Electronic – Read out tachometer from PC fan when manually turning the blades ( driven shaft )

fanfrequencytachometer

I have a problem. I can read out the tachometer of a PC fan and convert it to frequency with just using a simple microcontroller and labview.

Does anybody here have an idea how to read out the tachometer, without powering the fan? I want to measure the RPM with manually spinning the fan. I want to use the tachometer to measure the frequency of a shaft of a low rpm mini-engine. When I disconnect the power that makes the fan rotate, i do not get a reading anymore from the tacho, even when the blades are still spinning.
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Best Answer

Internally the tacho pin is wired as an open collector

enter image description here (source http://www.bearblain.com/fan_speed_control.htm)

With no supply connected to the red wire the base of the transistor can't be turned on so it never pulls down the output (collector). The only way I see is to open the fan and tweak the circuit to power only the tachometer section and not the motor itself.

You can always create an individual circuit to measure the RPMs.
One of the options is with a Reflective Optical Sensor like

enter image description here

Here is a sample project

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