Thank you. This is very helpful. To be safe, I'll probably go with batteries, or buy some separate 9 V wall warts for the Arduinos. I expect we will be using the Arduinos for other projects, so it might be good to have separate power supplies for them.
Next design question: Displaying data.
In my prototype I have the arduino connected to my computer, so I just send the rotation speed to the serial monitor where a student could read it. But the computers in my classroom are old. They are 10 year old iMacs running OS 9.2, which is not really supported by the arduino software. Here are the options I'm considering:
1) Finding a way to make these old iMacs work with arduino. I found one link about how to do this online, but it isn't really enough info for me, and I'm not sure it is an easy fix.
2) I have access to a set of old PC laptops, but these are running Windows 2000, which also does not appear to support Arduino without a fair amount of fiddling:http://arduino.cc/playground/Main/UnoWin2k
3) Buying an LCD display for each setup would probably work, but I'd rather find a cheaper alternative, since I have to build 8 to 10 of these.
4) A four-digit 7 segment display would be sufficient. Reading about these, I think I would also need a shift register chip. (Might I need four of these, one for each digit?) Or I could use a LED driver chip, but these seem to be more expensive. And if I go with LED displays, I think I would need extra transistors to provide the necessary current for all the segments in the four digits. So the cost here starts to add up, since I will be building 10 of these setups.
5) Another very elegant solution, in lieu of connecting directly to computers, would be to connect to students' TI calculators. The calculators have a link port built into them, and I have extra link cables in my lab. I found someone on youtube who is able to do this (http://www.youtube.com/watch?v=cMNOGHKQDQk&NR=1
), but I'm going to need to learn some z80 assembly language programming to interface the TI calculator to the arduino. In some ways this would be a great solution, though, because teachers at other schools would be able to duplicate it (since nearly all high school students have TI calculators, but many physics classrooms do not have computers) and students could also use the keypad on the calculator to send data to the arduino.
Any suggestions would be appreciated on how to make alternatives 1, 2, or 5 easier, or on exactly what hardware would be required to implement alternative 4.