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switch the brightness of an LED

Discussion in 'Electronic Design' started by [email protected], May 22, 2007.

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  1. Guest

    I want to use LEDs in a motor vehicle, but they must be dimmed to
    about 10% of their daytime brightness at night to avoid blinding the
    driver. I need a simple, cheap circuit to do this.

    My thinking was to use a light dependent resistor (ldr) to switch a
    transistor (tr) that then added a resistor (R2) in parallel with the
    LED. This resistor would reduce the current in the existing resistor
    R1 (as it is in series with it) thereby dropping the current available
    to the LED. But would it also change the voltage substantially at the
    resistor/LED junction so that the LED no longer lights?

    +V--\-----------------\
    | |
    ldr R1
    | |
    | --R2--+
    | / |
    +---Tr\ LED
    | \ |
    | | |
    0V---/-------/--------/

    Remember this is in a motor vehicle so there is plenty of spare power
    and heat sinking available.

    thanx, bye.
     
  2. DJ Delorie

    DJ Delorie Guest

    If you're using a uC to turn the LEDs on and off, can you just use PWM
    at 10% duty cycle? Hook the photocell to one of the uC's ADC inputs.
     
  3. Guest

    Already run out of I/Os (and code space)

    bye.
     
  4. D from BC

    D from BC Guest

    Maybe try a phototransistor..
    D from BC
     
  5. MooseFET

    MooseFET Guest

    Buy a bigger micro.
     
  6. JeffM

    JeffM Guest

    The *bigger uC* idea sounds apt.

    Alternately:
    http://www.google.com/search?q=LM555+source-or-sink-200-mA
     
  7. linnix

    linnix Guest

    I have 1 ADC input and 1 LED output (PWMed) in less than 2K (out of a
    16K AVR). I can't find any reasonable uC with less than 8K. How
    small can you get?
     
  8. linnix

    linnix Guest

    I have 1 ADC input and 1 LED output (PWMed) in less than 2K (out of a
    16K AVR). I can't find any reasonable uC with less than 8K. How
    small can you get?
     
  9. John B

    John B Guest


    This would work better.

    Set RA for minimum LED current and RA//RB for maximum LED current.


    -o-----o-------o----o----
    | | | |
    .-. .-. .-. .-.
    LDR | | | | RB | | | | RA
    | | | | | | | |
    '-' '-' '-' '-'
    | | | |
    | o--+ | |
    | | | | |
    | |/ | |< |
    o---| +--| |
    | |> |\ |
    | | | |
    .-. .-. +----o
    | | | | |
    | | | | V LED
    '-' '-' -
    | | |
    --o-----o------------o----


    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
     
  10. feebo

    feebo Guest


    forget resistors to do this stuff... use PWM to make them *appear* to
    be dim... your LDR idea is not the way to go - you have lights on,use
    this to signal to a PIC or similar to switch them
     
  11. Marra

    Marra Guest

    Why not hook into the lights circuit?
    Thats way you will know if it is dark without a sensor !
    PWM an LED from a simple micro.

    www.ckp-railways.talktalk.net/pcbcad21.htm
     
  12. Boris Mohar

    Boris Mohar Guest

    How about paralleling a CDs cell across R1? Set the R1 value for night time
    luminosity.



    Regards,

    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

    void _-void-_ in the obvious place
     
  13. Jasen

    Jasen Guest


    here's another way.

    // LED
    input ---------->|----.
    |
    park lights ----[R1]--+
    |
    0V ----[R2]--'

    that may look screwy on your screen, use a fixed pitch font.

    I've never seen a LED damaged by being reverse biased at a low voltage
    (as may be possible with this circuit) but if you're worried add a
    diode in series with the LED.

    Bye.
    Jasen
     
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