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2. Touchpad Lighting Control

Mechanical sliders and dimmers are used in a variety of applications, most predominantly for light dimming controls and in sound mixers. Their use is well-adopted in the physical world and the virtual world of graphical user interfaces (GUIs). However, physical sliders suffer from characteristics typical to mechanical devices with moving parts: frequent usage and age impact the effectiveness and reliability. Their virtual counterparts found in GUIs do not share the same problem and present additional features, such as setting discrete values instantaneously. The LED Lamp therefore implements non-mechanical physical sliders.

The design includes a touchpad as an input device, and two multi-colored LED light cluster as output devices. For the purposes of the LED Lamp, the touchpad is divided into 5 vertical regions (sliders), each of which represents a separate physical dimmer (Figure 1.13). The width of each slider approximates the width of the effective contact area of a finger used to control a mechanical dimmer or a pointing device. Each of the five sliders controls the intensity of one color: red, green, blue, yellow, and white (Figure 1.12).

The interface is implemented with a standard pressure-sensitive touchpad found in most notebook computers as a pointing device. Specifically, a Synaptics model TM1202SBU200-1 measuring 65 mm in width and 49 mm in height is used. The serial protocol touchpad is connected to a RabbitCore RCM2200 microprocessor module. The custom-designed program on this C-programmable device converts absolute coordinate values from the touchpad into commands understood by the light cluster. The raw horizontal range from 0 to 6143 is reduced to 5 horizontal positions including 4 spatial gaps, and the raw vertical range from 0 to 6143 is mapped to 23 discrete values: 0 to 22 for light intensity.

Figure 1.12: Using the touchpad embedded in a plastic casing (left upper corner), the intensity of each color in a multi-colored LED light cluster (red, green, blue, yellow, and white in one device) can be controlled individually. The interface between touchpad and LED cluster is established using the RabbitCore RCM2200 microprocessor module (right upper corner).
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2.1 Interaction

Sliding the index finger vertically over the touchpad changes the intensity of a color. Intensity of the color increases towards the top and decreases towards the bottom of the sliding region. Similar to mechanical dimmers and GUI-style sliders, the position on the slider is measured absolutely. Using this methodology, discrete intensity levels can be set quickly by simply touching the slider at the desired vertical position.

Figure 1.13: The touchpad is virtually divided into 5 vertical sliders, each of which controls one color. To increase the intensity of a color, the finger is moved towards the top of a slider.
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Since there are no physical boundaries between sliders for different colors, but instead virtual gaps, multiple sliders can be operated at roughly the same time with minimal additional effort per slider. In this scenario, the finger can be moved unobtrusively between the sliders in a horizontal manner to control several color intensities at the same time.

While there exists no indicator on the touchpad as to what values have been set, immediate feedback is present in the form of color brightness by the light cluster. With a processing rate of about 25 touchpad events per second and a cumulative transmission speed of approximately 40 milliseconds per event from touchpad to the light cluster, there exists virtually no lag between the input and the output devices.