How does iPod Click Wheel work?

The Click Wheel is a touch-sensitive ring that you use to navigate through all of iPod's menus and control all of its features. It provides two ways to input commands: by sliding your finger around the wheel and by pressing buttons located under and in the middle of the wheel.

Under the plastic surface of the Click Wheel, there are four mechanical buttons (Menu, back, forward, play/pause), and there's one button in the center. Its touch-sensitive function lets you move through lists, adjust volume and fast forward through a song by moving your finger around the stationary wheel. It works a lot like a laptop touchpad. In fact, the company that supplied the Click Wheel for the 4G iPod was Synaptics, most widely known for making laptop touchpads. For the 5G, Apple created its own proprietary Click Wheel design based on the same capacitive sensing principle as the previous Synaptics-designed Click Wheel.

He system controller supplies an electrical current to the grid. The metal channels that form the grid are conductors -- they conduct electricity. When another conductor -- say, your finger -- gets close to the grid, the current wants to flow to your finger to complete the circuit. But there's a piece of nonconductive plastic in the way -- the Click Wheel cover. So the charge builds up at the point of the grid that's closest to your finger. This build-up of an electrical charge between two conductors is called capacitance. The closer the two conductors are without touching, the greater the capacitance.

The Click Wheel controller (see above) is programmed to measure changes in capacitance. The greater the change in capacitance at any given point, the closer your finger must be to that point. When the controller detects a certain change in capacitance, it sends a signal to the microprocessor. As you move your finger around the wheel, the charge build-up moves around the wheel with it. Every time the controller senses capacitance at a given point, it sends a signal. That's how the Click Wheel can detect speed of motion -- the faster you move your finger around the wheel, the more compacted the stream of signals it sends out. And as the microprocessor receives the signals, it performs the corresponding action -- increasing the volume, for instance. When your finger stops moving around the wheel, the controller stops detecting changes in capacitance and stops sending signals, and the microprocessor stops increasing the volume.

The pen cap and the Silly Putty are not conductors, end of story. But what about the tip of the soldering tool, the paper clip and the charger prongs? Those are conductors! To solve this riddle, we contacted an expert in the electronics field, who recommended the following action: Wrap your finger in aluminum foil and try to work the Scroll Wheel. Our expert was thinking "surface area." This finger-wrapped-in-foil input worked perfectly.

Can it be that the surface area of the paper clip is not enough to trigger the conductive grid? To investigate this hypothesis, we tried to work the Scroll Wheel using the blunt end of a dinner knife (approx. 0.75 in x 0.5 in). It worked. We concluded that surface area matters. You need to be touching the knife or the orange in order for the Scroll Wheel to detect it. The determining factor, then, is you -- the human body is a very big conductor, providing a very big neutral area for a charge to jump to. The charge difference between your body and the Click Wheel's electrodes provides the voltage -- or electrical "pressure" -- that activates the Click Wheel system.

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