Here are 2 heat sensor circuits, first one is build with the 4060 IC, the second one is simpler than the one before and has fewer components. It is an ideal add on circuit to monitor heat in CD players, Amplifiers etc. It senses heat from electronic devices and gives a warning alarm.
The heat sensing element is the NTC Thermister and its resistance drops when the temperature increases. Resistance of Thermister is set by VR and if the temperature is below the preset level, transistor T1 will be out of conduction and keeps the buzzer and LED off. When the temperature near the Thermister increases, T1 conducts and buzzer beeps.