I need help writing a code to complete this function, and a diagram of how to set it up.
Create a device, using the Arduino and a photoresistor, that will measure the rate at which a three-holed pulley wheel rotates, and give the result in revolutions per minute (rpm).
Thank you all for your continuous help! 
we do frown on those homework assignments that seem like you post it here, then head to the bar and hope that while you have a few brewski's someone will do it for you.
this is a beginners task.
the photo resistor will open/close as the pulley spins.
give it a go to write some code and spin it a few times.
come back and post your code if you cannot make it work.
if you really cannot make it work.... consider knitting.
sorry if my humor seems boorish. just having fun.
seriously, give it a go and see how far you can get. learning is your experience and your work combined with your observations and drive.
we would not be helping if we just gave you a link to someone elses work.
using the Arduino and a photoresistor
You might need a decent light source too: I'm not sure that ambient light being (semi-)blocked by the pulley or let through by the hole will be enough of a difference. So you may need a light shining through the hole, but of course that will be part of your learning, experimenting with levels.
There's a good tutorial on light dependent resistors at adafruit.
ccampos112:
...last I checked 18 year olds aren't allowed in bars. Thanks for the guidance, asshole.
Thanks for behaving like an adult and not resorting to petty name calling.
Oh, wait. I got that backwards.
If you continue to behave like a child I will give you a timeout.
don't be too hard on him, maybe I am what he said.
after all, I started going to bars when I was 16. because it was easier than getting cases of beer delivered like I had been doing for a couple years before that.
never forget that time I went into the liquor store with my mom and the guy at the counter said " Hi Dave ! ' gosh, high school was much different back then.