The selected motor needs to be able to rotate a copper mechanical arm 360 degrees around the circumference of the resistor plate with a high degree of accuracy. Stepper motors can be controlled with high accuracy, have full 360 degrees range of motion, and can apply relatively high torque at lower speeds, which makes it a suitable motor for this application. The drawback of using a stepper motor, is that there is no indicator of the orientation of the motor at any given point, without using external means of determining the orientation of the shaft or rotor. Feedback will be applied later to overcome this limitation.

A bipolar NEMA-17 stepper motor with a step-angle of 1.8 degrees allows for 200 discrete steps per rotation. A motor such as Stepperonline's 17HS15-1504S-X1 or DFRobot's FIT0278, is a suitable selection for this design. The NEMA-17 class is large enough to produce enough torque at slower speeds to move a small arm assembly. The motor of this design has two windings with a rated current of 1.5 A per phase, and a winding resistance of 2.3 Ω. The DRV8833 is a dual H-Bridge motor driver that can be used to drive this motor as it can provide enough current for the motor in a small package. The motor driver will be configured to limit the current to 1 A per phase at VM = 5 V, by setting the sense resistor to 0.15 Ω, as illustrated in the DRV8833 Dual H-Bridge Motor Driver data sheet. The digital signals used to drive the motor are shown in Figure 3-3.