23-06-2016, 11:07 AM
Many fire alarm circuits are presented here, but this time a new circuit using a thermistor and a timer to do the trick. The circuit is also simple and straightforward so that it can be easily implemented. The thermistor offers low resistance at high temperature and high strength at low temperatures. This phenomenon is used to detect the fire here.
IC1 (NE555) is configured as an oscillator free audio frequency. The transistors T1 and T2 drive of IC1. The output (PIN 3) of IC1 is couples to base of transistor Q3 (SL100), that causes the speaker to generate an alarm sound. The NE555 frequency depends on values of resistors R5 and R6 and C2 capability. When thermistor gets hot, it gives a ride to low resistance to the positive voltage at the base of the transistor T1 through diode D1 and the resistor R2. capacitor C1 is responsible up to the positive supply voltage and increases the time during which the alarm is activated. Plus the value of C1, more the positive bias applied to the base of the transistor T1 (BC548). As the T1 collector is coupled to the base of transistor Q2, the transistor Q2 provides a positive voltage to pin 4 (reset) of IC1 (NE555). Resistance R4 is chosen s0 NE555 which maintains inactive in the absence of the positive voltage. Led D1 will stop the C1 capacitor discharge when the thermistor is connected to the positive supply voltage cools to and provides a high resistance path. It also inhibits the live polarization of the transistor T1.
