How Micro Cooling Fan in the tachometer signal to the control?

"Micro Cooling Fan" application has been very common in life. Then we know how to control signal DC cooling fan do? Xiao Bian tell you, DC cooling fan speed signal is generally output from the three-wire plug, three leads, yellow and black, respectively + 12V power and ground, another color of the line is the line speed signal output. It should be noted that some three-lead "Micro Cooling Fan" of a third lead is not speed signal output line, but line speed control signal input speed control signal by which the DC fan motor. "Micro Cooling Fan" circuit outputs an alarm signal from the "high" and "low" states, meaning two levels represented by the general system in accordance with positive logic, high indicates "failure", " low "for" normal.