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Introduction to How Slot Machines Work

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.

In this article, 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.

The classic slot machine design works on an elaborate configuration of gears and levers. The central element is a metal shaft, which supports the reels. This shaft is connected to a handle mechanism that gets things moving. A braking system brings the spinning reels to a stop, and sensors communicate the position of the reels to the payout system. A coin detector initially registers that a coin has been inserted and unlocks a brake so the handle can move.

There are any number of ways to arrange these elements, and manufacturers have tried dozens of approaches over the years, so we'll focus on one representative design. The basic 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.

When you pull the handle on a slot machine, these parts do a lot of work. We'll look at exactly what happens in the next section.

Pulling the Handle

Click on the three "click here" areas to see the different parts of the mechanism animated. Then read the description below for details.

A series of events takes place after a person pulls the handle on a slot machine. Here's a rundown:

  • 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.

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Payout

There are dozens of different payout systems used in slot machines. In one of the simplest designs, a jackpot is detected by measuring the depth of notches in the discs that drive the reels. For simplicity's sake, we'll look at this sort of payout system in a bare-bones slot machine. The machine only accepts one kind of coin, and there is only one winning combination of images.

When you put a coin in this machine, it falls into a transparent case. The bottom of the case is a movable shutter that is connected to a metal linkage, as you can see in the diagram. Normally, the linkage holds the shutter closed. But when the machine hits the jackpot, the third stopper shifts the linkage up, opening the shutter so the coins fall out of the machine.

Each of the three discs has notches for each stop position of the reel. The notch for the jackpot stop is deeper than the other stops. Consequently, when the first reel lands on the jackpot stop, the first stopper moves farther to the left than it would for any other stopper. If the second reel stops on the jackpot as well, the second stopper also moves farther left. Same goes for the third reel and stopper.

But if only the second reel stops on the jackpot, the second stopper will not move all the way into the notch. The first stopper has a catch that keeps the second stopper from moving past it. The second stopper, in turn, has a catch that holds the third stopper back. For the third stopper to lock all the way into the jackpot notch, then, the first and second reels would have to have landed on the jackpot image. When this happens, the shutter opens to dump all of the coins that have been played since the last jackpot.

Typically, slot machines will have more elaborate versions of this design in order to pay out partially on certain combinations of images and pay out completely on the jackpot combination.

In another popular system used in some electrical machines, the discs have a series of metal contacts attached to them. When the reels stop, one of the contacts engages a stationary contact wired to a circuit board. In this way, every stop on each reel will close a different switch in the electrical system. Certain combinations of closed switches (jackpot winners) will configure the machine's electrical circuit to operate the payout mechanism.

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.

Newer slot machines use computers instead of gears. We'll take a look at those next.

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Computerized Slot Machines

Most modern slot machines are designed to look and feel like the old mechanical models, but they work on a completely different principle. The outcome of each pull is actually controlled by a central computer inside the machine, not by the motion of the reels.

The computer uses step motors to turn each reel and stop it at the predetermined point. Step motors are driven by short digital pulses of electricity controlled by the computer, rather than the fluctuating electrical current that drives an ordinary electric motor. These pulses move the motor a set increment, or step, with great precision (see Introduction To Step Motor Systems to find out more).

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. 

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.

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What are the Odds?

In a modern slot machine, the odds of hitting a particular symbol or combination of symbols depends on how the virtual reel is set up. As we saw in the last section, each stop on the actual reel may correspond to more than one stop on the virtual reel. Simply put, the odds of hitting a particular image on the actual reel depend on how many virtual stops correspond to the actual stop.

In a typical weighted slot machine, the top jackpot stop (the one with the highest-paying jackpot image) for each reel corresponds to only one virtual stop. This means that the chance of hitting the jackpot image on one reel is 1 in 64. If all of the reels are set up the same way, the chances of hitting the jackpot image on all three reels is 1 in 643, or 262,144. For machines with a bigger jackpot, the virtual reel may have many more stops. This decreases the odds of winning that jackpot considerably.

The losing blank stops above and below the jackpot image may correspond to more virtual stops than other images. Consequently, a player is most likely to hit the blank stops right next to the winning stop. This creates the impression that they "just missed" the jackpot, which encourages them to keep gambling, even though the proximity of the actual stops is inconsequential.

A machine's program is carefully designed and tested to achieve a certain payback percentage. The payback percentage is the percentage of the money that is put in that is eventually paid out to the player. With a payback percentage of 90, for example, the casino would take about 10 percent of all money put into the slot machine and give away the other 90 percent. With any payback percentage under a 100 (and they're all under 100), the casino wins over time.

In most gambling jurisdictions, the law requires that payback percentages be above a certain level (usually somewhere around 75 percent). The payback percentage in most casino machines is much higher than the minimum -- often in the 90- to 97-percent range. Casinos don't want their machines to be a lot tighter than their competitors' machines or the players will take their business elsewhere.

The odds for a particular slot machine are built into the program on the machine's computer chip. In most cases, the casino cannot change the odds on a machine without replacing this chip. Despite popular opinion, there is no way for the casino to instantly "tighten up" a machine.

Machines don't loosen up on their own either. That is, they aren't more likely to pay the longer you play. Since the computer always pulls up new random numbers, you have exactly the same chance of hitting the jackpot every single time you pull the handle. The idea that a machine can be "ready to pay" is all in the player's head, at least in the standard system.

When you hit the slot machines in a casino, you'll have dozens of gaming options. Machines come with varying numbers of reels, for example, and many have multiple pay lines.

Most machines with multiple pay lines let players choose how many lines to play. For the minimum bet, only the single line running straight across the reels counts. If the player puts more money in, he or she can play the additional horizontal lines above and below the main pay line or the diagonal lines running across the reels.

For machines with multiple bet options, whether they have multiple pay lines or not, players will usually be eligible for the maximum jackpot only when they make the maximum bet. For this reason, gambling experts suggest that players always bet the maximum.

There are several different payout schemes in modern slot machines. A standard flat top or straight slot machine has a set payout amount that never changes. The jackpot payout in a progressive machine, on the other hand, steadily increases as players put more money into it, until somebody wins it all and the jackpot is reset to a starting value. In one common progressive setup, multiple machines are linked together in one computer system. The money put into each machine contributes to the central jackpot. In some giant progressive games, machines are linked up from different casinos all across a city or even a state.

Some slot-machine variations are simply aesthetic. Video slots operate the same way as regular machines, but they have a video image rather than actual rotating reels. When these games first came out, players were very distrustful of them; without the spinning reels, it seemed like the games were rigged. Even though the reels and handles in modern machines are completely irrelevant to the outcome of the game, manufacturers usually include them just to give players the illusion of control.

These are only a few of today's popular slot variations. Game manufacturers continue to develop new sorts of machines with interesting twists on the classic game. A lot of these variations are built around particular themes. There are now slot games based on television shows, poker, craps and horse racing, just to name a few.

To learn more about modern slot machines, including strategies to increase your chances of winning, check out the links on the next page.