Faking a fan's rpm signal

I want to use this...


To simulate a 120 mm fan rpm signal.  Does anyone have any ideas if this would be possible?
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Dr. KlahnPrincipal Software EngineerCommented:
You would need to attach a bipolar transistor or FET to the output of the pulse generator, as the standard fan tachometer output is open collector or open drain.  The motherboard provides a +12 pullup.

Note that the fan's tach output delivers two pulses per revolution.


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Tom_HickersonAuthor Commented:
Tom_HickersonAuthor Commented:
What would I need to do to hook a led to the rpm output of the fan to see it blink.
Dr. KlahnPrincipal Software EngineerCommented:
Hmmm ...

Get a fan extension jumper (usually under a buck on fleabay).  Splice into the +12, ground and fan tachometer lines.  Connect one end to the fan, the other end to the motherboard.

Build a little circuit with the base of an NPN transistor connected to the tachometer line.  Put an LED and dropping resistor in the collector line, and connect the transistor's emitter to ground.  The transistor should flip from cutoff to saturation.  I'm not sure that the blinks will be visible but this should drive the LED sufficiently to see if if the pulses are long enough.
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