Fire Alarm Circuit Diagram


Circuit Diagram
Description
An reverse biased germanium diode is used here as a heat sensor at normal room temperature the reverse resistance of the diode is very high in the order of over 10 kilo ohms so it produce no effects on the transistor Q1 which conducts and keeps the reset pin 4 of ic 555 at its ground level, and so the alarm doesn’t get activated.
When the temperature in the vicinity of the diode (the sensor) increases in case of fire, the reverse resistance of the germanium diode drops at about 70 degree its resistance drops to a value below 1 kilo ohms this stops Q1 conduction and the 555 ic pin 4 becomes positive through the resistor R1 which activates the alarm.

Note:

For installation of the alarm two or three reverse biased germanium diode can be connected in parallel at different locations in case of fire any of the diode can sense the heat and can activate the alarm.
DR25 Germanium diode is recommended for use but still it can be replaced with base emitter junctions of germanium transistors such as AC128, AC188 OR 2N360. 
 
Source - http://www.electronguide.com/circuits_pages/FireAlarm.html

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