fire alarm

A thermistor provides low resistance at high temperatures and high resistance at low temperatures. This phenomenon is used here to detect fires. IC1 (NE555) is configured as a free-running oscillator operating at audio frequencies. Transistors T1 and T2 drive IC1. The output of IC1 (pin 3) is connected to the base of transistor T3 (SL100), which drives a speaker to produce an alarm sound. The frequency of the NE555 depends on the values ​​of resistors R5 and R6 and capacitor C2. When the thermistor heats up, it provides a low-resistance path to the base of transistor T1 through diode D1 and resistor R2. Capacitor C1 charges to the positive supply voltage, increasing the alarm activation time.

The larger the value of C1, the greater the forward bias applied to the base of transistor T1 (BC548). Since the collector of T1 is connected to the base of transistor T2, transistor T2 provides a positive voltage to pin 4 (reset) of IC1 (NE555). Resistor R4 is chosen so that NE555 remains inactive in the absence of a positive voltage. When the thermistor is connected to the positive supply voltage and cooled, providing a high-resistance path, diode D1 stops the discharge of capacitor C1. It also suppresses the forward bias of transistor T1. This circuit can be powered by a 6V battery or a 6V power supply. The thermistor can be mounted on a heat-resistant material, such as mica, to prevent damage due to overheating. An LED serves as an indicator when the power switch is on.