The high temperature alarm will beep and LED blinks when the temperature of the device increases abnormally. This simple over heat alarm is to monitor heat generating devices such as motor, inverter etc.
The circuit is a simple astable multivibrator using the low power CMOS timer IC 7555 which is the low power version of the popular 555 IC. The reset pin 4 of IC1 is used to activate the alarm. The astable will work only if the reset pin 4 becomes high. The reset pin is connected to the positive rail through the 10 K NTC thermistor. The NTC (Negative Temperature Coefficient) thermistor offers high resistance in cold and its resistance becomes low to few ohms when the temperature in its vicinity increases.
So when the temperature is low (as adjusted by the position of the thermistor near the device) reset pin of IC1 remains low and astable is in off position and buzzer remains silent. When the temperature near the thermistor increases, its resistance decreases and provides voltage to the reset pin of IC1 and the astable starts working.
Note: Fix the thermistor near the device by adjusting the distance between the thermistor and the heat generating area of the device so as to keep the alarm silent when the heat is normal.