You can make a simple Heat Monitor using only a few components. It can be attached to heat generating devices like Amplifier, Inverter and Computer. The sensor is an NTC Thermistor always monitor the heat in its vicinity and activates the alarm LED if the heat increases above the normal level.
NTC Thermistor has high resistance at room temperature and the resistance decreases when the temperature increases. At normal temperature (as set by VR1), Thermistor will not conducts so the base of the PNP transistor will be high and it remains off. Thus LED remains dark. When the temperature increases, Thermistor conducts and pulls the base of T1 to ground potential and it conducts. LED then lights to indicate a rise in temperature.
Fix the Thermistor near the heat generating area like heat sink. Power to the circuit can be obtained from the device itself. Adjust VR1 till LED goes off at normal temperature.