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How to dim an LED array

  1. Jul 6, 2009 #1
    Hi. I've got a LED dimmer circuit. When the dc power turns on, the LED will slowly lights up.
    There is a switch in the circuit that if I "open" the switch, the LED will slowly turns off.

    The million dollar question is how do I remove the switch and the LED will slowly turns off when the power is cut off.

    Thanks!
     
  2. jcsd
  3. Jul 6, 2009 #2

    vk6kro

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    Hi. I've got a LED dimmer circuit. When the dc power turns on, the LED will slowly lights up.
    There is a switch in the circuit that if I "open" the switch, the LED will slowly turns off.

    The million dollar question is how do I remove the switch and the LED will slowly turns off when the power is cut off.


    I have read this many times and still don't know what you are asking.
    Please explain why you are doing this. Is the circuit doing something else or is it just dimming LEDs?
    Is the power going to be removed completely or is it still available to light the LEDs?
     
  4. Jul 6, 2009 #3
    is there a coil in series with the led? This would cause a delay on/off.
     
  5. Jul 6, 2009 #4

    negitron

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    Excuse me?
     
  6. Jul 6, 2009 #5
    This sounds very straight forward, but you need to describe the circuit completely. Is it a PWM (pulse width modulation) dimmer, and what is the circuit (e.g., NE555, etc)? How many LEDs? Series or parallel? I suspect a big capacitor someplace is sufficient.
     
  7. Jul 10, 2009 #6
    There is no analog dial for the dimmer. The LEDs are connected in parallel. There is a AC to DC power supply. 12Vdc & 8.5A.
    Basically it is AC converted to DC. When AC power is supplied, the LEDs will slowly light up. When AC power is cut off, the DC power will be cut off too and the LEDs will go off immediately.
    I'm trying to find a way allow the LED to "shut down" slowly. This project is for my fish tank.

    http://i160.photobucket.com/albums/t172/waterbottle_23/circuit.jpg

    The "switch" is normally closed. If you "open" the switch then, the light will slowly shut off. I would like to have the switch in always closed mode and when the AC power is cut off, the LEDs will shut off slowly. will adding a capacitor across the switch do the trick?
    Thanks for all the people who responds to my thread. Really appreciate it!
     
  8. Jul 10, 2009 #7
    There's a ceramic wrapped resistor in series with the LED.
     
  9. Jul 10, 2009 #8
    The power is removed completely and the LEDs will also shut off completely. The LEDs will light up slowly to full brightness. The only issue is I want the LEDs to slowly dim down, then fully off. But right now it will only shut off immediately when power is removed. There is a switch in the circuit that if it is opened, the LEDs will slowly dimmed n shut off. However, I want the leds to dim down n shutff "automatically" without me toggling the switch.
     
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