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

LEDs and Imaging
There are two main ways to display an image using persistence of vision. Both have useful applications and will be implemented at the convenience of the end user. The difference mostly comes with the preparation of the source image and its manipulation.

Imaging Display I
The first is a direct image display and typically results in a warped image. This is often used to display clocks and character based information. The content is physically wrapped around the display and in relation to the center of the wheel. Images are often described as donut shaped. If an image were displayed above the center, it would mostly appear normal. If it were displayed below the center, it would be flipped upside down. This is because of the nature of displaying quadratic pixel coordinates using a polar coordinate system.

Polar Conversion Display
The second method provides an unwarped image and can be displayed across the entire display without the consequence of warping or transformed relation based on the wheel's center. Getting this kind of image requires modifications to the source file. The basic concept is to treat a given image as if it were already drawn in polar coordinates, translating it to rectangular using software, and then allowing the illusion mechanics of the wheel's rotation to translate the image back into polar. This typically involves cropping the image into a circle, reducing its colors, and performing a translation to polar. Once done, the image can be displayed relatively clearly and across the entire POV machine's display.

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. Two major approaches stemmed from here. The second is the product of pursuing the first.

Power Concept I
Since our power requirements are fairly high, we should have a fairly large margin of error and the target we set was 5v at roughly 10A of continuous discharge as an acceptable power supply.

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.

It became apparent that a single, high ampere voltage regulator was more difficult to build than our team was capable. To mediate this, we came up with the concept of splitting the LED strip into smaller strips, and use several common low ampere regulators to power each individual section. This approach would employ 10 1A 5V linear voltage regulators and cut the LED strips into 8 pieces total. This supplies our target of 10A to the LEDs and meets our target settings.

Power Concept II
The power draw of an LED strip is relatively high while on at 100%. As previously mentioned it draws 8A at 5V. This concept does not hold for our situation however. Theoretically, by employing persistence of vision the LEDs draw a sixth or less than their maximum rated power draw. We only reach a sixth of the power draw when a blank white image is displayed at 100%. This is due to the precision blinking necessary to make an image look the way it does. At most, an LED is given power 60 times per second to create the POV illusion. Based on the time and power supplied to each LED at the time of illumination, the on-time for any given LED is almost exactly 1/6th of a second, and draws 1/6th of it's typical power draw.

This easily allows the use a single lithium cell to reach the current draw required. A 3.3V LED strip will be used in place of the 5V strip, and a power regulator will boost the cell voltage to 5V for the trinket microcontroller.