forward voltage change when the temperature change due to which you can make a good and working temperature sensor circuit with a low cost general purpose diode and without using a NTC thermistor or any other commercially available temperature or thermal sensors.
In the place of relay you can also connect other load like you can make it a temperature alarm circuit by simply connecting a buzzer in the place of relay or you can also connect some LEDs to get visual indication. Also do not use the diode D2 if you are not using relay with the circuit.
The circuit is using two 2N4401 transistors, which are working as switches. When the diode D1 which is used as a heat sensor in the circuit will receive heat then the transistor Q1 become switched ON and provides 0.7 volt at the base of the transistor Q2 due to which transistor Q2 will also switch ON and activates the relay switch. The 10K variable resistor is used to adjust the circuit to activate the relay on the desired temperature.
The figure below shows a project / schematic of a very sensitive, relaible and low cost temperature sensor using a 1N4007 diode. You might be thinking that how can a general diode perform a temperature sensing task? But it is true. A simple diode can sense temperature because their