Designing an electromagnet? (#of turns, diameter, etc)

[SuperJeep]

Hi, I've started a new project to learn about using feedback in microcontrollers and I've already hit a wall haha.

I'm trying to make an electromagnetic levitator to float an object like a coin, using PWM through an electromagnet and using a hall effect sensor for feedback on how hard to drive the solenoid.
(kind of like those floating globes one can buy)

My problem is i first need to know how to design an electromagnet. It needs to pick up something relatively small, like a coin, and be able to hold it in the air (roughly 3-4cm away).

I have different awg enameled wire which i plan to use but i dont know how to pick my design constraints:
-wire gauge
-number of turns
-length of the solenoid
-diameter??

Thanks for any help! I'm asking here because I've had nothing but great assistance from eevblog in the past.

[IanB]

It's going to be somewhat like designing transformers and coils, in as much as you will need to know the magnetic properties of the core and its area and relate that to the current in the windings. It will be complicated by the fact that an electromagnet is not a closed magnetic circuit but is usually open ended.

You might start at this very nice tutorial to get an idea of where to begin, but I doubt it will be the end of your journey:

http://ludens.cl/Electron/Magnet.html

[SuperJeep]

Thanks for the link it proved very useful! I simply took advantage of the linear relationship between number of turns and flux density and just wrapped 250 turns, using an Allen key  as the core. Works decently at picking up a coin at a 50% duty cycle with a 5v source and that gives me tons of room for tweaking,

thanks again.