I’ve been thinking about Persistence of Vision (POV) projects lately. I want to make a rotating cylinder POV device eventually that has a multi-color display.
In preparation for that project I have been watching some videos and reading up on POV. I’m presenting some of these references for your enjoyment as well.
Lets start off with Mr. Wizzard ,Don Herbert, introduction to POV.
The Wikipedia article is also pretty informative.
The Wikipedia article talks about POV displays constructed of 2D lights that repeat in the same place every couple of milliseconds. So lets start with a 2 millisecond refresh for an LED on a rotating device.
(1000 ms / s) / 2 ms = 500 Hz or 500 rotations per second. Sounds okay right?
Lets turn that into revolutions per minute.
500 Hz (or 500 revolutions / second) * 60 seconds / min = 30,000 RPM. What 30,000 RPM? Something must be wrong here. So lets keep looking for more info. The same article talks about modern film with a refresh rate (frame rate) of 24 Frames / Second and up. I’m not sure that comparing film, where the images is changing constantly and an led display, where we want character display persistence is fair. But lets try anyway.
24 FPS = 24 Hz = 24 Cycles / Second * 60 Seconds / Minute = 2,400 RPM. Now that sounds reasonable. But lets keep checking. Lets look at some videos and related articles if we can find any.
I like this one for the simplicity and that the parts are easy to obtain.
While this design is simple and seems easy to build the density of LEDs could be improved using LEDs based on the WS2812 device. At 5.0 mm per LED these chips can be tightly packed resulting in `2 LEDS / cm or 5 / inch. If you go looking for LEDs based on the WS2812 chip you will find that Adafruit calls their versions of these NeoPixels.
The above video references an article that lists the fan part as 3,800 RPM. So lets check this now:
3,800 RPM / 60 Seconds / Minute = 63 Hz.
At 63 Hz the refresh time is 1000 ms / 63 cps = 15.7 ms for the image passing by our eyes in the same place.
Okay so 2,400 RPM to 3,800 RPM might be more reasonable. I don’t think we need 30,000 RPM and have plenty of motors lying around that can spin at 3,000 to 4,000 RPM.
This video shows a nifty clock display and they say that they are using a hard disk drive motor (HDD) to spin the lights. Also the board has a nice LED density.
So how fast does a HDD spin? So 7,200 RPM and of course higher.
Now lets go looking for spinning vertical cylinder POVs.
When you watch this video you will see what is possible. To the best I can tell this is a commercial product. Regardless, this shows what a high quality vertical display would look like. If you pause the video at 7 seconds you can get a good look at the whole device from the side. It looks like there are three columns of LEDs in one vertical section. Advancing to 11 seconds you will see there are three vertical sections with three columns of LEDs. The extreme information (pixel) density of this display is a result of 9 total columns of LEDs.
This one looks monochromatic and 5 seconds in you can see how it is constructed. Very simple design from a spinning column standpoint. I wish I knew what the rotational speed is of this device.
The final version of the above POV device can be seen here. At 22 seconds you can see the base controls. Rotor, Motor and Hauptsch which is german for “Current Main”. So the builder is using main (house) current to drive the motor of this one. At 2:10 you can see 2 flat ones and a vertical one.
One more flat one. This one is packaged nicely.
And now a globe. I love the way he can “draw” on the globe.
The maker describes the project details in the Globe project information.
In the forum data is a statement about the rotational speed of the globe. “It’s round about 10-20 Turns per second (depending on the motor voltage).”
That would br 600 RPM to 1,200 RPM.
So now I can start to work on a design for a flat display (horizontal) so I can work out the electronics and the programming. Then, if time and money permit, maybe I’ll do a vertical display.