Trying to dim LED with POT

Discussion in 'The Projects Forum' started by Deric, Feb 8, 2010.

  1. Deric

    Thread Starter New Member

    Feb 8, 2010
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    I got myself into a little jam. I am a mechanical engineering student who tried to do some electrical projects, and got stuck. I have taken circuits classes, but a few years ago, and i am nowhere near an expert in the field, so i need some help.

    My girlfriend wanted some blue led strips in her car to make it more exciting. I talked her into buying rgb strips so she could use a remote to select between 12 preset colors. Then i talked her into buying a set without a remote (to reduce cost) and having me build a circuit with potentiometers so she could individually adjust the amount of red, blue, and green on her own, to make any desired color. At that point i was a hero and looked like the smartest guy in the world. Now i am alone doing some trial and error and feeling a bit foolish.

    The problem is that i get no adjustment in brightness for the first 85% of a turn of the pot. Then the brightness jumps up to full power. I was hoping to be able to make each color go from 0 to 100% with a single turn of the knob. I want to have three pots, one on the blue line, one on the red, and one on the green, to adjust each individually. Im sure that the pots might be the wrong size or there is more to the circuit.

    I guess my question is... How? I have read a few sites and tutorials and it looks like it wont be as simple as i wanted. but i still dont know what to do. What kind of circuit do i need to build? is it even possible to only use a pot and led in series?
    Here are the led strips 12v...... http://www.xmeteorlight.com/images/LLS-RGB12S.JPG
    and the pots.......................... Bourns 250K http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250561128770&viewitem=&sspagename=STRK:MEWNX:IT

    Thanks for any ideas or suggestions.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    How much current are these LEDs taking?

    You will likely burn up the pot. You need to interface it to some PWM circuitry.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    PWM circuitry is not too hard, you could use a variation of this to power up many LEDs, let me know if you need help and I'll redesign it for your application...

    [​IMG]

    This particular design is a throbber, it will slowly light and dim the LEDs over time. It uses the PWM approach I was mentioning.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Just reread your post, do you have the controller? If so you don't need the pots, it can be programmed. I've seen these at Tanner's, where they were glad to demo them.
     
  4. Deric

    Thread Starter New Member

    Feb 8, 2010
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    No, i do not have the controller, i just wanted to make my own that she could use knobs to make her own color. Looks like your first post has some good info though. Thank you. Ill have to look over it later when im not in a rush!
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    OK, just keep in mind what I said.

    I don't mind drawing schematics.
     
  6. Deric

    Thread Starter New Member

    Feb 8, 2010
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    Will my pots work for this? i dont know what the leds draw, they didnt come with anything but the strip itself.Will your setup allow the leds to go from 0% to 100% smoothly?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I tried to find that LED string on the website, but not certain if I found the right one.

    It would help a great deal if our original poster would provide a link to the page where the specifications are, or at least the order page.

    I did find a page for RGB LED tapes; it looks like they are wired in strings of three, common anode, with a current limiting resistor per string.

    So, you could use PWM on the three cathode lines.

    It would help if you could tell us what resistance your pots are. The other timing components (within certain limits) can be adjusted to what pots you have.
     
  8. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Also are your pots

    'linear' or 'Logarithmic' taper.

    Logarithmic is probably the better of the choice.

    You still can't just run the LED's from the pot alone, or you run into current isues loading the one resistor, the pot wouild be used in conjunction, with other drive circuitry, as explained above from the previous posters.

    You couod drive one or couple, but a string, of leds, should not be driven through one resistor alone.
     
  9. Deric

    Thread Starter New Member

    Feb 8, 2010
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    I posted links to the pots and the leds. those links are all that i know about them. the leds are a cheap hong kong brand. they work great, but come with no specs or anything, online or on paper. I bought the pots hoping they would be right. they are linear. but they were only like a dollar for 2 or 3. i know the pots are called Bourns 250k and the leds are rated for 12 volts. i know its not much to work with, but its all ive got.

    the leds do have spearate wires for red green and blue so i can get to each easily. the pots were cheap so i dont care if they go to waste. if i need something different then please let me know. i just want to be able to turn a dial to turn each color up and down indivually

    thanks guys, for all the help so far.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    What I'm thinking about is a lot like this.

    [​IMG]

    If this seems like something you're interested in I'll clean it up.

    The local electronics outlet sells those, I may be able to get specs from them. If you have them you could post them.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Well, here's Bill's circuit ready to go.

    You can get all of the parts at your local Radio Shack. Use poly caps for the .1uF and .01uF caps; they come in a 2-pack. I adjusted the timing resistor so you should still get about 200Hz out of the 555.

    The pull-up resistors on the output of the 339 were adjusted to 3.3k; that'll turn the MOSFETs on and off a bit more quickly.

    sch.pdf is the schematic.
    brd.pdf is the trace layout for the board. Print it on magazine print (or ad flyers in your newspaper) using a laserprinter and transfer it to a copper PCB using a 300° clothes iron. Use a Scotchbrite pad to clean the board first, and then wipe with acetone using paper towels.

