Simple Fire Alarm

Here is a simple schema which can be used as a Fire Alarm. 3 Volt is enough to operate. There is not much to the schema. The IC UM66 is connected to its supply and its output fed to a transistor for amplification.

Circuit diagram :

Simple Fire Alarm Circuit Diagram

UM66 is a complete miniature tone generator with a ROM of 64 notes, oscillator and a preamplifier. For amplification we have used a NPN transistor which is BC548. Here BC548 makes a common emitter schema. For limiting the base current we have used a resistance of 220 Ohms so that transistor will not get damaged even if IC is wrong connected.

For heat sensor we have used tube light starter in place of manual switch. In a starter there is a metal plate and a pin with small gap. When starter gets heated then metal plate of starter expands and get in contact with the pin and schema is completed and we get audio from speaker. For fast sensing we can use starter without its glass body by carefully breaking glass cover.

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Low cost fire alarm circuit

When there is a fire breakout in the room the temperature increases.This ultra compact and low cost fire alarm senses fire breakout based on this fact.







Transistor BC177 (Q1) is used as the fire sensor here.When the temperature increases the leakage current of this transistor also increases.The schema is designed so that when there is an increase in the leakage current of Q1 ,transistor Q2 will get biased.As a result when there is a fire breakout the transistor Q2 will be on.The emitter of Q2 (BC 108)is connected to the base of Q3(AC 128).So when Q2 is ON Q3 will be also ON.The transistor Q3 drives the relay which is used to drive the load ie,light,bell,horn etc as an indication of the fire.The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switche



Notes.

* The Preset R1 can be used to desired temperature level for setting the alarm ON.
* This is not a latching alarm,ie;when the temperature in the vicinity of the sensor decreases below the set point the alarm stops.
* The schema can be powered using a 9V battery or a 9V battery eliminator.
* All capacitors are electrolytic and must be rated at least 10V.
* The load can be connected through the C,NC,NO points of the relay according to your need.
* The calibration can be done using a soldering iron,and a thermo meter.Switch ON the power supply.Keep the tip of soldering iron near to the Q1.Same time also keep the thermometer close to it.When the temperature reaches your desired value adjust R1 so that relay gets ON.Done!

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