Ramping up brightness alternating flash LED Circuit

Discussion in 'The Projects Forum' started by rockenrock, Oct 24, 2010.

  1. rockenrock

    Thread Starter New Member

    Oct 24, 2010
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    Can the alternating flashing circuit of http://forum.allaboutcircuits.com/showthread.php?t=29092 be adjusted to make each LED alternately ramp up to full brightness, then turn off? So one LED ramps up, turns off and then the other LED ramps up, turns off... onward. I want ramp up time to be 1 to 1.5 seconds. I am only powering one LED on each side. I do not want an IC circuit. Thanks!
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Not ramp up, like a fader, it is a digital circuit. Each LED will turn on to it's full brightness, and stay there for its full duration. When the other one switches on the same thing will happen.
     
  3. rockenrock

    Thread Starter New Member

    Oct 24, 2010
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    Yes, I agree that circuit is either on or off. I need to have the LED slowly get brighter (in about 1 second). How can we change the circuit to control the current so the LEDs will light up slowly? Thanks.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    There isn't a simple way to "fade in" an LED, it requires PWM (Pulse Width Modulation) that sweeps from 0% to 100%.

    A microcontroller can do this fairly straightforward. Doing the same with discrete components would be a bit of a headache, though I'd guess there is some solution using a 555 to sweep the PWM range somewhere, I just haven't seen it.
     
  5. gizmoman0

    New Member

    Jan 21, 2010
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    you can implement an op amp as an integrator which would ramp up the current to the leds. Set it up in inverting configuration and put a capacitor in the negative feedback loop instead of a resistor. Note a positive input voltage would give you a negative output swing but set up in non inverting if you want it that way. You would also have to voltage divide the input to start low and then build up
     
    Last edited: Oct 24, 2010
  6. Wendy

    Moderator

    Mar 24, 2008
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    LEDs, 555s, Flashers, and Light Chasers (forum edition)

    Chapter 12

    Just finished the article. PWM is the way to go, but it isn't the only way to do it.

    You will need to use an IC however. I can think of one way to do it with your mulitvibrator, but I am out of time and have to go to work. I'll post it tomorrow. Basically it will be a variation of the 1st schematic in the links I posted.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    OK, using LEDS you can "see" the current feeding the bases. I think it will disappoint though, since the waveforms are not as clear cut as you may think they are.

    Remember, a transistor drops 0.7VDC on the base emitter. This is the total amplitude of that signal. In terms of voltage change it simply isn't that linear. Current is a different story, but it is on the small side where all the changes are happening. The collectors are simply on/off.

    [​IMG]
     
  8. mike8675309

    New Member

    Sep 25, 2010
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    Here is a link to a site with a discussion and some LED Fader circuits. These use some oscillators and comparators to create a PWM signal for the LED's generating a Random fade on 4 channels. Perhaps something there will give you an idea on how you could achieve the specifics of what you need.
    http://www.solarbotics.net/library/circuits/bot_ornament_qlf.html

    Note it follows a discussion with circuit revisions, so you get closer and closer to the final circuit as you move down the page.
     
  9. Wendy

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  10. rockenrock

    Thread Starter New Member

    Oct 24, 2010
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    Bill and Mike, Many thanks for your work and advice on this. I was pleasantly surprised by your efforts! I see there is a lot to study, and will be trying both IC and non-IC versions. The only reason for non-IC is cost savings.
     
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