This Slot Machine built with Lego is fully mechanical. It is powered by one motor.
How it works:
If you put in money the main-lever will be unlocked. When pulling this lever, an electric switch is activated. Now one cycle will run through:
1. The rolls will be accelerated.
2. The rolls will be decoupled from the drive train.
3. One roll after the other roll will be braked to halt.
4. Every roll is checked for an empty position.
An empty position represents a dollar sign.
If there are three empty positions, the money is given out.
5. The end of the cycle is reached and the switch will be turned off.
Probality and Win:
Every roll has six positions which it can halt. Three positions are dollar signs. So probality of a dollar sign is 1/2. Because you will win, if three dollar signs appear the probality to win the game is only:
1/2 * 1/2 * 1/2 = 1/8
So only every 8th game is win! If you win, you get 40 Cent but evere game costs 10 Cent.
Therefore the averegae win is:
1/8 * 0.4€ - 0.1€ = -0.05€
This means the automat is unfair and you lose half of your money you put in.