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?