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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    Dear sir,
    Thanks for giving such informations.

    Sir,I want to know can L14f1 be operated (activated) by laser beam or laser torch?

  • noxie

    mr mohankumar i am one of the third year student doing photodiode alarm , and would like you to help me

  • jenn

    hi there…let me know if the resistor used is 1watt? or 1/4 watt??

  • ravikumar

    sir iam made one ir circuit but ir led and phototransister position how to connect plz show me

  • Bharat

    Sir, I want to purchase ENTRY ALARM or any other security systems. Please let me know where can I get this. Please give your Cell No. as well so that I can talk to you.

  • D.Mohankumar

    There is nothing new in the circuit. When IR rays fall on the photo transistor, it conducts pulling the base of T2 to ground. When the IR beam is broken by the shadow, photo transistor turns off and its collector becomes high to give base bias to T2 and it conducts and activates buzzer.Observe the conditions
    1. IR rays should focus on the photo transistor
    2. Day light should not fall on the photo transistor
    3. Use only L14F1 photo transistor
    4. Adjust VR to set the base voltage of T2 below 0.6 volts in the standby mode
    5. Adjust the distance between IR LED and photo transistor by trial and error

  • nouman nayyer

    dear sir
    sir its not working sir
    light and buzzer is still emitting slightly:-(

  • D.Mohankumar

    Adjust VR to set the sensitivity of the phototransistor at the particular light level. If properly adjusted, base voltage of T2 will be less than 0.6 volt and it will not conduct.If IK preset is not sufficient, replace it with a 5K preset. Alternately reduce the value of R1 to 10R 1W so that the intensity of IR rays will be more

  • nouman nayyer

    and sir i m using 100R in quarter watt
    is this correct???

  • nouman nayyer

    dear sir
    first of all thanks alot for giving me your precious time for relying me.
    sir i did the same
    but buzzer voice becomes low and green led is also lit but buzzer and red led is also emitting voice and light.
    and distance is o more than 2 inches.

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