PWM Circuit Powering 5mm LED's

Discussion in 'The Projects Forum' started by Shift, Jan 6, 2011.

  1. Shift

    Thread Starter New Member

    Jan 6, 2011
    Hi All,

    Tried to find my answer among previous threads, Mr. Marsden blog and various other websites. Hours of time and research in my first attempt to build a simple PWM circuit to dim some LED's for my aquarium moon light.

    I am wanting to power 12 5mm blue leds over my aquarium for accent/moon lights. Each LED has a fw voltage of 3.5vmin-4vmax and a fw current of 20mA. I was planning on wiring 4 sets of 3 in series using a 12v ps and 75ohm 1/8w resistor.

    I've located a schematic that shows exactly what i want to build but the output is 10v not 12v. It's using a lm317 regulator(with resistors) to step down the voltage. If i have a 12v regulated ps that will output 12.05v-12.3v +-3% could i just exclude the lm317 and resistors? should I swap out the lm317 for a fixed regulator like a L7812? Should I get a 15v(+) ps and change the resistors?

    I think all would work but what is the best/reliable/efficient way to go about this?

  2. Wendy


    Mar 24, 2008
    If you want to help you need to be a bit more specific.


    For example, pin 7 should be left open. That is a major error.

    555 PWM Oscillator

    What the heck is DIM+ and DIM-? I don't think it will do what you want it to.

    You frequency is way too low, around 150 Hz. You need it around 1.5Khz. The reason is you can see the flicker, bet the fish can too. Bump up the speed and it goes away, so drop the variable resistor to around 10KΩ.

    If the LM317 is loaded with 10ma your design would work, but it is a bad assumption that the load will be present, especially since it is off the PCB. Here is a typical LM317 voltage regulator, note that R1 on this schematic is 120Ω, this will force the 10ma value the regulator needs.


    Another major error, you have the LM317 programmed for 9.6VDC. Each LED will drop around 3.6VDC. You have 3 LEDs in series, the total Vf for the string is 10.8VDC. Ego, you can not power the LEDs, you need at least a volt more than that for the resistor to limit the current to LEDs. You need a 12VDC power supply.

    LEDs must have a resistor, this is not optional. The resistor can be replaced with a current regulator, but that is a different story.

    Please draw this in schematic form and include the resistors/LEDs.

    What is the power supply you are planning on using? The 555 doesn't care, as long as it doesn't exceed 15VDC.

    I will be glad to help you polish your design if you give me the information. I understand the fun of designing it yourself, but the numbers have to jive. :D The core is there, but the design needs a lot of work.
    Last edited: Jan 7, 2011
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Shift, Why the need to dim the moonlight? Looks like the diagram you have used is for use with the dimmable meanwell eln drivers that people are using for their LED fixtures.. This provides the 0-10V dimming signal for the driver.

    Are you really going to be dimming/adjusting your moonlight all the time.. I doubt it..

    I would simply use a wallwart (ac/dc power supply) and the appropriate resistors to power your moonlight. Skip the there is NO need for it.
  4. wayneh


    Sep 9, 2010

    If you want nice dimming available, then continue with your PWM design. But if you just want a fixed light level, but with the ability to set that level, a power supply and a handful of resistors is all you'll need. Just tweak the resistor value to give you the light level you like (eg. 15mA for each string of 3 LEDs in series). For long LED life, avoid going all the way up to the rated max current. You may find even 5mA is plenty.
  5. Shift

    Thread Starter New Member

    Jan 6, 2011
    Thank you for the guidance Bill. I knew this circuit would require some modifications to work just wasn't sure where to begin. Although it's being to look more like a total overhaul!

    I was planning on using a 12VDC wall wart as a power supply.

    @mcgyvr Yes, this supposedly works to dim the Meanwell eln drivers. (thought i could use it for my application as well) Strange how there are major errors yet many have built this circuit for this very purpose and it's been successful...

    There may be NO need for dimming but I want that option. Aside from moonlights, if they are bright enough they will provide a nice accent to tank. However, at night I don't want to keep my fish (or my wife) awake....
  6. Wendy


    Mar 24, 2008
    Not if the OP has no clue what you are talking about. A criticism without a solution isn't help, it is just talking trash.

    Help is helping the OP make it right. If he chooses not to follow advise then you've done your part.
  7. Wendy


    Mar 24, 2008

    You may want to rethink the design a bit, the version of PWM you are using can be adjust manually, but not very well electronically. If you just want to dim the LEDs it can be done much simpler, PWM is overkill for the power levels you are talking about. It is linear, but generally that isn't a major requirement.

    A minor variation this would also work...


    The emitter resistors would be replaced with 33Ω at ¼W.

    If you want to continue with the design as you have it drawn I'll sketch up a schematic.
  8. Alberto

    Active Member

    Nov 7, 2008
    I gave a WARNING not a criticism! Changing deliberatly the meaning of someone action is really talking trash! :mad:

    End of any additional comment on the subject and end of my partecipation to the AAC forum.