Hooked the spinning water speed sensor up to 12v and oscilloscope with a 5k resistor between +ve and signal wire, captured this signal while wheel spinning.

Sensor is a hall effect sensor, my spinner had 3 magnets evenly spaced between the 6 spinner blades, there are several different sized spinners depending on what speed range you want to measure best.

http://www.furneauxriddall.com/acatalog/Sumlog_SL___HS_Spares.html

Change in speed effects frequency, period stays pretty much the same.

Sensor wiring is, green signal, white +12v, brown ground.