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Door Window Alert

Opened Window or Door Alarm

Economical circuit of an electronic alarm, notifies you when your door or window is open, is presented here. This circuit wired around the Quad 2-input NAND Schmitt trigger IC 4093 (IC1) can be powered from a standard 9V battery. Here, a normally-closed (N/C) push switch (S1) is used to monitor the door/window activity. For satisfactory operation,it is crucial to fix S1 in the door/window frame such that when the door/window is closed, S1 remains in open state.

Schematic of the Door Alarm

Door Window Alert

When the door/window is opened, capacitor C1 charges through R1 (and S1) until it reaches the point where output of IC1A switches from high to low. After this slight delay, output of IC1B then goes high to enable the gated astable realized using IC1C. Components C2 and R3 sets the astablefrequency. Note that here IC1D is used as inverting buffer/amplifier that gives single-ended drive to the 30mm piezo-ceramic transducer (electric-to-acoustic converter) BZ1. The monotone when IC1C is in “on” mode is, gated by a high input at pin 12 of IC1D. The signal reaching the piezo-ceramic element BZ1 is a square wave with peak-to peak amplitude equal to the supply voltage.

Ambient light sensor mechanism, built around the 150K preset pot (VR1) and the 5mm LDR, ensures that the alarm sounds only when ambient light is very low (ie night time). VR1 can be used to fine-tune the detection senstivity of this light sensor. In case of no interest you can remove this option by replacing the LDR with a 100K Resistor,and connecting pin 12 of IC1 to the positive rail via a 10K resistor.

This simple circuit works perfect with multiple switches. Just add desired number of N/C push switches in parallel with S1 (through low-voltage cabling) to cover all the entry and exit points!

5 Comments

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  • Brian

    Hello Mr Hareendran, I am looking for someone with your experience to help me undestand what the cost is for a 2 piece battery powered, wireless door or window sensor would cost to manufacture. This sensor would communicate with a base station that would then be able to send text messages through a GSM module in the base station.
    Are you in a position to offer your services?

  • Jordy Vlieghe

    Heey

    I’m using this circuit for a project at school. But I need a components list could you load it up, or tell me where I can find it ?

    Thanks
    Jordy

  • T.K.Hareendran

    Mr.David

    Thanks for the feedback. Will try to publish a modified version of this design with a ‘timed-alarm’ function as soon as possible!

    • John H

      Mr. Hareendran,

      We are building a new church building that has several entry/exit doors. When we leave at the end of the day we want to look at a “panel” that signifies that a door may be open or unlocked so as to avoid a visit to each location. Do you know of anyone that has just a simple set up that would give such simple information – preferably wireless? If not, can it be build by simple humans like me?

      Thanks – John

  • David

    Hello Mr Hareendran,

    Thank you for this circuit the LDR is a nice touch. May I ask if there is a modification that can be made so that it will stop alarm after a given time? Possibly 1 minute.

    Thank you
    David.

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