September 26th, 2006


Whenever I make a selection on a vending machine, the USE CORRECT CHANGE sign lights up for a moment. I have a theory about why this happens:
  1. The USE CORRECT CHANGE light is wired to light when there is no money in the machine at all
  2. After you make your selection, the machine disables the count of money in the machine and the light goes on
  3. If you hit the refund lever while the light is on, nothing will happen because the machine thinks there is no money available to refund
  4. Your snack or drink comes out and the money count is enabled again

This is to prevent the machine issuing both a product and a refund; it's a race-condition avoidance mechanism.

However, this is just a theory, and surprisingly I can't find any information about it online. Anyone have any competing theories or authoritative info?