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12V LED dimmer/timing circuit advice required, please!

Discussion in 'Electronic Design' started by Dungeon Dave, Dec 3, 2008.

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  1. Dungeon Dave

    Dungeon Dave Guest

    Hi, all. Relative newb here - relative in the sense that I used to dabble
    in some electronics as a youngster and did some electronics theory in A-
    level physics but that don't make me an expert as such.

    Okay, so onto my problem. I bought a set of these the other day:

    (actually, that doesn't look like the right package, but it'll do for the
    purpose of this query)

    - they're a string of white "nail" LEDs that are intended to mount in
    thin plinths, powered by a plug-in transformer that delivers 12V to a
    matchbox-sized distribution point. Since the DB just consists of 13
    powered sockets in parallel connection, I'm guessing each LED has a
    resistor in series to provide the correct voltage across the LED itself
    (they may be bright, but I ain't heard of any LED that can take the ommpf
    that 12V will pack and survive for too long!)

    During the day, these lamps are bright enough, but I want to subdue them
    for evening and night usage (I'm planning on leaving them on as
    nightlights on the stairs, etc), so thought of just using a 12V dimmer of
    some kind. Bill Bowden's site came up in a few google searches, and was
    thinking of employing something like:

    (the site directed me to this forum)

    However, it had me thinking: firstly, I have a transformer to drop
    240AC->12DC, then another circuit on to lower the 12V... would I benefit
    from combining the two into one? Build something that delivers a
    rectified 2-12V output (looks like 5W max, BTW)? If so, are there
    diagrams/kits (Vellmen, RS Electronics etc) readily available?

    Secondly, I'd like to combine this with a timer circuit in some way, so
    that it'd fire up at 100% brightness at one set time, dim to 20% (or
    whatever preset I use) at a second time, then turn off completely at a
    third time. I was toying with the idea of linking in a serial feed from a
    USB controller then firing control information from my server (Linux,
    running 24x7) so I could easily schedule something that determines what
    brightness at what times of day I require, letting that ramp up/down the
    lighting as I see fit. I've seen various projects online that cover this
    info, but before dipping my toe in there, are there any more readily-
    available circuits that can do this kind of timing-based control? Could I
    hack away at a central heating controller? I'd prefer to have the self-
    contained wall-mounted jobby if possible.

    Okay... I know expecting someone to design something that exactly fits my
    needs is asking a lot, given I've just strolled in here. However, I'm
    guessing that if I'm going ot be reinventing the wheel through my
    ignorance, I'm hoping someone can at least put me on the right road.

    Thanks in advance, people!
  2. Guest

    You already have the 12V supply, so I would keep it as part of the
    project. For one thing, this prevents you from having to deal with the
    mains. There are probably several LEDs in series, plus a resistor. I
    don't think they would use 12V per LED since the drop across the
    resistor would be high.

    What you want to do is pulse width modulate the power to the LED. That
    is, let duty cycle control the brightness.

    Most cheesy PWM circuits consist of a sawtooth generator which you can
    make out of a 555, and a comparator.
  3. Jasen Betts

    Jasen Betts Guest

    555-based PWM lamp dimmer, a good choice.
    Bill's circuit doesn't reduce the voltage (much) what it mainly does
    is reduce the on-time of the LEDs (by blinking them at a rate that's
    too fast to see) this is a very power-efficient way of dimming lights.
    an irrigation controller perhaps ? this is way more complicated than
    Bill's PWM dimmer. it may be easier to hack a few electronic timer-plugs
    and use their timers to control on-off and brightness.

    OTOH it would be easy to wire a light-sensor a variation of bill's dimmer
    circuit so that when it sees "dark" it dims the lights and just use
    the timer plug to turn them off when you want off.
  4. Guest

    One thing to consider when chosing between a switched (pulse width
    controlled) dimmer, and some linear system, is that in pulse width
    LED supplies you get a sort of stroboscopic effect when your eye scans
    the LEDs (you see more then one, some distance apart).
    If you do not like this, then use DC.
    As you are not very experienced, maybe it would be simpler to use the
    serial port.
    For example to write to an antique UART, and use the 8 data lines to
    example drive a resistor R2R network to make an analog voltage to
    drive the brightness,
    for example drive a 2N3055 emittor follower.

