A couple of issues I have: I'm having a hard time figuring out how to calculate the buoyancy, though that may have to come later once I decide on all the components.
I don't want to use any lighter-than-air materials like Helium, just regular air heated up. I've been trying to find some small ceramic heaters, but I haven't had much luck. Ideally, something like this seems like it would work perfectly, but I need to find some that I can buy in quantities less than 20,000: http://cdsk.manufacturer.globalsources.com/si/6008801115398/pdtl/Heating-element/1070197186/Surface-Heating-Elements.htm Whatever I choose would also need to heat up and cool down pretty quickly so the rising/falling would have a stronger effect. I might be asking too much here.
I'd need the whole circuit to be pretty small and lightweight. If I use an Arduino Uno, could I just transplant the ATmega328 into the balloon? If I used a small watch battery, would that even provide enough juice to operate an LED and a heating element? Anything bigger seems like it would be too heavy.
As for the casing, I plan on whipping something up in Blender and printing it out in lightweight plastic. It would have to hold the circuitry and be airtight to keep the balloon sealed.
Any thoughts or ideas would be greatly appreciated!