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Did not know where else to ask about this, so if it is off-topic I would gladly take some pointers as to where the correct place would be. I would like to make one of those spinning LED fans that can project an image that looks like a hologram. I am not looking for anything with big resolution, just want to try it out, see if it is possible. So I have a couple of questions:

  1. Is 30 Hz frequency for the fan enough to be able to see images without lag? I'm basing this off of the fact that 30 fps is needed for a video to be smooth for a human, so if a fan is spinning 30 times in a second, that means a human won't be able to see it? If this is correct, would a 6VDC 340mA motor which can spin at 14400rpm (thats 240 Hz) be enough?

  2. What LED density should the LED matrix on the fan be in order for the images be recognizable? Or is a LED strip a better idea?

  3. I'm thinking that the controls should go to an Arduino. Is that at least possible? I think I should be able to divide a circle into 30 parts and control which LED's should be on in each section of a revolution.

EDIT: Let's say I want to control an 4x8 RGB LED matrix (something like this https://www.adafruit.com/product/2945). I want to plot simple pictures like an arrow or a drawn cat face. While searching I thought the MAX72XX ICs would work for this? The picture I think would be sent via USB (or something similar because MAX72XX support SPI) form the computer, one at a time.

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  • \$\begingroup\$ I have no idea what kind of item you are talking about (a pic or video would be nice), but if it's about setting n RGB leds in a matrix to a certain value once every 33ms, then even an Arduino would keep up. If it involves some manner of live decoding of a video feed or such, then you should go for a modern MCU instead of Arduino. It all boils down to what the source of the picture/video is coming from. \$\endgroup\$
    – Lundin
    Commented Feb 6, 2020 at 12:47
  • \$\begingroup\$ @Lundin Googling the title yields a lot of results, but an commercial example would be this \$\endgroup\$
    – Daniel Zaksevski
    Commented Feb 6, 2020 at 12:51
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    \$\begingroup\$ Ok that one with SD card, file formats, Wi-fi and the whole ballet included is likely running some PC-like computer similar to Rasp Pi, or possibly a DSP. The file decoding will be done live. No corn flakes 8 bit MCU in that one. \$\endgroup\$
    – Lundin
    Commented Feb 6, 2020 at 12:56
  • \$\begingroup\$ Ok well what if the image will be processed on a PC and sent through a USB cable? \$\endgroup\$
    – Daniel Zaksevski
    Commented Feb 6, 2020 at 13:18
  • \$\begingroup\$ I wouldn't have picked Arduino regardless. It's possible that a mid-range MCU could do it, but you need to work through the whole design spec and determine enough details before picking a MCU. How to drive the leds, if/how they should be PWM:ed, etc etc. \$\endgroup\$
    – Lundin
    Commented Feb 6, 2020 at 13:34

1 Answer 1

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Here is a picture of a simple toy clock that runs off a USB port.

enter image description here

The picture was taken at 1/40 sec, I didn't quite get one revolution. So about 1/30 would be one revolution.

The LEDs are on only one blade. The minimum arc that I see is about 2.8 degrees, so the LEDs are updated about 128 times per revolution (360/2.8).

Therefore, the LEDs are updated about 3840 times per second (30 * 128).

Now this is just a small toy. Something larger will need to update at a smaller angle. You will need to do some geometry calculations to get an update rate for a whatever "pixel" size you choose.

Realize that making something close to photo-realistic from scratch is a massive job.

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    \$\begingroup\$ +1 for working out all the timing details from one photo exposure time. \$\endgroup\$
    – Transistor
    Commented Feb 6, 2020 at 13:50
  • \$\begingroup\$ Would using 4 blades with separate drivers be easier? It would divide the number of updates per second by 4, right? If not, are update speed like this achievable with consumer grade electronics? \$\endgroup\$
    – Daniel Zaksevski
    Commented Feb 6, 2020 at 14:01
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    \$\begingroup\$ No this would multiply the number of updates per second, since you're driving four times the number of elements. An FPGA could easily do this type of speed, but that has a very steep learning curve (a whole other, advanced field of electronics.) \$\endgroup\$
    – rdtsc
    Commented Feb 6, 2020 at 14:08
  • \$\begingroup\$ @DanielZaksevski - You need to define your end goal better before people can guess at what processing capability you need. This toy could be done with an Arduino. Something larger, with still images, close to photo-realistic, probably with a higher-end microcontroller. Video? I won't even speculate. \$\endgroup\$
    – Mattman944
    Commented Feb 6, 2020 at 14:16

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