Here is a simple fire alarm circuit built around 555 timer ic, A reverse biased germanium diode i used here as a heat sensor. At room temperature the reverse resistance of the diode-being very high (over 10 Kilohm), it produces no effect on transistor T1 which conducts and keeps the reset pin (4) of IC1 at "ground"level, and so he alarm does not sound When temperature in the vicinity of diode D1 (the sensor)increases in case of a fire the reverse resistance of D1 drops.At about 70 c its resistance drops to a value below 1 -kiliohms. This stops conduction and the IC's reset pin (4)becomes positive through resistor R2,which sounds tha alarm.