POV display

From OpenCircuits
Jump to navigation Jump to search

POV display[edit]

A "persistence of vision display" (POV display) has only a single line of LEDs blinking on and off. The display mechanically sweeps it across a person's field of view, giving the illusion of a 2D display.

(Do I need a diagram or a picture here?)

(Nearly all LED displays in microwave ovens and alarm clocks electrically sweep the digits across a person's field of view, giving the illusion that it's displaying all 4 digits of the time "12:55" when actually only one digit is illuminated at any one instant).

User:DavidCary is building yet another "POV display".

I plan on putting a few features into my display that I haven't seen in any other so far:

  • bright blue LEDs (even though blue is usually the most expensive color ... although sometimes "true green" is even more expensive than blue).
  • no slip rings -- just pumping energy across an air gap
  • ... and a few other features that I'm keeping hush-hush.

I want lots of lumens. Unfortunately, when I go to pick LEDs out of the catalogs, they're not rated in lumens, they're rated in candelas.

Is it even possible to estimate "lumens" from the catalog information?

You can use this table to get an approximate conversion from candelas to lumens. Find your LED beam width in degrees, and divide the candelas number in your specs by the cd/lm factor listed for that beam angle to get lumens.
beam angle cd/lm
beam angle = 5 divide candela value by 167.22
beam angle = 10 divide candela value by 41.82
beam angle = 15 divide candela value by 18.60
beam angle = 20 divide candela value by 10.48
beam angle = 25 divide candela value by 6.71
beam angle = 30 divide candela value by 4.67
beam angle = 35 divide candela value by 3.44
beam angle = 40 divide candela value by 2.64
beam angle = 45 divide candela value by 2.09

Hope this helps.....Dr. Andrew Thomas...

Do you know what the difference is? Does your flashlight really produce 10 times the light ( in lumens)? Or does the flashlight merely focus its light on a tiny spot, so that that spot gets 10 times as many candelas ?

For example, check out these 2 LEDs: $8.75 SSP-LX6144C7UC : 4000 mcd at 120 mA $8.75 SSP-LX6144D7UC : 1800 mcd at 120 mA

From the mcd rating, it appears that the first one is more than 2wice as bright -- and it is, if you're directly in front of it when you look at it. However, if you're even the tiniest bit off-center, the second one is much brighter -- in fact, the total lumens that second LED puts out (2500 mlm) is slightly more than the total lumens than the first one. If you diffuse the light and try to light up a whole room with an array of them, the second one will make the room brighter.

I'm currently planning on using these in my first POV display:

  • $1.32 Telux TLWB7900 : blue : 330 mlm, 231 mcd at 50 mA. (price in ones from http://Newark.com/ )

Other LEDs I considered using in my POV display (and may re-consider for my next one):

  • $18.89 "Lamina light engine" BL-22B1-0140 : 22000 mlm at 420 mA. (this price includes the required heat sink -- price in ones from http://Digikey.com/ ). This has the most lumens per dollar I've found so far (for blue LEDs).
  • $8.750 Sunbrite LuxLEDs SSP-LX6144D7UC blue: 120 mA, 2500 mlm, 1800 mcd (Odd that the red Sunbrite LuxLEDs are cheaper at Newark, the other colors cheaper at Digikey.) cheapest $/lumen blue LED, except for the "light engine"
  • $2.550 Lumex "DSP LED" 67-1876-ND: blue, 5 mm, 2500 mcd ( DigiKey ) apparently have some kind of internal chip. All colors run at 2.0 V nominal (1.5 V minimum) (rather than running on current like most LEDs. unfortunately, the data sheet doesn't suggest how much current they take -- I presume more than 20 mA).
  • $1.600 Sunbrite "based LED" SSP-01TWB7UWB12 (441-1007-ND) blue 10 mm 20 mA 7000 mcd
  • $5.980 GM5WA06270A SMT RGB (35 mA red, 35 mA green, 35 mA blue) 3000 mcd (full color range -- apparently used for digital camera flash ?) (Digikey)
  • Mouser: hi-power LEDs ( http://www.mouser.com/search/refine.aspx?Ntt=LEDs+hi-power ) shows "lumens" directly -- exactly what I wanted.

further reading[edit]

Many POV devices are based on the PIC, see the PIC Links and search on POV.
Spoke-POV and propeller clocks links:

While most POV displays spin the LEDs in a complete circle, some "wiggle" the LEDs back and forth:

Is this http://led-display-and-design-swicki.eurekster.com/ relevant?

I think so-- 18:06, 22 June 2007 (PDT)

  • "the Orbital Rendersphere"[4][5][6][7][8], aka the "Big Giant Spinning Limb Slicer".

The Orbital Rendersphere displays images and videos on a four-foot diameter spherical surface using persistence of vision (POV). Uses a (stationary) Mac Mini to render each frame, and a (spinning) BeagleBone Black to update the LEDs. Slip ring for power; Wi-Fi to transfer data. The Orbital Rendersphere displays video at 30 frames per second by spinning four vertical LED strips spaced 90 degrees apart at 450 RPM. 36 feet of WS80211 LED strips. (But it looks like 2 complete circles, each 4 ft diameter -- isn't that closer to 25 feet?) 102x224 pixel resolution. Several people comment that this is the biggest POV globe they've ever seen. It's mentioned on p. 1-44 of John Baichtal, "Robot Builder"[9]. Uses LEDscape[10].

  • Mike Szczys. "Full-color video on a spinning POV display". Built by Félix, Sylvain, and Jérémy at at Telecom ParisTech. highest-resolution POV display I've seen so far. "The time it took us was roughly two months full time". (Does this have a name?)
  • "Borg Ventilator". Twice the resolution of the Telecom ParisTech display? A total of 244 RGB LEDs (spread over 4 wings). Has standard VGA input; a Xilinx FPGA and a bunch of support chips convert to circular coordinates. Power through slip rings. Data through non-contact inductive transfer (recycled VCR head). "The speed is infinitely adjustable between 0 and 2500 rpm / min. The FPGA adapts to the actual speed"
  • Collin Cunningham. "Ceiling fan POV display"[13][14]. A single ATMEGA328 and 20 74HC595 shift registers to drive the LEDs. 32 LEDs per fan blade x 5 blades = 160 LEDs.
  • Gareth Branwyn. "MonkeyLectric 3D spherical POV display"[15][16]. by Dan Goldwater / MonkeyLectric.

Mentions "we recently decided to move this project to the Raspberry Pi board away from Arduino. The capabilities and processing power of the Pi are much greater than Arduino. ... [but] ... the refresh rate is too slow, much slower then the Arduino was. We need some help figuring out how to turn up the SPI speed with our current software arrangement. ... used Phillip Burgess' at Adafruit 'Light Painting' Python script to process images and output to the LPD8806 http://learn.adafruit.com/light-painting-with-raspberry-pi/software "

  • PixelPi: RGB Pixel Array Driver for the Raspberry Pi[17]. Based on Phillip Burgess 'Light Painting' Python script. Apparently supports many different POV hardware displays.

(FIXME: make a separate section for bicycle POV displays)

  • Hazard. "RGBike POV - Open project". Arduino compatible; 16 RGB LEDs; hall effect sensor; through-hole components and single-layer PCB for easy fabrication.

It might be nice to make a spinning electronics page to describe the many weird and wonderful ways that people (a) get power to the spinning electronics, and (b) transmit data to and from the spinning electronics. POV displays are the most popular kind of spinning electronics, but the same approaches are useful for other kinds of spinning electronics such as run-time tire pressure sensors and bioreactors.