frequency count

i would like to ask the input signal,if the project is used to count the motor frequency,

how can i connect the 3pin or 4pin fan to the 8051.

thanks alot.

KM

Is this a project listed on this site ?

yes, but i need more information to connect the fan. and the project also including feedback(frequency count) to monitor the rpm.

thanks alot.

I am assuming you're talking about the small CPU fan. You cannot measure RPM of 3-Pin fan but you can measure RPM of 4-Pin CPU fan as it has sense signal which gives pulse output to measure RPM. The sense signal goes into the frequency counter. Please do check the voltage of the signal before connecting it to MCU directly.
http://www.allpinouts.org/index.php/Motherboard_%28CPU%29_4_Pin_Fan

Try this link.
Basically find the tacho output pin and connect it to the input of the micro.
Add a pullup resistor to the micro's supply voltage (3 or 5 volts) of about 2.2k

http://pcbheaven.com/wikipages/How_PC_Fans_Work/