Homebuilt Wind Turbine

I think that you will have a hard time finding a motor that will produce meaningful voltage and current at such a low RPM. A 24-inch blade turning at 8 rpm would produce power in the milliwatt range, so you would need to be thinking in terms of a hobby motor, the vibrator motor from a cell phone, etc. These, however, operate at far higher rpm (often a few thousand rpm/volt) so produce millivolts/rpm, when used as a generator.

I think you may need to wind you own motor (or use a gearbox to get rpm up to a reasonable value). Many sites describe how to design your own wind turbine generator.

8 rpm with a 24 inch blade (of perhaps 12 inch pitch) suggests a wind speed of about 8 x 12 or 96 inches per minute. This is 60 x 96 or 5760 inches per hour: . 09 mph. At this speed, a gearbox would absorb virtually all the power. For meaningful power, you would need to design for a wind speed about 100 times higher.

Is this for instrumentation rather than producing usable power? If so, then some form of counting mechanism might be appropriate (via photocell, magnetic pulse, etc.).