How to make a remote switch?

I want to make a remote switch out of a cell phone but i need to build some sort of circuit to react to the current from the phones ringer to turn on a switch, does anyone know how i would go about making such a circuit? I am also wondering how to make a timed switch circuit.
Update: I didn't say I was making a bomb detonator. A remote switch can be used for many things other than explosives.
1 answer 1