I remember reading in MAKE about a gumball machine that only dispenses gumballs when a secret knock is performed. I'm sure something like that can be adapted fo

I remember reading in MAKE about a gumball machine that only dispenses gumballs when a secret knock is performed. I'm sure something like that can be adapted for this purpose. http://grathio.com/2010/05/secret_knock_detecting_gumball_machine/ http://grathio.com/2010/05/secret_knock_detecting_gumball_ma...