The Fire Alarm circuit presented here is very simple, a diode reverse-biased germanium is used as a heat sensor low cost. In this project you can use two or three diodes connected in parallel at different locations in case of fire any of the diodes can activate the alarm.
The germanium diode at normal room temperature, its resistance inverse diode is very high, in the order of more than 10K ohms no effect on transistor Q1 conducts and maintains to the reset pin 4 of the IC 555 in its ground level , and so the alarm is not activated.
Fire Alarm Circuit Diagram
When the temperature in the vicinity of diode (sensor) increases, in case of fire (about 70 degrees) reverse resistance of diode germanium falls below a value of 1K ohms, Q1 driving IC 555 and pin 4 becomes positive through the resistor R1, which activates the alarm.
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