There are different elements to electronic temperature measuring such as LM35, NTC, PTC, 18B20 and even diode 1N4148, some of them have very high precision in temp. measuring and some other have low precision and used only for detect if the temperature is cold or hot.
Following circuit is simple way to using NTC, when the temperature raise over a certain temp.(more than 41C when the potentiometer set in mid level ) NTC's electric resistance decreased and run the transistor for running the buzzer or any other load. Potentiometer using for set the temperature level that buzzer active. you can use any other NPN transistor for this circuit.
The circuit works by 5v and consume lower than 10mA when the buzzer be turn ON.
Parts list:
Transistor S9013
NTC 10K
Buzzer
Potentiometer 1K
Resistor 470 ohm
