which are two transistors, one NE555 IC, a buzzer and few other discrete components, for sensing the temperature a 10K NTC thermistor is used, the temperature level on which you want to activate the buzzer can be selected with the 1M variable resistor.  The circuit will activate an alarm and flashes an LED when the temperature around the thermistor rises above the preset level.

The beeping and flashing rate of the buzzer and LED can be adjusted with the 100K variable resistor connected between the pin6 & 7 of the 555 IC through the 1K resistor. The circuit can be operated from 5V to 12V DC.

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This article describes a very useful project of a high / over temperature sensor or detector alarm circuit, the circuit will sense the temperature and activate an alarm and flashes an LED when the temperature rises above the normal level.
The circuit shown here is using very few components
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High / Over Temperature Sensor Alarm Circuit Diagram