    That will save you from programming a PIC.
    Here is a simple serial communications program for Linux:
  5. Dungeon Dave

    Dungeon Dave Guest

    okay. It's one less headache, I guess.
    Nor me, but the transformer claims it's chucking out that and these are
    somewhat superbright LEDs. I'll get a multitester on the DB and measure
    the voltage at the parallel end, see if it really is that much.
    okay.. I recall some diagram that had a strobe-like effect to simulate
    dimness, although it looked more like a square wave (I could probably pop
    some caps across the output points to smooth out the wave).

    Ta for the help!
  6. Dungeon Dave

    Dungeon Dave Guest

    One thing to consider when chosing between a switched (pulse width
    Given they're not going to be near anything else strobing (overhead
    lights @50Hz, monitors/TVs etc) I wasn't too bothered about the strobing
    effect, but I know what you mean. Would upping the PWC frequency to
    something much higher (maybe in 200Hz range) disguise the strobe effect?

    [OS-based controller]
    snagged, untarred and read the README... mmm.. nice! Thanks for that, I
    may toy with a few proof-of-concept experiments and that util.
  7. Dungeon Dave

    Dungeon Dave Guest

    [Bill's diagram]
    Can't go wrong with a 555... where would we be?

    (don't say "taking a pair of scissors to a 556"...)
    I read that reducing the power output (strobing) is the more efficient
    method than splitting it and dissapating the unwanted power elsewhere,
    but as we're talking a total of 1W for my lights I didn't know if it was
    worth it. I'll have to do some costings.. perhaps I should have done that

    (/me ponders a blog...)
    mm... never thought of that. Will investigate, ta.
    Essentially I wanted to have the lights at 100% during evening/dusk so
    some LDR-based circuit could trigger the "on" cycle, but then wanted them
    to dim to "comfort light" after a pre-determined point (20% from midnight
    onwards?), so what was an illuminating effect doesn't become too dazzling
    to sleepyheads. Bit of a strange requirement, I know.

    okay, that's given me some pause for thought - ta for that info!
  8. PeterD

    PeterD Guest

    I can just see one preparing a ham sandwich with a strobe light. The
  9. ehsjr

    ehsjr Guest

    Yes. Build the Bowden circuit - it runs at 200 Hz so there
    is no need to up the frequency. You were also concerned about
    the square wave output - that is not a problem, at all. Get
    the dimming portion of what you want working first, sort out
    the timing part later.

    Be sure to check that the voltage coming from the supply
    included with the lights is 12 volts DC.


  10. Rich Grise

    Rich Grise Guest

    This is my recommendation, but I wouldn't use a 2N3053 and 2N2955 - I'd
    use something more like a PN2222 and a TIP32, because they're much more
    recent designs, with lots of beta and stuff. If you want to switch the low
    side, lose the 3053 and just drive a TIP31. They're less than a buck at

    And yes, 200 Hz is high enough to make the flicker go away. :)

    Have Fun!
  11. neon


    Oct 21, 2006
    LEDS are not resistive lamps whereby linear in resistance. they are exponential devices therefore yes you can controll them some of the way but never like lamps. use to control brightness is just plain unsound idea.
  12. Guest

    No, a rectangular pulse is what you want. Your eye will average the
    intensity. The 200Hz suggested frequency is plenty fast.
  13. Dungeon Dave

    Dungeon Dave Guest

    okay.. so just replace 3053->2222 and 2955->TIP32?

    Roger that.
    I don't know why, but I'd prefer to have the switch at the high side -
    something about having floating grounds worry me.
    And yes, my idiotic eyesight never actually READ the site carefully and
    noticed that! Major DOH! Excuse my noobness/impatience; it's strangely
    nice to be on the other side of the clueCurtain once in a while!
  14. Nope - they're paralleled up.. and just checked the voltage. 13.55V at the
    supply end!
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