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Build Yourself This Simple Sound Activated Alarm

written by: Swagatam • edited by: Lamar Stonecypher • updated: 4/6/2010

Using just three transistors and few other passive components, a simple sound activated alarm circuit can be built. It may be installed over critical areas or objects to raise a warning signal if somebody tries to break-in.

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    Introduction

    In the present modern world, alarm systems have become an indispensible part of our life. Right from your mobile phone to the various security systems, they can be seen everywhere. Even the horn of your motor car is a kind of alarm used to warn people and vehicles in front of its presence. The sole purpose of an alarm is to warn an individual of an incoming danger. They may be in the form of a visual indicator, for example a flashing light, but an alarm in the form of an audible tone or siren is more widely accepted as it is able to wake up even the unaware and the oblivious. In this article we will learn how to build a simple sound activated alarm.

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    About the Present Circuit

    The circuit described here may be used to safe guard specific areas like a cupboard or a safe from intruders. It can be also fitted on doors to detect a possible intrusion.

    The circuit basically consists of a sound sensor and an amplifier which can detect any vibration made over the thing on which it has been fitted. For example if the sensor is attached to a door, it will be able to detect any noise made over its surface, may it be in the form of a knock or a manipulation made to pick lock.

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    Circuit Description

    The following circuit description will help you in building this DIY sound activated switch project and to understand its functioning:

    Sound Activated Alarm, Circuit Diagram, Image 

    • Two NPN transistors T1 and T2 and a PNP transistor T3 are configured as a highly sensitive voltage amplifier.
    • This configuration is still used in the preamplifier stage of cassette players to sense and amplify the tiny magnetic pick-up signals from the tape head. It performs the same function here and is used to amplify the small electrical signals from a piezoelectric buzzer B1.
    • A piezoelectric buzzer normally converts an electrical oscillating voltage into a sound output. But in this circuit it is wired to function in just the opposite way. Here it is used to sense the external sound vibrations and convert them into electrical pulses.
    • After getting suitably amplified through the transistor stage, these pulses are filtered and smoothed by capacitor C1 and ultimately reaches a latching circuit stage comprised of transistors T4 and T5.
    • On receiving the sensed input pulse, transistor T4 receives the required biasing and immediately conducts.
    • Transistor T5 also instantly switches ON due to the received base voltage through T4. At the same moment a feedback voltage is sent back to the base of T4 by T5 and the circuit latches to trigger and sound the external alarm circuit. The siren type of horns used in cars may be used here.
    • The sound activated alarm can be stopped only by switching OFF the unit.
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    Construction Hints

    The circuit design is very simple and can be built within an hour or so. Just procure the required parts as shown in the schematic, insert and solder all the components on a piece of general PCB in an organized manner as per the circuit schematic.A Typical Door Alarm System 

    Cover the whole circuit along with the power supply inside a good looking plastic enclosure. Remember to stick the piezo on the inner surface of the box.

    Adjust the preset to an optimum level so that the system is not disturbed by external stray noises and only responds to the desired sound levels.

    The project ” DIY build sound activated switch” is now ready and may be installed over a particular place or an asset which is to be secured from a possible theft. You may customize the circuit as per choice and use it for different applications.

    Image credit: http://www.walkabouttravelgear.com/safety.htm