Fire Alarm

Working of NE555 timer. Application of germanium diode as fire sensor. Working of trnsistor as amplifier. The fire alarm circuit employs a simple mechanism to detect fire in the vicinity and sound an alarm. The heat sensing is done by a germanium diode (D1) with a high reverse resistance, while the alarm is sounded by the NE555 timer IC (IC1). The bridge between these two prime components is the BC548 NPN transistor (T1). In case of fire, the temperature in the vicinity of germanium diode D1 increases and its reverse resistance decreases. To decide the threshold value of the detectable temperature, the variable resistor (or potentiometer) VR1 is used. Typically at a temperature of about 70 degree Celsius, the reverse resistance drops from about 10 Kohm to 1 Kohm. This change in the reverse resistance of the diode causes the base voltage of transistor T1 to fall below the collector voltage and hence, it stops conducting. The collector terminal of the transistor is connected to the reset pin (pin no.4) of the NE555 IC and is responsible for switching its logic level. When base voltage of T1 falls below collector voltage, the reset pin is held high and oscillations are generated at the output pin (pin no.3) of the NE555, the frequency of which is decided by the value of resistors R1, R2 and capacitor C1. This electrical signal (oscillations) is input to the loudspeaker through the capacitor C3 and is further converted into a loud and continuous alarm sound. Brand: Kits'n'spares, Marketed by: Kits'n'spares