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I am working on an electronic field mill with a computer fan, and am following the design on this instructable.

The arduino needs to know what rate the fan is spinning to know when the shutters are open and closed to measure e-field voltage. We are using the tach output on the motor to do so.

When powering up the computer fan, I first tried to probe the device by placing the ground of the oscilloscope to the negative lead on the fan and the voltage probe to the tach output of the fan, but I only saw some spikes, no readable square wave. I only got this square wave when I placed the ground wire of the scope on the VCC wire of the motor and left the voltage probe on the tach output.

Why is this output referenced to the input voltage, and will it still work with the arduino? If this is something not unique to only the fan I have, what is the reason for building the fan this way?

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  • \$\begingroup\$ I think you messed up with the scipe trigger. Maybe the output is open collector and it doesn't go down to zero. So if you had your trigger on zero or below, this is exactly what you would see. Always is a wave is visible referenced to one voltage, it will be visible referenced to another voltage (if they all are referenced to the same ground). So check your setup and post some screenshots please. \$\endgroup\$ – Gregory Kornblum Jan 8 '16 at 6:00
  • \$\begingroup\$ Ok, I'll check it tomorrow at school. It is not my scope so I probably had a trigger issue. I'll try it with mine too. \$\endgroup\$ – skyler Jan 8 '16 at 6:04
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The tach output of a fan is open collector which means that it either pulls to ground or allow the line to float. The proper way to read it is to connect a pull-up resistor to high (maximum would be the voltage fed to the fan on the red lead), 4.7K or 10K ohms would be good. This way, when the output is floating, the resistor will pull it high.

If you don't have some kind of pull-up, the line will never get much above zero volts.

I only took a quick look at the instructable, but I'm assuming that they are programming the processor pin to be an input with weak pull-up enabled.

Here's a reference on the fan I/Os: http://www.formfactors.org/developer/specs/REV1_2_Public.pdf

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