Here is a simple circuit which can be used as a heat sensor. In the following circuit diagram thermistor and 10K Ohms resistance is connected in series and makes a potential divider circuit .
If thermistor is of N.T.C (Negative temperature Coefficient ) type then after heating the thermistor its resistance decreases so more current flows through the thermistor and 10K Ohms resistance and we get more voltage at junction of thermistor and resistance. Suppose after heating 10K ohms thermistor its resistance value become 9K ohms.then according to potential divider circuit the voltage across one resistor equals the ratio of that resistor's value and the sum of resistances times the voltage across the series combination. This concept is so pervasive it has a name: voltage divider. The input-output relationship for this system, found in this particular case by voltage divider, takes the form of a ratio of the output voltage to the input voltage.
This output voltage is applied to a NPN transistor through a resistance. Emitter voltage is maintain at 4.7 volt with a help of Zener diode.This voltage we will use as compare voltage. Transistor conducts when base voltage is greater than at leat .3V then emitter voltage. Transistor conducts as it gets more than 5 base Voltage and circuit is completed through buzzer and it gives Sound.
Heat Sensor Circuit
Parts to buy:
b) Transistor BC548 or BC547
c) 4.7V Zener Diode
d) 10K Thermistor (NTC)
e) 10K and 3.3K Resistance
f) 9V Battery
* please use 4.7K Ohms thermistor and 4.7 K Ohms resistance in case 10K ohms thermistor not available.
You may also be interested in Project using Precision Temperature Sensor .