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Over-temperature Alert

A universal alarm circuit

Over-temperature Alert
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.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor Members can exclusively download these files for free as part of their Membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Resistors:
R1 = NTC 10k? *
R2 = 10k? *
R3 = 2k?2
Capacitors:
C1 = 100nF
Semiconductors:
D1 = LED
IC1 = TLC555
Miscellaneous:
S1 = pushbutton, 1 make contact
K1 = 2 solder pins
* see text
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