*2.3. Materials*

We have developed a large-scale hardware prototype based on the ColorWatch concept which exhibits the represented color while exploring different objects. Figure 4 displays the working principle of a large-scale prototype. The two rotating disks in the round and square shape are used to represent both levels of primary colors and color tones as shown in Figure 5a. Figure 5b provides a demonstration for the prototype in use. The hardware prototype is a 110 × 170 × 70 cm encased acrylic box that encapsulates the control electronic circuit board, two stepper motors, and their motor drivers (Figure 5c). A set of wires is extended from the box which connects the color sensor module (Figure 5d). The ISL29125 RGB color sensor is used in the color sensor module for color data acquisition. The color sensor is designed to operate in diverse luminance environments ranging from darkrooms to sunlight by rejecting IR in light sources. The color sensor has low-power and needs 56 and 0.5 μA current for operation and power-down mode, respectively. The integrated ADC of the color sensor also rejects flicker caused by artificial light sources. The 2.25–3.6 V logic levels of the color sensor need to be converted if used with a 5 V Arduino board, and a bi-directional logic level converter is used for this purpose. The color sensor is housed in an acrylic casing that limits the incoming light from the target angles only. Four LED lights have been installed into the color sensor module for providing balanced luminance for target objects. An Arduino Uno microcontroller (Arduino, Somerville, MA, USA) is used as a control unit, which takes its inputs from the color sensor module. The 16 bit RGB color values are then converted to Hue, Saturation, and Value-based color model. The color gamu<sup>t</sup> of ColorWatch is calibrated on color samples and these calibration values are compared by a nearest-neighbor based algorithm for classification of any target object color and color tones. The target object color and color tones then index the corresponding angular position of motors from a lookup array. Based on the difference of current and target angular position, the required angular rotations are then conveyed to stepper motors via motor drivers.

**Figure 4.** Block diagram of the prototype's working principle.

**Figure 5.** The tactile perception and color sensor modules of prototype with a color identification training relief paper (**a**); a demonstration of the prototype in use (**b**); the internal hardware for tactile perception and color sensor modules (**<sup>c</sup>**,**d**).
