how about a magnet at the midpoint of a stiff (carbon fibre etc)rod fitted inside the balloon. (top to bottom)
with a reed switch on the outside attached to the lower end of another length of rod..
with the top end attached to the top of the internal rod (or the plug/bung or whatever is there
positioned so that upon bursting the 2 rods will swing together allowing magnet and reed switch making the contact.
im sure you could improve on the basic idea with some thought.
Mark.
Biting the hand that feeds IT
