4 Part Selection

While the system consists of relatively few parts, consideration had to be made about the cost and power consumption of the components. First, the selected MCU is the MSP-EXP430G2ET. The MSP430 platform was considered an excellent choice for this application due to the low cost and ease of use. The DFRobot SER0053 was selected as the motor due to the PWM interface, low power consumption, and low cost. Additionally, the low current draw of the motor means that a dedicated motor driver was not necessary, further reducing the cost and complexity of the system.

The PWM interface allows for precise positioning by varying the duty cycle. On this motor, one duty cycle corresponds to one position. For example, a duty cycle of 1% corresponds to 0° of rotation, while a 50% duty cycle corresponds to 150° of rotation. Other servo motors considered are controlled by the applying voltage to one of the two pins on the motor. A high voltage applied to the clockwise pin results in a fast rotation clockwise, while a lower voltage applied to the counterclockwise pin produces a slower motion in that direction. While simple, this makes accurately positioning the motor, and therefore the mirror, significantly more difficult.

For powering the device, there are two options: line and battery. A rechargeable battery increases the price of the unit and adds the issue of battery life, but makes the device significantly more portable and easier for home use. For the battery life calculations in the Power Consumption section, a 3.2-V, 1.5-Ah rechargeable lithium iron phosphate battery was chosen. This offers a good battery life at an economical price.