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Mini POV Speed
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Mini POV Speed

by crusaderabbit on Mon Jul 23, 2018 11:58 pm

Hi, I want to incorporate a mini POV into a project I'm making. I'm hoping you can tell me how fast the mini POV needs to be going, in cm/sec or inches/sec, for the images to be intelligible to the human eye. I would like to use it in real time, not to make photographes. Thank you.

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Re: Mini POV Speed

by kcl1s on Tue Jul 24, 2018 7:50 am

According to this page the speed is adjustable https://learn.adafruit.com/minipov4-diy-full-color-persistence-of-vision-light-painting-kit/more-usage-tips. I have not used this product but have created simple handheld POV wands from scratch. Those needed to be moved quite fast to see the text by eye. I can only estimate but I think 50 - 100 in/sec is needed. You will not be able to slow it down too much or the POV effect will be lost.

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Keith

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Re: Mini POV Speed

by crusaderabbit on Tue Jul 24, 2018 8:38 am

Thanks Keith, It is adjustable, which implies a range. I guess the faster it is blinking the slower it needs to be going, and vice versa.
The range you mention sounds about right. I have a mini POV that I assembled. From waving it in circles in front of a mirror until I could read it, I guessed it would need to be going approx 60 in / sec for the blinking rate I had it set at. That's right in your range. Thank you for taking the time to respond. Let's see if Adafruit comes up with a more precise answer, but your's seems like a good one.

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Re: Mini POV Speed

by kcl1s on Tue Jul 24, 2018 9:00 am

I guess the faster it is blinking the slower it needs to be going, and vice versa.


I think just the opposite. At each refresh (blink) the wand needs to move ahead one pixel width. More blinks more moving (faster). Lower the refresh and less moving (slower). At some point as you reduce the refresh your eyes/brain are going to stop being fooled and just see flashing dots.

Keith

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Re: Mini POV Speed

by crusaderabbit on Tue Jul 24, 2018 9:24 am

I wondered if my assumption was correct. I was thinking that it was a series of blinks that made up the different letters in my message, which it is, but it is not only the blinking by itself that is creating the letters. It's the refresh plus the motion. Once the light sequence has changed (a blink), then some travel is required to create the illusion, right? Thanks.

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Re: Mini POV Speed

by kcl1s on Tue Jul 24, 2018 10:17 am

OK I did a little Googling on the subject of POV. There are a lot of different ideas out there. Most agree our eyes retain a light image (dot in our case) for 1/25 of a second or 40 ms. Those doing rotating POV try to get the whole image in that 40 ms by rotating at 1500 rpm (25/sec x 60 sec). This is mainly done to prevent flicker in the image. TV and film use 25 frames / sec or more for that reason. I think with wands it is different. All we need in those 40 ms is enough information for our brain to interpret the flashes as a group of letters. How many letters that is the question. I remember being somewhere and having an obstructed view of a scrolling marque and having a hard time figuring out the message with maybe 6 to 8 letters visible as it scrolled by. OK enough rambling got to get to work.

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Re: Mini POV Speed

by crusaderabbit on Tue Jul 24, 2018 10:23 am

Good job! I looked for some info but didn't find anything like you did. Great!

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Re: Mini POV Speed

by adafruit_support_carter on Tue Jul 24, 2018 10:51 am

Once the light sequence has changed (a blink), then some travel is required to create the illusion, right?

Correct. That's the general idea for how it works. So the faster the blink rate, the quicker you must move it so that you're in a different position to show the next set of "pixels".

The rate of travel doesn't need to be super precise. You could get a rough estimate using:
MOVE RATE = (PIXEL WIDTH) * (BLINK RATE)
Where pixel width would be the physical size of the LEDs of the POV and blink rate would be how many times per second the LEDs change. The mini POV uses the piranha LEDS:
https://www.adafruit.com/product/1451
which are ~0.3" wide. The blink rate is adjustable:
https://learn.adafruit.com/minipov4-diy ... -speed-5-1
but the details of how the rate is set in the software are a little complex. It uses a timer compare to drive an ISR:
https://github.com/adafruit/Adafruit-Mi ... e.ino#L133

*this is based on a quick scan of code, I may have missed a detail or two so may not be 100% correct*
driven by 12MHz external crystal
prescaler is set to clkio/1024 (0x05)
compare value ranges from 100 to 1000

@100:
BLINK RATE = 12000000 / (1024 * 100) = 117Hz
@1000:
BLINK RATE = 12000000 / (1024 * 1000) = 11.7Hz

@117Hz:
MOVE RATE = 0.3" * 117Hz = 35 inches / second
@11.7Hz:
MOVE RATE = 0.3" * 11.7Hz = 3.5 inches / second

So, you need to move it somewhere between ~4 to ~35 inches / second.

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Re: Mini POV Speed

by crusaderabbit on Tue Jul 24, 2018 11:08 am

You are awesome! What a great tutorial! I'll be doing homework for a month to get to the bottom of everything you said. I love it! Thanks for showing me how to calculate this for myself... and the answer. At that speed I can totally use it in my project. Thanks.

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