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led dimmer circuit

Discussion in 'Electronic Design' started by [email protected], Jun 4, 2006.

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  1. Guest

    i would like a simple circuit that could be used to dim a high
    brightness white led for installation inside a bird box for a colour
    camera. i don't want to scare the birds by a simple on / off . A
    gradule increase in brightness would be good. i have tried a resistor
    in series followed by another variable resistor which works to a degree
    but there comes a point when the brightness suddenly increases with a
    small turn of the pot. could anyone post a simple cicuit please ?
     
  2. Nico Coesel

    Nico Coesel Guest

    The LED brightness depends on the current. It is easy to make a
    voltage to current converter with an opamp:

    http://www.allaboutcircuits.com/vol_3/chpt_8/7.html/conversion

    Substitute Rload by the LED.
     
  3. mc

    mc Guest

    i would like a simple circuit that could be used to dim a high
    But there should also be no problem using resistors. What resistances are
    you using (fixed and variable) and is the variable one linear-taper or
    audio-taper?
     
  4. Don Bowey

    Don Bowey Guest

    Resistors won't do the job. Post this on the basics board, or better yet,
    google for led dimmer. You need a circuit that varies the length of time
    the LED is fully powered, reducing it gradually to zero.

    This has been answered a jillion.5 times.
     
  5. Fred Bloggs

    Fred Bloggs Guest

    This will convert the variable resistor into a linear control over full
    off to on range, start out with 100 ohm pot fully CCW for LED off:
    View in a fixed-width font such as Courier.
     
  6. John Fields

    John Fields Guest

    ---
    Sure they will: (View in Courier)


    +V
    |
    [POT]<--[R]--[LED>]--+
    | |
    GND GND
     
  7. Pooh Bear

    Pooh Bear Guest

    That's shockingly inefficient. you might at least have used a transistor.

    Graham
     
  8. Fred Bloggs

    Fred Bloggs Guest

    This one might be a little easier, less battery drain and more readily
    available parts- use any small signal transistors- the variable resistor
    is totally linearized:
    View in a fixed-width font such as Courier.
     
  9. Don Bowey

    Don Bowey Guest

    Well, sure if you don't care that it doesn't work well.

    You are becoming a regular troll.
     
  10. James Waldby

    James Waldby Guest

    Here's a "more-efficient" approach, in that all of the current
    used (except a few hundred uA) goes through the LED:

    .. VR
    .. +-------I O-----------+
    .. | A |
    .. | | R1
    .. | | |
    .. | | +--+----+
    .. Bat | | | |
    .. | | | D |
    .. | +---->R2 | |
    .. | | | |
    .. | +--+ Led
    .. | R3 |
    .. | | |
    .. +---------------+-------+

    VR is a 3-terminal regulator, like LM317, although I used
    a LX8383, having a bunch on hand. I used: R1 = 11 ohms,
    R2 = 5K pot, R3 = 1K, D = a diode.

    -jiw
     
  11. John Fields

    John Fields Guest

     
  12. Nico Coesel

    Nico Coesel Guest

    PWM may cause interference when using a digital camera.
     
  13. Fred Bloggs

    Fred Bloggs Guest

    How do you figure that? If the LED goes to 3.3V, you end up with
    3.3-0.7=2.3V across R3 for 2.3mA. And you need to make R1 more like 39
    ohms, since the LEDs do not like continuous operation at more than 30mA.
    Then because 'D' has an orientation, you might show it.
     
  14. Fred Bloggs

    Fred Bloggs Guest

    yeah-right...
     
  15. James Waldby

    James Waldby Guest

    I forgot to show input and output capacitors for the VR -
    .. VR
    .. +---+---I O-----------+---+
    .. | | A | |
    .. | | | R1 |
    .. | | | | |
    .. | | | +--+-D1-+ |
    .. Bat | | | | | |
    .. | | | | D2 | |
    .. | | +---->R2 | | |
    .. | | | | Led |
    .. | | +--+ | |
    .. | C1 R3 | C2
    .. | | | | |
    .. +---+-----------+-------+---+

    Also, the circuit I posted before showed D2 but not D1; the
    circuit I tested included D1. D1+D2 = 1 dual-diode MMBD7000.
    D1 can be left out if R1 is made slightly larger.
    -jiw
     
  16. John Fields

    John Fields Guest

    ---
    What do you mean? 0 mA into the LED at the bottom of the pot, 20mA
    at the top, and something between that as the pot gets rotated.
    Maybe not all that efficient, but who cares? The OP wants something
    simple, and it's just 3 parts.
    ---
     
  17. James Waldby

    James Waldby Guest

    I think you're right, the loss would be a couple of mA rather than
    a few hundred uA. The 100 uA I was thinking of (by mistake) was the
    current in the Adjust pin of the VR.

    The circuit I built is shown more accurately in a post I made about
    the same time you were posting your comment, and still more accurately
    below. (My other post showed D1 and D2 but not D3, which was included
    to isolate multiple LED's)
    .. VR
    .. +---+---I O-----------+---+
    .. | | A | |
    .. | | | R1 |
    .. | | | | |
    .. +| | | +--+-D1-+ |
    .. Bat | | | | | |
    .. -| | | | D2 | |
    .. | | +---->R2 | | | D1: |<
    .. | | | | LED |
    .. | | +--+ | | D2, D3: v
    .. | C1 R3 | C2 -
    .. | | D3 | |
    .. +---+-----------+-------+---+
    Here, the drop across D1-D3 is about 2V, so there would be
    1.3V across R3 (for a 3.3V LED), hence 1.3mA for R3=1K.
    To cut down that loss without D3 in the circuit, increase R3
    a bunch.

    You suggested 39 ohms for R1, vs the 11 ohms I used. With
    R2 slid to the D1 end, and 1.25 V at the A pin, and .65V
    across D1, there's .4 V on R1, or 36mA through it, so about
    34mA through the LED, a bit high. 15 ohms would limit LED
    current to 25mA; 39 ohms limits to 8mA. So 11 is too low
    and 39 too high, for the circuit above. With D1 not in there,
    your 39 limits LED current to about 30mA, so is good for the
    earlier circuit.

    Using D1 removes about .65V overhead from the LED current path,
    allowing the circuit to work on that much less battery voltage.
    -jiw
     

  18. Because transistors only light up ONCE? ;-)


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  19. Don Bowey

    Don Bowey Guest

    BFD. Lightning might be attracted to it too.

    Take your comment to the basics board.
     
  20. mc

    mc Guest

    The LED brightness depends on the current. It is easy to make a
    Will someone explain *why* resistors "won't do the job"?

    This is not a basic question. I am not the original poster. I understand
    about PWM and its advantages (saving power). But if an op-amp adjustable
    current source (which is not PWM) will do the job, surely so will a humble
    rheostat.

    Is there something strange about the current-vs-brightness relation of these
    LEDs?
     
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