I am assuming that you cannot use an actual piece of visco
fuse, so i will go in the non-pyro direction. I would say its gonna be pretty custom but what you really need is a
foot long piece of 1/2 fiber optic bunch. A simple flickering red and/or amber
led inside the "bomb" will make the
fuse look likes its burning. The actor can turn the
unit off when they "blow" it out.