Joysticks pull off a really neat trick. They take something entirely physical the movement of your hand and translate it into something entirely mathematical a string of ones and zeros (the language of computers). With a good joystick, the translation is so flawless that you completely forget about it. When you're really engaged in a game, you feel like you're interacting with the virtual world directly.

The Simplest System: Design

The basic idea of a joystick is to translate the movement of a plastic stick into electronic information a computer can process. Joysticks are used in all kinds of machines, including F-15 fighter jets, backhoes and wheelchairs.

The various joystick technologies differ mainly in how much information they pass on. The simplest joystick design, used in many early game consoles, is just a specialized electrical switch.

This basic design consists of a stick that is attached to a plastic base with a flexible rubber sheath. The base houses a circuit board that sits directly underneath the stick. The circuit board is made up of several "printed wires," which connect to several contact terminals. Ordinary wires extend from these contact points to the computer.

The printed wires form a simple electrical circuit made up of several smaller circuits. The circuits just carry electricity from one contact point to another. When the joystick is in the neutral position when you're not pushing one way or another -- all but one of the individual circuits are broken. The conductive material in each wire doesn't quite connect, so the circuit can't conduct electricity.

Each broken section is covered with a simple plastic button containing a tiny metal disc. When you move the stick in any direction, it pushes down on one of these buttons, pressing the conductive metal disc against the circuit board. This closes the circuit it completes the connection between the two wire sections. When the circuit is closed, electricity can flow down a wire from the computer (or game console), through the printed wire, and to another wire leading back to the computer.

The Simplest System: Communication

When the computer picks up a charge on a particular wire, it knows that the joystick is in the right position to complete that particular circuit. Pushing the stick forward closes the "forward switch," pushing it left closes the "left switch," and so on. In some designs, the computer recognizes a diagonal position when the stick closes two switches (for example, closing the forward switch and the left switch simultaneously would mean a forward/leftward diagonal position). The firing buttons work exactly the same way when you press down, it completes a circuit and the computer recognizes a fire command.

This design communicates joystick motion in a sort of shorthand it processes movement as absolute values instead of subtle gradations. In other words, it can't distinguish between pressing forward on the stick a little bit and pushing it as far as it will go there is only one value for forward.

This is fine even ideal for some games. It's the perfect design form something like Pac Man or Tetris, for example. But it can be fairly limiting for other games, such as flight simulators. In the next section, we'll look at the conventional analog joystick design that can pick up on subtle shifts in position.


Conventional Analog: Design

In order to communicate a full range of motion to the computer, a joystick needs to measure the stick's position on two axes the X-axis (left to right) and the Y-axis (up and down). Just as in basic geometry, the X-Y coordinates pinpoint the stick's position exactly.

In the standard joystick design, the handle moves a narrow rod that sits in two rotatable, slotted shafts. Tilting the stick forward and backward pivots the Y-axis shaft from side to side. Tilting it left to right pivots the X-axis shaft. When you move the stick diagonally, it pivots both shafts. Several springs center the stick when you let go of it.

To determine the location of the stick, the joystick control system simply monitors the position of each shaft. The conventional analog joystick design does this with two potentiometers, or variable resistors. The diagram below shows a typical arrangement.

Conventional Analog: Potentiometers

Each potentiometer consists of a resistor, in the form of a curved track, and a movable contact arm. The computer power supply conducts electricity to the input terminal, through the curved resistor, through the contact arm and back to the joystick port on the computer. By moving the contact arm along the track, you can increase or decrease the resistance acting on the current flowing through this circuit.

If the contact arm is on the opposite end of the path from the input connection terminal, electricity will have to flow through a long length of resistor, so it will face maximum resistance. If the contact arm is near the input terminal, the potentiometer will have minimal resistance.

Each potentiometer is connected to one of the joystick shafts so that pivoting the shaft rotates the contact arm. In other words, if you push the stick all the way forward, it will turn the potentiometer contact arm to one end of the track, and if you pull it back toward you, it will turn the contact arm the other way.

Varying the resistance of the potentiometer alters the electrical current in the connected circuit. In this way, the potentiometer translates the stick's physical position into an electrical signal, which it passes on to the joystick port on the computer.

This electrical signal is totally analog it's a varying wave of information, like a radio signal. In order to make the information usable, the computer needs to translate it into a digital signal a strict numerical value

Conventional Analog: Into Digital

In the conventional system, a card (a printed circuit board) inside the computer handles this with a very crude analog-to-digital converter. The basic idea is to use the varying voltage from each potentiometer to charge a capacitor, a simple electrical device that stores electricity (see How Capacitors Work for more information). If the potentiometer is adjusted to offer more resistance, it will take the capacitor longer to charge; if it offers less resistance, the capacitor will charge more quickly.

By discharging the capacitor and then timing how long it takes it to recharge, the converter can determine the position of the potentiometer, and therefore the joystick. The measured recharge rate is a numerical value the computer can recognize. The computer performs this operation whenever it needs to get a read on the joystick.

You can potentially apply this system to an infinite variety of controls by connecting a potentiometer to different rotating components. For example, conventional steering wheel controllers work exactly the same way, with the wheel rotating the potentiometer contact arm directly. Some joysticks use an additional potentiometer for a Z-axis, activated by rotating the stick itself.

Some joysticks also have a "top hat" a thumb-activated miniature controller on top of the stick. This small joystick uses the same switch system as the simple joystick in the last section.

The conventional analog system works okay, overall, but it does have limitations. In the next section, we'll find out what the major problems are and then look at some recent solutions.