Persistence of Vision Machine

Persistence of vision (POV) refers to the optical illusion whereby multiple discrete images blend into a single image in the human mind and believed to be the explanation for motion perception in cinema and animated films. Like other illusions of visual perception, it is produced by certain characteristics of the visual system (link). Our objective is to build a large display by taking advantage of POV using precision activated LEDs and rotational mechanics.

Problem Definition
The University marching band is aiming to improve their halftime shows using LED based devices, and requires a large, low cost display that is both entertaining in form and execution of display content.

Project Requirements

 * The diameter of the visual display should be at least two meters.
 * Capable of maintaining rotational speeds necessary for POV.
 * The machine should be weather resistant.
 * It should not require more than two people to transport safely.
 * The electronics should not require any external power sources.
 * LED colors should be capable of University theming.
 * Display content should be programmable by anyone given instructions.
 * The display's on-time should be at least 10 minutes in duration.
 * A single POV machine should cost around $250.
 * Write instructional documentation to assemble a POV machine.
 * Build six POV machines to display the letters "UIDAHO".

Background
Currently, the University of Idaho marching band has been using LED electronics to improve their half time shows. As their performances become more extravagant, so do their LED props. Our task is to build a visual display using ordinary LEDs that boasts an entertaining design. POV is particularly elegant in this respect, as it can provide an avenue of visual entertainment without using a typical store bought display. The project will implement the concept of POV by spinning a strip of LEDs in a circular motion at high speeds. The rotational movement combined with precision blinking of LEDs creates a visible image, similar to a regular TV screen. This design however is much cheaper, more robust, and a unique display of entertainment. Our objective will be to create six 6' diameter POV machines. These machines will be used by the University of Idaho Marching Band for the purposes of halftime football game performances to display entertainment content.

Project Learning
As our project progresses in development, we solve one problem after another. The following is a categorial breakdown of the project learning for each major component.

Mechanics

 * Spokes
 * Mobility
 * Rotational Mechanics

Electronics

 * LEDs and Imaging
 * RPM, Calculations, and Code
 * Supplying Power
 * Half of our electronics are composed of a power circuit which safely supplies the LED strip and trinket with the juice they need. Our first step was to match up the power requirements of the electronics with the specifications of a battery, and build a battery pack from there. Typically two meters of LEDs will draw 5V at 8A of continuous discharge if they're on at 100% brightness and using the color white. Our microcontroller used to send data to the LEDs which display the image draws about 250mA at 5V. Since our power requirements should have a fairly large margin of error, the target we set was about 5v at 10A of continuous discharge as an acceptable battery pack.
 * Lithium Ion batteries' are very power dense and are excellent at storing and discharging extreme amounts of electricity. A typical lithium ion cell runs at an average of 3.7V however, which is not exactly our target voltage. To reach 5V, a voltage regulating circuit is required. It's possible to regulate 3.7V up to 5V, but in a similar way we can use two lithium ion cells in series to regulate 7.4V down to 5V. The latter is often more efficient and easier to do. The lithium cells available can supply 3400mAh, with a safe discharge rate of 2C at about 6A. A pack of four cells with two sets of batteries in series, and the two sets connected in parallel, can deliver 12A at 7.4V for over 30 minutes.
 * Our power delivery circuit was originally going to be a pre-designed circuit from TI, and would cost more than we thought necessary. This circuit would discharge exactly 8A of the 12A capable from the battery pack, and would be enough for the LED strips. Because of it's cost and because the output of 8A did not allow for much margin of error, we decided to come up with another solution.
 * The next power circuit is far cheaper and could be built in house. It's composed of several smaller linear voltage regulators, each which power their own segment small segment of LEDs. Essentially our LED strip is now cut into 8 total pieces, each of which are flush and lined up next each other. The benefit being we can supply each portion individually with a single linear voltage regulator. Using a total of 10 regulators, we can supply 10A of power throughout the strip, and be comfortably within our margin of error.