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Inside a Slot Machine

About the Author: Devin Wong

Devin Wong was a senior at USC majoring in Civil Engineering with a minor in Management Consulting. He plans to attend Graduate School to study Architectural Acoustics.

Gambling is a billion dollar industry that attracts millions of people around the world. While games such as Texas Hold ‘Em poker receive more fanfare, by far the most profitable and available attraction in these casinos is the slot machine. While familiar in appearance, these devices are a mystery to the layperson. Dating back to the late 1800s and with a history rooted in the California Gold Rush, “slots” use the principles of probability and chance to beguile players and create a profit for owners. The design of slot machines has evolved from mobile mechanical parts to mostly electronic components over the course of the last century, but their charm and attraction remain strong. With the advent of computer servers and live streaming of data, slot machines are poised to intrigue and befuddle a new generation of gamblers hoping to hit the jackpot.


The gambling industry brings in billions of dollars every year all over the world. Places such as Las Vegas and Monte Carlo have built their reputations on gambling establishments with hundreds of games ranging from poker to electronic horse racing and with all kinds of luxurious accoutrements from flashy shows to nightclubs. People travel from many different places, sometimes bringing a month’s worth of savings, so that they can participate in these games of chance and experience the casino atmosphere. More often than not, they leave having lost more money than they have won, but having enjoyed the experience nonetheless.
Since its introduction in 1893, the most popular form of gambling has been the slot machine. Slots are simple, giving rewards when the correct combination of symbols has surfaced. They rarely cost more than five dollars and require only the push of a button or the pull of a handle to play. Flashing lights, exciting sounds, and large grand prizes entice players who hope to strike it rich. Because they do not require skill or practice to play, these machines give everyone the chance to win. Immortalized in movies such as Casino and Ocean’s 11, slot machines continue to be the most profitable game in any casino. In Las Vegas alone, slot machines accounted for 67% of total casino profits in 2006 [1].
Various models of slot machines are in use today, but the most popular contains a combination of electronic and mechanical parts. Using the fundamentals of probability, slots are uniquely engineered to appear simple while disguising their true inner workings from the public. The slot machine combines engineering acumen, mathematical know-how, and psychological deceit in a single, attractive package.


In San Francisco in 1893, Gustav F.W. Schultze invented a small countertop gambling machine to capitalize on the strong gambling tendencies that resulted from the Gold Rush. This device accepted a nickel, which caused a colored disk to begin spinning. After the disk stopped, a connected star wheel determined the payout. A slide then cut the correct number of coins and dropped them into a payout cup. During the late 1890’s, Charles Fey perfected this machine by adding three wheels instead of one and changing their orientation so that they flashed symbols of playing cards through a window. Using more springs, cams and levers, Fey contrived a mechanism that allowed the reels to stop in succession, creating an atmosphere of suspense [2]. With over a thousand different possible combinations and a 75.6% payout of money played (meaning the owner kept 24.4%), Fey’s contraption known as the “Bell” became the standard archetype for slot machines and a popular attraction in saloons.

Herbert S. Mills further perpetuated the popularity of the Bell machines. By increasing the reliability and decreasing the cost of manufacturing slots, Mills was able to ship these machines throughout the new territories of the United States. He also pioneered the inclusion of a pack of gum with play in order to skirt the growing number of laws restricting the use of these gambling attractions. In essence, gamblers could only play once they performed the legal transaction of “buying” the gum. In 1906, Mills introduced his own slot machine with the now-classic bell and other symbols (Fig. 1) supplanting the original card signs on the slot reels [2]. Furthermore, he expanded the window opening from one to three lines to add another factor to entice people to play, as players could now see how close they were to winning.

Slot innovation continued in the twentieth century. During the 1920s, slots were revamped to accept quarters and even silver dollars. Sometimes skill elements were added, such as buttons that allowed the player to attempt to stop each specific reel at a moment of his or her choosing. Most importantly, the concept of the jackpot was incorporated. Windows showing a buildup of coins proved to be effective bait for many players.
In 1963, Bally Manufacturing Company introduced the first electronic slot machine. Electromechanical sensors replaced the mechanical slides and allowed for brand new features. All three lines in the slot window could offer payouts, as well as diagonal matchups, allowing for many more winning combinations [2]. Furthermore, multiplier machines could accept multiple coins for larger prizes. Soon, logic cards and integrated circuits replaced the switches and wires. In 1981, Michael S. Redd founded International Game Technology (IGT), a public company that pioneered a radical new invention in slots. Redd took all the recent electronic breakthroughs and coupled them with a computer that replaced the original wheel randomizer. This computer generated random numbers that instantly decided the payout and where each “reel” would stop. A motor simultaneously carried out these directions on the slot screen. These machines remain the basis for slots in today’s casinos.

The Modern Electronic Reel Slot Machine: A Piece by Piece Breakdown

Slot machines produced today are a far cry from their mechanical ancestors. Computer chips have replaced spinning wheels, and electronic credits have replaced coins. This fusion of new age electronics with old fashioned nostalgia creates a machine that, while simple in theory, beguiles players on a regular basis.
At the heart of all modern slot machines is a computer chip called the Random Number Generator (RNG). This device replaces the original spinning wheel found in gambling machines from the first half of the twentieth century. The RNG is capable of continuously generating thousands of random numbers and, in the case of the three-reel slot, thousands of three-number combinations every second [3]. Each of these three numbers in the combination serves to designate where the reels will stop.

To simplify the thousands of combinations into discrete slots on reels, each random number in the combination is divided by a set value (typically, slot machines use 32, 64, 128, 256, or 512 as that value). The computer records the remainder of this quotient, which by mathematical law cannot exceed the set value [4]. The remainder is mapped to a certain symbol that is physically distributed among the number of slots the reel contains. A virtual representation of this mapping using a set value of 128 and a reel with 22 slots may be seen in Fig. 2.

This is how the odds swing in favor of the casino. The quotient remainders are not all distributed equally among the actual slots on the reel. Blank spaces are weighed heavily, as seen in the Percent Chance of Choosing Symbol column, as are those that have a lower payout. Furthermore, each of the three reels is weighted differently. For instance, the first reel is most likely to hit the jackpot slot, while by the third reel the likelihood is minuscule. Along with a heavy weighting to blank spaces surrounding the jackpot, this creates a “near miss” effect and keeps the player in suspense [3].
The RNG is part of a larger Computer Processing Unit (CPU) that controls other various functions of the slot machine. The basic functions include the Start switch, Max Bet switch, token or credit sensor, and RAM to store the various hit combinations, reel stops and payouts. Depending on the style of machine the CPU also may control the hopper, which stores tokens or coins, or the credit account of the player [5].

For a machine with actual spinning reels, once the random numbers have been generated, the machine uses a stepper motor to turn each reel and stop it at the predetermined point [4]. A stepper motor is an electromechanical device that uses electrical pulses to drive a shaft or spindle in discrete increments. These incremental movements are related to the frequency and sequence of the electrical pulses, allowing for total movement control [6]. If the machine only has a screen, then a piece of software animates a virtual spinning reel.

Overall, the slot machine’s actions are controlled by a simple algorithm based on basic programming principles like IF switches and loops. The algorithm begins once the on switch is triggered (usually by the insertion of a token or credit). At this point, the start button or “lever” is pulled, which leads the RNG to choose the random numbers that will control the positions of the reels. The step motor simultaneously spins the reels until they arrive at the correct position. If the selected combination corresponds to a payout, many machines will trigger a program of flashing LED lights and sounds, and the payout will be awarded. Otherwise, the game ends, and the player must trigger the start switch once more by adding a new token or credit. In addition to this simplified loop shown in Fig. 3, some machines include special jackpots or other attractive motions and sounds in order to create a more satisfying experience for the gambler.

Slot-Machine Innovations

By today’s standards the technology employed in slot machines has not advanced very significantly. Fig. 4 shows some of these newer slot machines. The Random Number Generator central to the slot machine’s function has existed for decades. Even newer slots with more than three reels, multiple payout lines and interactive games are child’s play when compared with a current video game system. However, new innovations to the world of gambling games are cropping up and copying many existing prototypes of networked machines.
In recent years, slot machine designers have worked closely with video game architects in order to bring some of the visual appeal found in virtual gaming to the gambling arena. Video monitors, 3D graphics, and group competition are all making their way into slot construction [7]. Furthermore, slots have taken on a pop culture persona in order to keep up with a younger generation of gamblers. Games featuring “Lord of the Rings” and “Sex and the City” themes have already appeared at gaming expos. Designers have even added multi-level games to help bring gamblers back.

One of the most intriguing developments in slot technology involves connecting slots to servers, much as office computer networks are designed. This innovation would allow casinos to easily manage minimum bets, percentages paid, and the type of game available on a machine. Furthermore, these activities or factors could be adjusted based on the time of day, level of activity at each floor, or even the players’ personal information [8]. These networks will also allow different games to be recommended for different players as well as the streaming of live television to built-in screens.

The appeal of slots has always been the atmosphere of suspense it creates with its spinning reels coupled with the appeal of “easy” money. The average person can easily set the minimum bet and playing is as simple as pressing a button. Recently, slot machines have been overshadowed by other table games such as poker, which is extensively televised, and the live excitement of craps. Changing with the times and utilizing new technologies will allow popular gambling machines from the past to remain as attractions to new generations of gamblers.


    • [1] “Nevada Reporting Huge Casino Profits.” Online Gambling Paper. n.d. Web. 16 Jul. 2010. <http://www.ogpaper.​com/news/news-0222.h​tml>.
    • [2] Jack Kelly. “Slots: Perfecting a machine to take away your money.” American Heritage of Invention and Technology-Online 16.2 (2000): 34. Web. 13 Jul. 2010.
    • [3] Michael Bluejay. “How Slot Machines Work.” Top Ten Las Vegas Tips. n.d. Web. 16 Jul. 2010. <http://vegasclick.c​om/games/slots/how-t​hey-work.html>.
    • [4] Tom Harris. “How Slot Machines Work.” HowStuffWorks. 26 Feb. 2002. Web. 15 Jul. 2010.<http://enterta​inment.howstuffworks​.com/​m>.
    • [5] Haruo Inoue. 1997. Slot Machine. U.S. Patent 5,609,524, filed July 12. 1995, and issued March 11, 1997.
    • [6] “Stepper Motors.” Omega Engineering Web. 16 July 2010. <​m/prodinfo/stepper_m​otors.html>.
    • [7] Christopher Palmeri. “Face-lift for the One-Armed Bandit.” Business Week 21 Dec. 2009: 77. Web. 15 July 2010. <http://www.thefreel​​+FOR+THE+ONE-ARMED+B​ANDIT-a01612094149>.​
    • [8] Quentin Hardy. “Well-Armed Bandits –” Forbes Business News. 19 May 2008. Web. 16 Jul. 2010. <http://www.forbes.c​om/global/2008/0519/​054.html>.
    • [9] “Old Downtown Las Vegas History.” EarlyVegas. n.d. Web. 20 Jul. 2010. <http://www.earlyveg​​>.
    • [10] Rusman. “Playing in an Online Slot Machines for Players.” Games and Entertainment Reviews. 6 Aug. 20101. Web. 11 Oct. 2010. <http://www.komiks-d​​playing-in-an-online​-slot-machines-for-p​layers.html>.

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