32 Led Chaser Resistor Question

Discussion in 'The Projects Forum' started by wwalch, Feb 4, 2012.

  1. wwalch

    Thread Starter New Member

    Jan 10, 2012
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    0
    Hello,

    I’m new to all this. My goal is to create a rotating chaser circuit to light a large scale model of the Jupiter 2 spacecraft from “Lost in Space”.

    The idea is to have 32 Led’s in a chaser effect with 4 channels of 8 Led’s at a time. The Led’s are laid out in a circle of with 8 groups of 4 Led’s. Based on the arrangement it would be challenging to wire them in series. I will be using a 9-volt plug- in power supply with an output of 1-amp. There will be other lights… for the interior of the ship.

    My question is resistors on the positive side of the Led’s. I’m unclear if I should treat each Led as if it’s wired separately and use 1 resistor per Led or if I’m actually using a parallel arrangement. My 680 ohm resistors are based on a 10 mA led that drops 2.2 volts.

    A very basic question but I’m confused. Finally any other issues or advice on my circuit would be appreciated.

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    First, you have multiple outputs of the 4017 wired together - that's a no-no. Leave Q5 through Q9 disconnected. All INPUTS to CMOS logic ICs must have a current path to Vdd or Vss, but unused OUTPUTS are left disconnected.

    If you're operating LEDs in parallel, you need one resistor per LED.

    However, since you are using a 9v supply and your LEDs have a Vf of 2.2v, you could wire them in strings.
    Two strings of three, and one string of two LEDs per 8-LED group.
    The 3-LED strings would need 240 Ohm resistors.
    The 2-LED strings would need 460 Ohm resistors; that's not a standard value, but you can use 470 Ohms instead.
     
    Last edited: Feb 4, 2012
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want the leds to have equal intensity, you could use only 2 led strings, as SgtWookie suggested.

    Bertus
     
  4. wwalch

    Thread Starter New Member

    Jan 10, 2012
    3
    0
    Thanks all. I will change the schematic so that Q5 - Q9 are not connected. I'll experiment with the Led's in string but was I correct that each Led would need the 680 resistor?

    Thanks again - this forum is a great resource.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, 680 Ohms was correct for one LED.

    Bertus,
    4 LEDs in series leaves only 200mV for the resistor, which really is not enough. Current regulation would not be good over tenperature.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    I was meaning to use 4 ledstrings of 2 leds with the 470 Ohms resistors..

    Sorry for the misunderstanding.

    Bertus
     
  7. wwalch

    Thread Starter New Member

    Jan 10, 2012
    3
    0
    Thanks again. Based on the - I think I'm going with 1 resistor for each Led. More work but I don't want to have to tear the model apart if something goes wrong.
     
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