Here’s another cool improvised switch hack from Forrest Mims’ 1996 Sensor Projects mini-notebook for RadioShack (now available as a compilation with two other classic Mims books in Electronic Sensor Circuits & Projects).
All it requires is a spare tip/shield male connector with a removable housing, and a BB or other conductive metal ball. The version I built and photographed below uses RadioShack’s #274-286 1/8” mono phone plug (2-Pack) and a 7/32″ chrome steel ball (such as Amazon Supply’s BS-0219-C). The dimensions of the ball are not too critical.
To put it together, all you have to do is remove the housing, set the ball in place between the contacts, as shown, and then put the housing back on. I built two of these, for testing purposes; the first worked like a champ, and second one was a bit wonky until I polished the plug contacts with a Q-tip moistened with rubbing alcohol and then touched to a pile of table salt.