How to Make Your Own EMP Jammer
The YouTube channel FPS Weapons comes up with plenty of good material for protecting yourself. Previous videos have included tactical knives, bushcraft knives, and Iron Man gloves (just in case).
This video shows you how to craft your own electromagnetic pulse jammer. EMP jammers emit short bursts of electromagnetic energy to disrupt surrounding technology. Basically, this is the tool to build if you want to mess with your friends' cell phones. He even includes links below the video to buy all the parts for the jammer.
EMP Jammers can run up to $179 USD, so FPS provides a slightly cheaper alternative for those willing to put in a little effort.