This fire alarm circuit created based on thermistor and a timer circuit 555. The IC1 (NE555 Timer IC) is configured as an oscillator operating at an audio frequency. The transistors T1 and T2 IC1 unit. The output (pin 3) of IC1 is at the basis of the transistor pairs T3 (SL100), which drives the speaker to generate the alarm sound. NE555 frequency depends on the values of resistors R5 and R6 and C2 capability.
When thermistor is heated, it gives a low resistance path to the positive voltage to the base of transistor T1 through diode D1 and resistor R2. Capacitor C1 charges to the positive supply voltage and increases the time for which the alarm is activated. The higher the value of C1, the higher the positive bias applied to the base of the transistor T1 (BC548). As the collector of T1 is coupled to the base of transistor T2, the transistor T2 provides a positive voltage to pin 4 (reset) of IC1 (NE555).
Resistance R4 is selected remains inactive NE555 s0 that in the absence of positive voltage. Diode D1 stops discharging the capacitor C1 when the thermistor is related to the positive supply voltage is cooled out and provides a path of high resistance. It also inhibits the bias of the transistor T1.
When thermistor is heated, it gives a low resistance path to the positive voltage to the base of transistor T1 through diode D1 and resistor R2. Capacitor C1 charges to the positive supply voltage and increases the time for which the alarm is activated. The higher the value of C1, the higher the positive bias applied to the base of the transistor T1 (BC548). As the collector of T1 is coupled to the base of transistor T2, the transistor T2 provides a positive voltage to pin 4 (reset) of IC1 (NE555).
Resistance R4 is selected remains inactive NE555 s0 that in the absence of positive voltage. Diode D1 stops discharging the capacitor C1 when the thermistor is related to the positive supply voltage is cooled out and provides a path of high resistance. It also inhibits the bias of the transistor T1.
