This is a circuit sound alarm when an engine start spreading the heat. Although every car will have a temperature gauge. But in some cars. May be no warning if the engine is overloaded with heat. Or the driver may not have noticed the temperature gauge can cause problems.

The circuit in figure 1 When the temperature of the engine increases. The resistance of the thermistor decreases. Thermistor is connected to R1 for divide the voltage of the power supply to pin 2 of an op-amp IC1-LM741. This voltage will decrease as the temperature rises, by at VR1-100K is used to adjust a reference voltage at pin 3 of IC1. When a voltage at pin 2 lower than pin 3, IC1 will be switch close makes has voltage at output pin 6 to send to IC2.

The IC2 is connected as an astable multivibrator to the sound signal generator circuit Temperature level to cause the sound can be adjusted with VR1. The VR2 will determine the frequency of the sound. Thermistor used in this circuit, as beads. Has a negative temperature coefficient.(NTC) Thermistors are devices that are heat resistant and can pass it quickly. Suitable for adoption. Used in this circuit currents up to 10 mA.