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POV globe

Discussion in 'General Electronics Discussion' started by Joho, Nov 28, 2010.

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  1. Joho

    Joho

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    Nov 28, 2010
    Hello everybody,

    For my first appearance on this forum I have a lot of questions and it would be kind of you if you could give me a few answers. Also forgive my bad English (I'm French, and everyone knows that French people aren't very good in English :)).

    So like I've said in the title I have to do the following project: a Persistence Of Vision (POV) globe. You may find quite a lot of videos on the net, also I've found this link where someone explains how he did his POV globe : here.

    So I gave it some thought and I've come out with a few ideas but also with question marks:

    In the link I've put, he uses an atmega328P, however I would like to be able to command a maximum number of LEDs so I want to use a µcontroler which has a lot of outputs, is the atmega328P the best choice?

    Forgive my ignorance but I'm quite confused concerning the functions of a µcontroller, for instance with the atmega328P can we save the picture we want to display in the µcontroler, and if so is it saved in the RAM? The program being saved in the ROM is that right?

    Also another question concerns the choice of the LEDs. If I use a Led with only one colour, would it be possible to control the intensity of each LED (for instance have dark blue and light blue).

    Is it possible to use as much RGB LEDs as mono-colour ones? I guess not because for the RGB LED I would have to control the intensity of the three-colour whereas with the mono-colour led I would only have to control the intensity of one colour.

    There's also a problem with the feeding of the rotating system (because the circuit will be turning). In the link he uses a jack cable to do so however I would like other suggestions, for instance I've found this link sorry it's in French, but if you've got an idea of what it is in English is it reliable? Feeding a rotating system

    That's all :D sorry it's a bit long...
     
    Last edited: Nov 28, 2010
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    It's not the best design I've seen.

    I would recommend a hall effect sensor rather than a reed switch for sensing position, and coupling the power inductively rather than via a 3.5mm plug and socket. I will admit the 3.5mm plug and socket is an interesting solution, but if I went that way I would make sure that both parts were easily replaceable.

    I would suggest that the image is part of the program, stored in the eeprom that stores the program.

    You could control the apparent brightness of the LEDs by varying the ON time. For esample, suppose the LEDs are turned on for 100uS each ms (they are multiplexed in groups of 8). You could change the programming to turn on the LED for the full 100uS, or change the bit pattern to make it 10uS on and 90 uS off. This will be more tricky if you want several LEDs to be at different intensity in the same group.

    You could use RGB LEDs. Remember that each LED will then require 3 signals, effectively increasing your need for more control pins. You should probably consider having larger banks of LEDs rather than more banks as the former reduces the skewing effect of rotation, and allows higher average brightness.

    Remember that since you are turning each bank on one at a time, you need to cycle through the banks very quickly (and also several times for each dot column or you will see a staggered row of dots rather than a nice clean line.
     
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