The schematic shown here is a project of a simple temperature sensor circuit or we can also say it a heat sensor circuit, which will activate an LED when receive heat. The circuit is easy to make and using only few components. The two BC547B transistors are connected as a darlington pair to increase

the sensitivity of the circuit. Other components of the circuit are an LED, a current limiting resistor for LED, a 20K variable resistor and a thermistor. A thermistor is a device that limits the passing of current through it according to the temperature. In the condition of low temperature they have higher resistance and in the opposite condition when they receive heat their resistance starts decreasing rapidly and current starts to flow.
Working of the circuit is simple when the thermistor will receive heat its resistance will decrease due to which the transistors will become switched on and the voltage will starts passing through the transistors which will activate the LED. The 20K resistor is used to adjust the circuit to activate the LED on the required heat or temperature.   

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Temperature Sensor Circuit