    Soak the magazine print off using hot water, and scrub it gently with an old toothbrush.

    After you etch and drill the board (use a #66 drill for most of the holes, a #30 or 1/8" drill for the corner mounting holes, and maybe a #56 drill for some of the larger holes) use acetone or isopropyl to get the laserprint off the bottom of the board traces.

    Then lightly sand the top of the board a bit to roughen it up, and iron on top.pdf matching up the corner holes. That'll give you the parts placement on the top of the board.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Here's the same idea, but eliminating the 555 from the circuit.

    I simply utilized the unused comparator as an astable multivibrator.

    Documentation in the schematic is a bit better, as the pot connections are explained.

    It takes a few more resistors than the 555 version, but this enabled removing two capacitors and the 555 IC. The resulting board is smaller; under 2" square.

    There was no real point in having three pads each for the pot terminal 1 and 3 connections; so now there's just one each for all three pots, plus a pad for each pin 2.
     
  13. Deric

    Thread Starter New Member

    Feb 8, 2010
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    Is that something i could buy to contol the leds ?
    That looks like more than the supllies or time that i have.
    If thats the simplest it gets, how do i go about building that?
     
  14. Deric

    Thread Starter New Member

    Feb 8, 2010
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    i guess i know how to build it after reading all your posts. Just seems overwhelming to me cause i have never built any type of circuit. I took circuits class, but it was taught very closely. Im going to look over all this info and the schematics and see what i can do. I go to a new engineering school called Iron Range Engineering, and we just hired the chair of electrical engineering dept. at NDSU as our director. So im sure he can help me out. I also have access to a 3D solids printer. Do you think that would be a good way to make a base for the circuit board?
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    It's more of a do-it-yourself kit. You'll have to make the circuit board, drill the holes, buy and solder in all of the components, and test it.

    If you want to buy a kit that comes with an already made circuit board, check out Big Clive's nifty RGB controller:
    http://www.bigclive.com/rgbcont.htm

    Note that 1 GBP is about $1.57 USD.
    You'll still need to solder all of the components to the board. You'll need a soldering iron, solder, flux, isopropyl alcohol & brush to clean things.

    If this sounds like too much trouble, why not just buy the controller module?
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Well, I've been using a laserjet printer to print boards on magazine paper, then ironing it on the copper side of a blank PCB.

    I've been etching the copper off using a mix of 1 part muriatic acid (available at swimming pool supply stores) to 2 parts hydrogen peroxide (available at pharmacies/drugstores) - add the muriatic acid to the peroxide, not the other way around. You must mix and use these chemicals in a very well-ventilated space, most preferably outside. Don't breath the vapors. Wear rubber gloves and protective eyewear, and keep a bucket of water handy in case of emergency.

    I use a drill press to drill the holes. A hand drill will be difficult to use.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    Dremels also sell an accessory that will allow their units to be used as a drill press. Nowdays I have to wear magnification glasses instead of googles.

    Come on Wook, a PCB???

    I thought of eliminating the resistors around the pots all together, it would create a dead space top and bottom (about 1/3 of the rotation for each).
     
  18. Deric

    Thread Starter New Member

    Feb 8, 2010
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    Thanks for all the help guys. Its amazing how complete strangers will put that much time to help me out. these all look like great ideas. i have looked at all of them and chose the last one from Wook, and one of Bill's to look at deeper. Thank you guys for all the help and advice
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    Deric,
    You sent me an E-mail requesting help, but I don't help people via PMs nor E-mail, as is stated in the signature line of every post that I have made on the forums.

    It took extra effort for me to track down this thread so that I could make sense out of your request.

    You wrote:
    In that circuit, the bottom comparator is not used. Connect both inputs to ground, otherwise you may have odd problems.

    The etchant (a chemical) does the work. Some Radio Shack stores sell "PCB Etchant" in bottles; it is ferric chloride. Place the board in a container, pour in some ferric chloride, and rock the container back and fourth slowly. It will take 5-10 minutes to remove the excess copper.

    Another etching solution is made from 2 parts hydrogen peroxide to 1 part muriatic acid. (pool acid; available in hardware and pool supply stores)
    You measure out 2 parts of hydrogen peroxide and pour it in a container, and then slowly add 1 part muriatic acid to the hydrogen peroxide. Never the other way around.

    You must use this 2nd etchant in a very well-ventilated outside area. Don't breathe the fumes from the acid. Wear rubber gloves and protective clothing. Keep a bucket of water handy; if you splash the etchant on yourself or on something, dilute it with the water immediately.

    While the board is in this 2nd etchant, wipe it very gently and constantly with a piece of soft rag or soft paper towel. This speeds up the etching process quite a bit.
     
  20. t06afre

    AAC Fanatic!

    May 11, 2009
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    As you are beginner I think you should go for a pre drilled prototype board. At the point you are now these boards are best. You can get them in many variants. With solder pads or stripes in one side. I like stripe variant best, and I use a drill bit to cut tracks. You can even get free programs helping you with the layout.
    [​IMG]
     
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