Fire Detection System raises an alarm when temperature increases due to fire. Fire alarm circuit is used to sense the fire as well as temperature. Here reverse bias Germanium (GE) diode is used as a heat sensor. At temperature about 60^oC and above, reverse resistance of GE diode drops and makes pin4 (reset) of IC 555 at logic high level. Hence IC generates output pulse and raises the alarm. It has household as well as industrial applications.
Circuit operation :

• At room temperature reverse resistance of diode D1 (the sensor) being very high (over 10KW) it produces no effects on transistor T1 that conducts and keeps the reset pin4 of IC555 at ground level, so the alarm does not sound.
• When temperature in the vicinity of diode D1 increases in case of a fire the reverse resistance drops to a value below 1KW. This stops T1 conduction and reset pin 4 becomes positive through resistor R2. Hence it gives output pulses, which sounds the alarm. Alarm can be heard from 8W speaker.
• For installation of alarm two or three reverse biased Ge diodes connected in parallel can be kept at different locations. In case of a fire, any of the diodes can sense the heat and raise the alarm. Here diode D2 is used as a rectifier diode. Capacitor C4 is used as a filter capacitor, C3 as a coupling capacitor.
• Calibration : VR1 is a preset used for temperature sensitivity.

Fire Detection System raises an alarm when temperature increases due to fire. Fire alarm circuit is used to sense the fire as well as temperature. Here reverse bias Germanium (GE) diode is used as a heat sensor. At temperature about 60^oC and above, reverse resistance of GE diode drops and makes pin4 (reset) of IC 555 at logic high level. Hence IC generates output pulse and raises the alarm. It has household as well as industrial applications.
Circuit operation :

• At room temperature reverse resistance of diode D1 (the sensor) being very high (over 10KW) it produces no effects on transistor T1 that conducts and keeps the reset pin4 of IC555 at ground level, so the alarm does not sound.
• When temperature in the vicinity of diode D1 increases in case of a fire the reverse resistance drops to a value below 1KW. This stops T1 conduction and reset pin 4 becomes positive through resistor R2. Hence it gives output pulses, which sounds the alarm. Alarm can be heard from 8W speaker.
• For installation of alarm two or three reverse biased Ge diodes connected in parallel can be kept at different locations. In case of a fire, any of the diodes can sense the heat and raise the alarm. Here diode D2 is used as a rectifier diode. Capacitor C4 is used as a filter capacitor, C3 as a coupling capacitor.
• Calibration : VR1 is a preset used for temperature sensitivity.