It’s probably the worst nightmare for all PC owners: The fan has been quietly doing its job keeping the PC internals nice and cool but over time the bearing lubricant dries out and friction increases until one fateful morning you turn on the PC and the fan sticks. Now it’s only a question of time before something expensive pops. This neat circuit gives a warning when things start to get too hot.The circuit shown in Figure 1 uses a simple and economical thermistor as a temperature sensor. The sensor should be insulated and positioned on the PC case. When the temperature exceeds 40° C a warning LED lights or a buzzer sounds or a cooling device can be switched on. The alarm will not be reset when the temperature dips below 40°C again so it ‘stores’ any brief over-heating event. Pressing pushbutton S1 will reset the circuit.