How Slot Machines Work Click here to print this article.
Originally, casinos installed slot machines as a diversion for casual gamers. Unlike traditional table games (such as blackjack or craps), slot machines don't require any gambling knowledge, and anyone can get in the game with a very small bet.
This idea proved to be a monstrous success -- slot machines eventually moved off the sidelines to become the most popular and the most profitable game in town, bringing in more than 60 percent of the annual gaming profits in the United States.
The technology of slot machines has also changed a lot over the years. The classic mechanical designs have been almost completely replaced by computer-controlled machines. But the game has remained the same. The player pulls a handle to rotate a series of reels (typically three) that have pictures printed on them. Winning or losing is determined by which pictures line up with the pay line, a line in the middle of a viewing window. If each reel shows the same winning picture along the pay line, you win (certain single images are sometimes winners as well). The amount you win -- the payout -- depends on which pictures land along the pay line.
Now, we'll find out what sets the reels in motion in modern slot machines as well as in the old mechanical models. We'll also see what determines the odds of winning on a slot machine and look at some popular variations on the traditional game.
There are any number of ways to arrange these elements, and manufacturers have tried dozens of approaches over the years. The diagram below shows one representative design.
Click on the three "click here" areas to see the different parts of the mechanism animated. Then read the description below for details
This design includes three reels mounted on a central shaft. The central shaft also supports three notched discs, which are connected to the three reels. A second shaft below the central shaft supports a kicker, a piece of metal comprising three paddles. The kicker paddles are lined up so they can push against the notches on the three discs. The second shaft also supports a series of connected stoppers, teeth that lock into the notches on the discs.
The kicker and the stoppers are both connected to springs, which hold them in a standby position. The kicker is held in place behind the discs, while the stoppers are held up against the discs, locking them into place.
Here's what happens when a player pulls the handle:
The handle rotates a hook mechanism, which grabs hold of the kicker, pulling it forward (toward the player).
A catch on the opposite end of the kicker grabs a control cam piece and pivots it forward. This rotates a series of gears connected to the control cam. A spring pulls the control cam back to its original position, but the gear assembly slows it down considerably -- the gears act as a mechanical delay.
When the control cam is pivoted forward, it releases a spring-mounted cam plate extending across the back of the machine.
The control cam also pulls the stoppers away from the notched discs. As the kicker keeps moving, it pushes the stoppers against several catches on the cam plate. These hold the stoppers in place, so the discs and reels can rotate freely.
As the handle continues to move the kicker, the kicker paddles push the discs forward briefly. When the handle is pulled all the way back and the kicker has passed the discs, the bottom of the hook mechanism moves up against a slanted surface. The slant pivots the hook forward, which causes it to release the kicker.
The kicker spring jerks the kicker backward at a good speed. The kicker paddles hit the notches on the discs, spinning the reels rapidly.
While all of this is happening, the control cam is slowly returning to its original position. When it does return, it pushes the cam plate back, which releases the stoppers. The different catches holding onto the different stoppers are positioned so that the cam plate will release the stoppers one at a time. Each stopper springs forward and locks into a notch, holding the reel in position.
From the player's point of view, here's how it looks. The player pulls the handle. There is a clunk, and the three reels start spinning. Then the three reels stop abruptly one at a time, followed by the payout (if necessary). The "stopping one at a time" part builds suspense. If the first reel stops on the jackpot symbol, then you have to wait for the next reel to stop to see if it is a jackpot, and then finally the third. If all three display the right symbol, the player wins.
Conventional mechanical slot machines eventually gave rise to electrical machines that worked on similar principles. In an electrical machine, the reels are spun by motors and the stoppers are generally activated by solenoids, but the game basically plays out the same way. Electrical machines have more sophisticated money-handling systems, like those you might find in a vending machine, and flashier light and sound displays.
In both types of systems, once the reels have come to a stop, the slot machine needs to read whether the player has won or lost. In the next section, we'll examine some systems for making this determination.
A more advanced system uses photoelectric cells (also known as photo diodes), devices that generate a current when exposed to light, to detect the position. In this system, a series of holes are drilled through the rotating discs, all around their outer edges. The photo diode is positioned on one side of the disc, and a light source is positioned on the other side. As the disc turns, the light shines through the holes onto the photo diode. The pattern of holes in the disc causes the photo diode to generate a similar pattern of pulses of electricity. Based on this pattern, an electronic circuit can determine the position of the reel.
In the past 15 years, electric machines and fully mechanical machines have both been eclipsed by computerized machines. In the next section, we'll see how these modern slots work.
But even though the computer tells the reels where to stop, the games are not pre-programmed to pay out at a certain time. A random number generator at the heart of the computer ensures that each pull has an equal shot at hitting the jackpot. (Click here to find out how random number generators work.)
Whenever the slot machine is turned on, the random number generator is spitting out whole numbers (typically between 1 and several billion) hundreds of times a second. The instant you pull the arm back (or press the button), the computer records the next few numbers from the random number generator. Then it feeds these numbers through a simple program to determine where the reels should stop.
Here's how the complete process plays out in a typical three-reel machine.
You pull the handle, and the computer records the next three numbers from the random number generator. The first number is used to determine the position of the first reel, the second number is used for the second reel and the third number is used for the third reel. For this example, let's say the first number is 123,456,789.
To determine the position of the first reel, the computer divides the first random number by a set value. Typically, slot machines divide by 32, 64,128, 256 or 512. In this example, we'll say the computer divides by 64.
When the computer divides the random number by the set value, it records the remainder of the quotient. In our example, it finds that 64 goes into 123,456,789 a total of 1,929,012 times with a remainder of 21.
Obviously, the remainder can't be more than 64 or less than 0, so there are only 64 possible end results of this calculation. The 64 possible values act as stops on a large virtual reel.
Each of the 64 stops on the virtual reel corresponds to one of the 22 stops on the actual reel. The computer consults a table that tells it how far to move the actual reel for a particular value on the virtual reel. Since there are far more virtual stops than actual stops, some of the actual stops will be linked to more than one virtual stop.
Computer systems have made slot machines a lot more adaptable. For example, players can now bet money straight from a credit account, rather than dropping coins in for every pull. Players can also keep track of their wins and losses more easily, as can the casinos. The operation is also simpler in modern machines -- if they want to, players can simply press a button to play a game, rather than pull the handle.
One of the main advantages of the computer system for machine manufacturers is that they can easily configure how often the machine pays out (how loose or tight it is). In the next section, we'll see how the computer program can be configured to change the slot machine's odds of hitting the jackpot.