This is the tough one, and you need to have at least one of the game's Easter eggs to get it.
Once you have an egg, go to the tool room. Did you notice something in here that you could look closely at but which had no other use?
Go toward the table where you assembled the battery, but this time look at the right side of the table.
There you'll find a small scale. Put the egg on the scale --
-- and the scale swings up to reveal the charm at the base.