Follow the instructable for LED Throwies
, but just leave the magnet out of the equation. We also decided to make an internet guestbook, print out the URL and tape that to the LED units that were going inside the balloons.
There is also some maths you can do to make sure your balloons are going to fly:
The volume of a sphere:
V = 4/3(pi)r3
And we used 12" latex balloons, so r = 6
inches, which gives ~904 cubic inches or ~15 liters of helium per balloon.
You can approximate for every liter of helium used, you can get about 1 gram of lift, the battery and LED were about 4 grams, the balloon itself was around 3 grams and we can give another gram for the tape and URL paper. The payload for the helium was a total of ~8 grams, which is a ready for launch Houston!