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dimming LED indicator lights

Discussion in 'LEDs and Optoelectronics' started by Bob CT, Jan 16, 2017.

  1. Bob CT

    Bob CT

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    Jan 16, 2017
    Hi guys, first post here and it's a dumb one just to get things started!

    I am replacing a bunch of switch indicator lights on the instrument panel on my boat. My issue is that they are too bright when I'm running at night and I would like to cut the light output by around 75%. I exchanged emails with the manufacturer and they suggested a 470ohm resistor, no change. I progressively tried more and more and I think I had a total of six 470ohm resistors in series but virtually zero change. I really only have room for one resistor but I was just trying to get to a value and go from there.

    I already installed a 12v rotary dimmer switch for my instrument backlighting so I hooked that up to one of the lights as a test. That works great if I have it set at almost max dimming, it would be perfect. The problem is that each indicator light picks up 12v+ from the switch itself when I turn it on. I can't figure out a way to wire all of the indicator lights (11) to the single dimmer and still have them work independently with each switch.

    This is a great option if someone has a wiring suggestion. The switch just has 12v +/- IN and OUT. I have a common ground for each of the indicator lights and as I mentioned, they get 12v+ from each of the eleven switches.

    I emailed the company that makes the switch and asked them if they knew what the approx. resistance would be at the almost all the way dimmed location (or close enough). Their response was:

    "Our dimmer switch works by pulsing the current. It turns the current off and on 135 times per second and then the potentiometer adjusts the ratio between how long it is on versus off in each of those cycles. So there is no resistor that will produce a similar effect. You can dim LEDs by dropping the DC voltage but it is imprecise and usually, for 12v DC LEDs, is a range between 12v and about 8.5 to 9v where the light drops out."

    Any ideas appreciated.

    thanks
    Bob

     
  2. BobK

    BobK

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    What you need is a PWM circuit between the common of all the indicators and ground.

    PWM is the switching on and off method that you menton in you post.

    Bob
     
  3. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Good idea Bob. I was thinking the common could also be tied to a voltage divider to give similar results.
     
  4. Bob CT

    Bob CT

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    Jan 16, 2017
    Hi Bob,

    So, I do have a PWM already, that's what dims the backlighting on my gauges. They're all connected in series though.

    How would I wire up an additional PDM so that each indicator light gets 12v+ from the switch it's connected to? I don't need them do dim individually. Crude diagram attached.

    thanks
    Bob
     

    Attached Files:

    Last edited: Jan 17, 2017
  5. Bob CT

    Bob CT

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    Jan 16, 2017
    orientation fixed
     

    Attached Files:

  6. Alec_t

    Alec_t

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    Jul 7, 2015
    Like this
    ExtraDim.PNG
     
  7. BobK

    BobK

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    Exactly. The beauty of using PWM like this is that the brightness will be the same no matter how many LEDs are on.

    Bob
     
  8. Bob CT

    Bob CT

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    Jan 16, 2017
    Hi Alec, thanks for posting the diagram. Since it's apparent I don't do this everyday, wondering if you could help translate a bit. Can you tell me what "M1" and "NMOS" are?

    In what I drew below, I can easily connect the PWM to two terminal blocks I already have installed. That could take care of the input side. I'm having trouble envisioning the switch side though. Also, it looks like the PWM in your diagram only have 3 connections, where mine has four.

    Thanks,
    Bob
     

    Attached Files:

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  9. BobK

    BobK

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    Nothing changes on the switch side. We are just interrupting the ground connection for all the LEDs.

    Bob
     
  10. Alec_t

    Alec_t

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    M1 = "MOSFET number 1" (purely a label in the schematic).
    NMOS = N-channel MOSFET.
    PWM is just a label for the input of a second PWM source for switching the MOSFET on and off. I haven't shown the PWM source, but it could be an off-the-shelf module or a home-brew circuit. This circuit/module does require that the existing ground connection of your lamps can be broken to insert the MOSFET. Is that possible?
     
  11. Bob CT

    Bob CT

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    Jan 16, 2017
    so, just to clarify. I would be wiring the PWM only on the ground side of my indicator lights (ignore the 12v+ posts on the PWM) and leave 12v+ from indicator light to switch as it currently connected.

    I thought the PWM would require a 12v+ input to operate. If it works with only the ground side connected, that works for me and yes, I can wire it that way.
     
  12. BobK

    BobK

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    Jan 5, 2010
    The PWM controller, which we have not yet discussed, will require a supply voltage as well. 12V would be a good one since that is available.

    Bob
     
  13. Bob CT

    Bob CT

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    Jan 16, 2017
    I'm wondering if this helps. The PWM is not installed but I've tested it with one of the LED lights like you see in the pic. Since the PDM requires a 12v input, I can accomplish that. Where I think I'm stuck is the "output" side of the PWM.

    Going back to my first post, is there a way I can use one PDM to control multiple Indicator lights that are wired individually to multiple switches. The switch provides the 12v+ to the light when the switch is thrown.

    thanks
    Bob
     

    Attached Files:

    • pic.jpg
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  14. KTW

    KTW

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    upload_2017-1-20_20-24-21.png
    I see it like this, using a couple of terminating strips to terminate all the wires .
     
    Last edited: Jan 21, 2017
  15. BobK

    BobK

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    Connect the output of the PWM to the line labelled PWM on the schematic in post #6.

    Bob
     
  16. Alec_t

    Alec_t

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    Jul 7, 2015
    :confused: I'm confused. I assume that thing in the top right of your post #14 pic is the existing rotary dimmer? I thought you were using that for the backlighting; not the indicator LEDs?
     
  17. Bob CT

    Bob CT

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    Jan 16, 2017
    Thank you all for the replies (and editing my diagram). It looks like I have this working just trying a few lights w/alligator clips. This is great and I'll post back and confirm once it's 100% complete.

    Alec, I added an add'l rotary dimmer now. One for my backlighting and one dedicated to the indicator lights. I'm sure this seems crazy on paper but anyone who runs a boat at night will tell you that bright lights completely throw off your vision.

    Thanks!
     
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