When the heat near the NTC Thermistor above the preset level, Red LED lights. Adjust the 100 K preset to set the heat level. Ideal to use in Amplifiers and heat generating devices. When the heat increases above the set level, Thermister conducts and pulls the base of PNP transitor BC557 to ground. Transitor conducts and LED lights. Circuit can be connected to the same power supply of the device itself working in a voltage range of 5 to 12 volts.