The later electromechanical 4 player pinball machines from Bally and Williams used a common circuit to track the Ball in Play and current Player Up using three different step units. Two player machines used a simpler circuit that just used a relay to keep track of whether a 2nd player was playing. The examples below illustrate how the three step units, Ball Count Unit, Player Up Unit and Coin Unit work together to determine which ball or player to change to each time the ball drains.
The basic problem this circuit solves is how to count out the balls to play given the number of players. In a one player game the solution is simple. The Ball Count should advance to the next ball whenever the ball drains into the Outhole. On a 2, 3 or 4 player game the problem is more interesting. When the ball drains in these cases the Ball Count should only advance if the last player just drained. Otherwise the Player Up should advance and the Ball Count should remain unchanged. Once the last player has drained the Ball Count can advance and the Player Up should reset back to player 1.
Here are examples of the Ball Count circuit from 1976 Bally Old Chicago and 1974 Williams Star Pool:
This schematic diagram represents a simplified version of the Ball and Player Count functions in later Bally and Williams 4 player pinball machines that solves the basic problem:
The Score Motor activates switches (whose schematic symbols include a circle around the switch) in the circuit at various times as shown by the timing chart on the left and the waveforms that are drawn as the simulation advances.
The animation below shows what happens when the ball drains on a single player game.
Ball drains on a 1 player game
Note: you can stop the animation above or drag the slider to any time to study what is happening.
In a single player game the Coin Unit is in the lowest position to indicate a one player game, and the Player Unit is in the lowest position to indicate that player 1 is up. Note that in a one player game the Coin Unit is in its lowest position and there is a path to the Ball Count Unit Step Up solenoid that bypasses the Player Unit entirely. This means that any time the Outhole relay fires (and the Score Motor turns) the Ball Count unit will advance.
The simulation above starts when the ball drains and closes the Outhole switch. This in turn fires the Outhole relay which closes a switch that starts the Score Motor turning.
When the Score Motor has turned 30 degrees the Motor 1 switch closes which completes the circuit to the Ball Count Unit Step Up solenoid. This advances the Ball Count unit and also fires the Player Reset relay.
When the Score Motor has turned 60 degrees the Motor 2 switch closes which fires the Player Unit Reset solenoid. This is unnecessary in a single player game since the Player Unit is already in its reset position, but this will become important in the multiplayer situations below.
The timing diagram on the left shows that the Ball Count Unit Step Up solenoid fired at 30 degrees. The Ball Count Unit however didn't advance from position 1 to position 2 until 45 degrees when the Step Up solenoid relaxed. Steppers generally only advance once their step up solenoids relax since it's the return spring and not the solenoid that advances the unit.
This animation shows what happens on two consecutive ball drains when two players are playing the game.
Ball drain in a 2 player game
The first half of this animation shows what happens when player 1 drains:
It starts the same as the single player animation above except that the Coin Unit is in the 2nd position which indicates that two players are playing the game. Note that the path around the Player Unit is now a dead end and the only way to fire the Ball Count Unit Step Up solenoid is to go through both the Player Unit and the Coin Unit.
This time when the ball drains and the Motor 1 switch closes after 30 degrees nothing happens to the Ball Count Unit because the Player Unit and Coin Unit aren't lined up - the game recognizes that player 1 just drained in a 2 player game and that the Ball Count shouldn't change.
Instead after 60 degrees the Motor 2 switch closes which fires the Player Unit Step Up Solenoid. The Player Unit advances to the next position (from player 1 to player 2) at 75 degrees when the Player Unit Step Up solenoid relaxes.
In this scenario when player 1 drains the ball the Player Unit advances instead of the Ball Count Unit as it would have in a single player game.
The second half of the animation shows what happens when player 2 drains:
Note that after player 1 drains in a 2 player game the Player Unit and Coin Unit are lined up so that there is a path from the Motor 1 switch to the Ball Count Step Up solenoid.
The animation from here is virtually the same as the 1 player game animation above. The only difference is the path used to fire the Ball Count Unit Step Up solenoid. In a 1 player game the path around the Player Unit was used. In a 2 player game a path through both the Player Unit and Coin Unit is used. When the ball drains the Motor 1 switch will advance the Ball Count Unit and the Motor 2 switch will reset the Player Unit (from player 2 back to player 1).
The final animation below shows what happens when player 4 drains the ball in a 4 player game.
4th player drains in a 4 player game
Note that the Player Unit is in the last position to indicate player 4 is playing and the Coin Unit is also in the last position to indicate a 4 player game. In this case the path to fire the Ball Count Unit Step Up solenoid uses only the Player Unit and bypasses the Coin Unit entirely. That's because the Ball Count should advance any time player 4 drains.
Apart from using a different path to fire the Ball Count Unit Step Up solenoid, this animation is the same as the first animation above for a 1 player game. The Ball Count will advance when Motor 1 switch closes and the Player Unit will reset (from player 4 back to player 1) when Motor 2 switch closes.
Problems with this circuit can arise when the step units don't advance or reset reliably. For example, if a game always plays through multiple players even when a 1 player game was started it's often because the Coin Unit hasn't reset at the start of the game. Looking at the schematic you can probably see that if the Coin Unit is stuck in the last position for example, there's no way to advance the Ball Count until the Player Unit steps up to its last position too.
Follow these links to other animated schematic diagrams: