How do those wacky fruit machines work, anyway...?

From the coin slot to a coin detector to the brake system to the hook mechanism to the kicker to a catch to the control cam to notched shaft to ... it's everything you've always wanted to know about how a traditional slots machine works but were afraid to ask.
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Slot machine mechanics: The Kicker and the Stopper

Slot machine mechanics: The Kicker and the StopperAre you someone who likes to understand what makes things “tick”?  How about what makes a slot machine spin? 

Classic slot machines function on a gear-and-lever system. Gears and levers are what make the movement inside a slot machine possible. The main apparatus within the machine is a metal shaft that upholds the reels and is connected to the handle. There are a few main components to the slot machine:  the shaft, the handle, the kicker, the stopper, the brake system, the coin detector and the payout system. 

So let’s begin with what happens when someone pulls a slot machine lever: What do you think happens? Well, nothing really at first, that is until a coin is dropped in and the coin detector loosens the lever by unlocking a brake system. A hook mechanism then starts the whole event in motion with the pull of the handle. The hook grabs hold of what is called the “kicker” and pulls it toward the player. A catch on the opposite end of the kicker grabs a control cam, which pivots forward to move the mechanism of gears that creates a mechanical delay.  When this control cam moves forward, it releases a spring-mounted cam plate that extends along the back of the machine. 

The shaft generally has three notched discs which are connected to the reels. The second shaft supports the kicker, a piece of metal with three paddles that can push up against the notches on the three discs. The second shaft also has a variety of connected stoppers and teeth that lock into the notches on the discs. A slant in the surface of the hook mechanism pivots a hook forward that releases the kicker. 

Both the kicker and the stopper are connected to springs. The kicker is held in place behind the discs at which time the stoppers are held up against the discs, locking them into place. When the kicker paddles hit the notches on the discs, this is what makes the reels spin quickly. As the control cam slowly moves back to its original position, the cam plate is pushed back and this releases the stoppers one at a time into different notches. The brake system brings the reels to an abrupt stop and sensors communicate the position of the reels to trigger the payout system. 

What appears simple on the outside is much more intricately managed within the slot machine’s mechanics. Most machines are created as a means to an end; slot machines were made with the idea of generating money – Winning: now that’s the part of the machine we really want to get inside and master! --written by Natalie Jaro
07 October, 2009

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According to Wikipedia, the precursor to the slot machine was invented by the now-mysterious “Sittman and Pitt of Brooklyn” way back in the last half of the 19th century. Something of a similar-looking contraption with five “reels,” the machine sported a deck of cards in each. A nickel got the player a spin, and winning poker hands won various prizes from the host, typically a pub.