Fan Speed Measurement

I have an application that requires the fan speed measurement.  We can use some photo detector and a light source between the fan blades as an encoder.

The CdS photoresistor cell does not have the speed.  A solar cell salvaged from a broken timer does respond adequately.

The solar cell is enclosed in a box with a small hole exposing the solar cell.  A LED flashlight is pointed at the solar cell through the hole.  The fan blades interrupts the light periodically.  The voltage generated by the solar cell is digitized and the variations are detected and measured to determine the rate.  The fan has 10 blades, at 1000RPM, the pulse rate is about 170Hz.  The scope captures the signal,

If we use a laser point, which has a small beam spot, we get,

Note the rising edge is faster, but the falling edge still has the long tail.