The click wheel of the iPod is the navigation component of several iPod models. It uses a combination of touch technology and traditional buttons, involving capacitive sensing technology, which detects the ability of the user’s fingers. The wheel allows a user to find music, videos, photos and play games on the device. The wheel is flush with the face of the iPod and is under the screen. The design was first released with the iPod Mini, and was last used with the iPod Classic.
The click wheel detects user input via its touch ring. Due to four mechanical buttons below, the ring is able to perform several commands. For example, when browsing the music, after selecting a particular song, the click wheel is used to adjust the volume. Pressing the selection button can be used to switch to a specific part of the song. The primary technology that the wheelwheel demonstrates is that of capacitive sensing. This technology dates back to 1919, when it was used for the first time in a musical instrument called theremin. It allowed to control the height and the volume of the instrument by the distance between the hands of the musician and two antennas. When two metal plates are placed very close to each other, without coming into contact, a current flows through the plates, the energy is stored, but once the current is removed, the stored energy creates a current through herself. This is how a capacitor collects and stores energy. This same principle is applied to the iPod Classic and the first to fifth generations of the iPod Nano. The “brain” behind the ratchet wheel is the conductive membrane behind the plastic liner. This membrane has “channels” that, when connected, create a set of coordinates. These channels are conductors that, when connected to another conductor (a finger in this case), try to send a current through the user’s finger, but are blocked by the plastic covering the click wheel. So, instead of going through the plastic, the current creates a charge at the place closest to the finger, which is also known as the capacitance. The component that detects this capacity change is the controller. Whenever the controller detects a change, it sends a signal to the microprocessor, which performs the desired action. The faster a finger moves around the steering wheel, the more the signal flow is concentrated. The moment the finger leaves the wheel, however, is when the controller stops detecting the change of capacity, thus stopping the current process.