iPod Click Wheel

iPod Click Wheel

The Click Wheel is a touchsensitive ring that you use to navigate through all of iPods 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 theres one button in the center select.

Youve got five buttons and five corresponding contacts on the motherboard. When you press, say, the right side of the wheel while youre listening to a song, the wheel pushes down the forward button. The underside of each rubber button is metal, so pressing it completes the corresponding circuit on the motherboard. The motherboard tells the processor this circuit is complete, and the processor tells the operating system to fastforward through the song.

The Click Wheels touchsensitive 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 Synapticsdesigned Click Wheel.

Under the plastic cover of the Click Wheel, there is a membrane embedded with metallic channels. Where the channels intersect, a positional address is created, like coordinates on a graph.

At its most basic, a capacitivesensing system works like this: The 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 theres a piece of nonconductive plastic in the way the Click Wheel cover. So the charge builds up at the point of the grid thats closest to your finger. This buildup of an electrical charge between two conductors is called capacitance. The closer the two conductors are without touching, the greater the capacitance.

The sensing part of the system comes in with the controller. 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 buildup moves around the wheel with it. Every time the controller senses capacitance at a given point, it sends a signal. Thats 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.

Now, in discussing the workings of the Click Wheel, a particularly curious HowStuffWorks staffer raised the following question: If your finger controls the Click Wheel because your finger is a conductor, why cant you control the Click Wheel with a paper clip?

While we scratched our heads, we embarked on a experiment.

Now that weve checked out the iPod hardware, lets take a look at the software its supporting.

Experiment: How About an Apple?

What can you use to control the touchsensitive Click Wheel? Heres an abbreviated list of what we tested:

Finger: YesOrange: YesApple: YesPlastic pen cap: NoSilly Putty: NoPaper clip: NoTip of Cold Heat soldering tool: NoProngs from iPod charger: NoThe yesses are easily explainable fruit and flesh can conduct electricity. The nos, however, are a bit more mysterious. 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 fingerwrappedinfoil 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 thatsurface area matters.

But theres another factor, too, because holding the dinner knife between two plastic pens and moving it around the Scroll Wheeldoesnt work. Same with the apple and the orange. 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 Wheels electrodes provides the voltage or electrical pressure that activates the Click Wheel system.