light fence circuit schematic

Light Fence Security Beeper

General purpose hobby circuit of a simple light fence security beeper is presented here. This circuit can be used as a door alarm, gate alarm, pathway alarm, etc. Any 12 Volt dc power supply can power the whole circuit.

Working of this circuit is straight forward. In standby mode photo transistor T1 receives light from the green LED (D1) and T1 conducts to disable the gated low-frequency astable built around IC1(4093).

When this light path is interrupted by any object in the path, T1 stops conducting and IC1 is switched by the low level at its input point (pins 12&13). As a result the piezo-buzzer starts beeping at a slow rate determined by the in circuit values of R3, R4 & C1. Red LED (D2) is a visual warning indicator.

With the help of suitable reflector and lens assembly, length of the light path can be increased. A minimum of 1 to 2 metres is possible with a high-bright green LED as D1. A laser pointer can also be used as the light source.

Light Fence Alarm Circuit Schematic

light fence circuit schematic


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  • dhyanendra

    thanks to post this nice trick ..i need this for my agriculture form to save crops frm animals .so i think to cover a big pathway and to connect with soler power source it may need some change .. i want to purchase this as readymade and if success this is revolutionary trick for formers .. so plz help .thank you

  • sures dawsr

    I want to make it as a project. but I don’t know how to make it. so please tell me details on this project

  • vishwajeet

    i want to make it as a project. please me anyone send me the details how to do it ??????

  • Charanpreet

    i want to make a minor project on this,so can i get full information on this.

  • T.K.Hareendran

    It is a multi-purpose circuit.First of all assemble the circuit and test its performance.Then mount D1 on one side of a doorframe and T1 on the opposite side,so that T1 receive enough light from D1.When anybody crosses this light path circuit is activated.Now a Door/gate security alarm is at work!As mentioned in text,with suitable lens&reflector performance can be increased.

  • pradeep

    how can i build a LIGHT FENCE SECURITY BEEPER. it sounds ineresting and i want to make it as a project. please me anyone send me the details how to do it ??????

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