This is the simple

*circuit. It can be used to control any device using heat sensor. In this circuit a***heat sensor***and a resistance is connected in series. This arrangement makes a potential divider***thermistor****circuit.**
Here the thermistor is

*type. So when the room temperature is increased its resistance decreases simultaneously and more current flows through the resistor and the thermistor. We find more voltage at the junction of the resistor and the***Negative Temperature Coefficient***.***thermistor**
Our thermistor resistance value is 110 ohms. Suppose the resistance value becomes 90 ohms after heating the 110 ohms thermistor. Then the voltage across one resistor of the

**voltage***circuit equals the ratio of that resistor’s value and the sum of resistances of the voltage across the series combination. This is the concept of voltage divider.***divider**
The final output voltage of the voltage divider circuit is now applied to the npn transistor

*through the base resistor (3.3K ohms). Here the emitter resistor is replaced with a***(BC548)***. Emitter voltage is maintained at 4.7volt with the help of zener diode. This voltage is used to compare voltage. Transistor conducts when base voltage is greater than the emitter voltage. Transistor conducts if it gets more than 4.7volt of base voltage. Then the circuit is completed through***zener****diode***and it gives sound.***buzzer**