a Fire Alarm Circuit

When a fire broke out at room temperature increases. It is very compact and low cost fire alarm senses fire based on this fact.
BC177 transistor (Q1) is used as a sensor fire here. As the temperature increases transistor leakage current is also increased. This circuit is designed so that when there is increased leakage currents Q1, transistor Q2 will be biased. As a result there is a fire escape when the transistor Q2 will be in. The emitter of Q2 (BC 108) is connected to the base Q3 (AC 128). Thus, when Q2 is ON Q3 will also be ON. Q3 relay drive transistors used to drive the load ie, lights, bells, horns, etc. as an indication of the fire. Diode D1 is used as a diode to encourage free to protect her from the back EMF generated when the relay is activated.

Circuit diagram with Parts list.

Note.

* The preset R1 can be used for the desired temperature level to set the alarm ON.
* This is not an alarm attached, ie: when the temperature around the sensor decreases below the set point the alarm stops.
* The circuit can be supported by using 9V battery or 9V battery eliminator.
* All electrolytic capacitors and must be given a minimum value of 10V.
* Load can be connected through C, NC, NO points of the relay according to your needs.
* The calibration can be done by using soldering iron, and thermo-meter. Switch ON the power soldering iron tip supply.Keep near Q1. The same time keep the thermometer close to it. When the temperature reaches the desired value to adjust R1 so that you will relay ON