My Dad's led project

Discussion in 'The Projects Forum' started by Doc Brown, May 7, 2012.

  1. Doc Brown

    Thread Starter New Member

    May 1, 2012
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    Hi Fellas, new here and new to the cool world of electronics but have been reading and studying my butt off.

    I built a fading display case out of acrylic for my Dad. I purchased a led driver/controller from ebay that comes with led ribbon. I installed everything and it works as it should except my Dad wants it to fade in and out with variable duration on just one color(blue)...it's an rgb led strip.

    I figured the best way to do this would be to bypass controller and just use the supplied power supply 12v into a project box with a pot for controlling fade speed. My reading and studying has led me to this logic which could very well be wrong:

    power supply>555 timer>n channel mosfet>led strip ?????

    The led strip is common anode as the strip has +12v on one contact next to the red green and blue contacts on end of strip which hooks to controller.

    My thinking would be to use the 555 timer to pwm the gate on the transistor which would in turn vary the current available to the led strip???? I'm thinking 5 to 10 second fade in/ fade out duration.

    I thought I could use a pot to control the fade in/ fade out time of the led strip...

    My other thought was to come out of timer to mosfet to pulse width modulate the +12 volts that is the common anode to all the led's ????

    I am not sure which direction to take, but I think it would be best to apply pwm to mosfet gate to vary the +12v from high to low( I am assuming the Vf of the blue led's in the string is low, or whatever the minimum voltage needed is)

    Can I use a power mosfet in this fashion? Am I on the right track? I really appreciate any input from the pros here as I am learning as fast as I can and really enjoy electronics. It's pretty fascinating to me. I am a master electrician and own an electrical contracting company but I am finding out just how dumb I am when it comes to electronics the more I learn and dig into it. Thanks guys/gals. Nate.:)
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    To fade in/out LED u need to PWM drive the LED, there is no other way.

    The 555 > some logic > mosfet drive > led cathode will work.

    This method is for beginners.

    Something I did. Read this. U might be interested, It's simple and very effective. U can use any no. of leds. as much as u want.

    U can even modify the code for ur need if u know programming .
     
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  3. Wendy

    Moderator

    Mar 24, 2008
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  4. wayneh

    Expert

    Sep 9, 2010
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    You might take a look at using a second, slow moving 555 (such as the second timer on a 556) to control the "control" pin on the PWM timer. Changing the voltage applied to the control pin changes the frequency - which really wouldn't matter to you much - but also changes the duty cycle. Changing the duty cycle would in turn give you the fading in and out that you want. No idea if the effect would be the pleasant one you're looking for, but it's a simple circuit to implement. [edit] Last time I calculated this out, I got a max possible fade range of from about 20% to 70% duty cycle.
     
    Last edited: May 8, 2012
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  5. Doc Brown

    Thread Starter New Member

    May 1, 2012
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    Thank you fellas! I will do some more research and studying. I really appreciate the input from all. Nate.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Boworing from The Sgt. & wayneh gives a 0- 100% duty cycle using two sawtooth generators; fast about 500 Hz, slow 20 sec. Change C2 to 1μF for 2 sec range. Use driver compatable with load- logic level MOSFET?
    Darn -Idid it again.
     
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  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Maybe I'll slow down & think which key to hit.
     
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  8. Doc Brown

    Thread Starter New Member

    May 1, 2012
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    Hi Bernard. The voltage of the led ribbon is 12 volts. Would the output of this circuit be used to drive the gate of a power mosfet? Which in turn would be supplied by 12volts? Thanks Bernard...
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Do you know the current drain of the Blue strip? #1 posibility: Connect anode to +12V,logic level FET, Drain to Blue, Source to ground, Gate to U1b output. Open for comments- I do not believe gate capacity should be a problem at approx. 500Hz
    #2: Raise Vcc from 5V to 12V, increase R4,6,& 10 to 3.3K to cut drain, use std. FET.
     
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