Is this the best way to make a timer auto-off feature for a CPU fan?
With a few modifications this can be made to work. This will create a delay in the 15 minute range. I've added R4+R5 to provide &quot;snap action&quot; hysteresis so that Q2 doesn't overheat as it slowly turns off. R4 should be at least a 1W-2W resistor because it sees a fairly large pulse when the switch is pressed. As well as providing snap action R4 prevents C1 from welding the switch contacts when it is pressed.
R3 prevents leakage in Q1 from being multiplied by the gain of Q2.
If the power is interrupted during a timing cycle the B-E junction of Q1 can break down but that should be infrequent and low enough current not to cause a problem.



simulate this circuit &ndash; Schematic created using CircuitLab
Frankly, though &quot;best&quot; .. not really. I'd use a 78L05, a PIC microcontroller such as PIC12F509 and a logic-level MOSFET. And maybe a flashing LED that increases in flash rate as it counts down for giggles.