if i could use this cct in conjunction with a solenoid that will trigger something else instead using a speaker.
    i also was wondering on using 12volts dc instead.
    thank you

  • AMIT

    sir, can i use l14g2 phototransiter instead of phototransister l14f1

  • lakshmanarao

    Dear sir, thank you very much for giving us this mini project.

    sir, i completed this project successfully. but, i want a documentation for this.. please help me in this…

    thank and regards


    • Ramakrishna

      Hello Mr.Laxmanrao…
      I am also working on this project.Can you say what is VR1 and preacautions for this project.
      My mail is
      I should submit in my college…so mail me as soon as possible…
      waiting for your reply

  • aakash

    thank u so much sir for d information…

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