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Light sensitive switch

Discussion in 'Electronic Basics' started by Jan Nielsen, Mar 20, 2007.

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  1. Jan Nielsen

    Jan Nielsen Guest

    I would like to turn off a supply when the light level drops below a
    certain point.

    I have 5VDC and the circut (LCD) requires 5VDC, so I cant waste too much
    power in the on/off circuit.

    Sofar I have tried with a LM311 comparator and a phototransistor, but
    that dosent work with the lcd (it keeps running even when gnd is

    How would you solve the problem ?

  2. sirkituk

    sirkituk Guest

    The voltage is NOT the same thing as POWER. How much current or power
    is your supply capable of giving? How much current or power does your
    circuit need?
    An LCD (liquid crystal display) is a component and not a circuit.
    It would help if you can post the circuit schematics you are working

    My site has a system for learning
    basic analogue electronics and includes stuff like light sensitive
    circuits and relay outputs which would isolate your light circuit from
    whatever you want to switch. It would also enable you to double pole
    switch ie isolate both + and - side of supply to your controlled

  3. Lionel

    Lionel Guest

    Looking at the rest of your post, I'm guessing that you want to switch
    off an LCD display module at night?
    It's connecting to ground via the ESD protection diodes on the
    data/control lines. You should disconnect its +5V supply instead, &
    run all the control lines you're using on it via open-collector
    buffers, or diodes, depending on what kind of circuits you're
    connecting to it, that will still be powered u;p when the LCD isn't.
    If you're doing it to save power, maybe use a photo-transistor or an
    LDR to to drive a FET, & use that to drive an opto-relay. Use the
    relay to switch the 5V to everything else. Shouldn't take more than
    half a dozen discrete parts.

    If you just want to blank the display at night (or in a cupboard,
    etc), I'd wire a photo-transistor & a resistor into the
    contrast-control pin on the LCD panel to set the contrast to zero.
  4. Jan Nielsen

    Jan Nielsen Guest

    sirkituk skrev:
    The comparator circuit is here
    (I used the A part)

    I have 5VDC supply from usb (100-500mA), I dont know how much the LCD is

  5. Jan Nielsen

    Jan Nielsen Guest

    Lionel skrev:
    I would prefer not to have to modify the LCD module, I brought it in a
    finished state, and all I really have access to it the +5V, lpt port,
    and perhaps the wires from the lp port to the LCD circuit.

    I tried to run the + from the comparator instead, but that didnt work,
    so I need some way of disconnecting the + supply when the - supply from
    the comparator is missing.

    What about a transistor, with - to base and + to collector and emitter,
    would that work?
    I want it to turn off at night, since I sleep in the same room and the
    computer is often on.

    So a relay is the best solution?

  6. Lionel

    Lionel Guest

    Ah. Now it all becomes clear. You're talking about one of those
    specially made modules that connect to your PC, & have a built-in
    backlight, yes?

    Well, your first problem is that an LM339 is an open-collector output,
    & a wimpy one at that, so you can only sink current with it, & even
    then, it's only rated for 16mA max. So I doubt that it'd be able to
    reliably drive your LCD+backlight anyway.

    It seems to me that what might be simplest would be to disconnect both
    the backlight (usually one or more LEDs) connections from the rest of
    the LCD module, & drive that separately with your comparator (16mA
    /might/ be enough to give you a useful amount of light out of your
    backlight), or use the comparator to drive a small transistor, & use
    that to power the backlight.

    On most LCD modules that have a backlight, it's generally wired
    straight to two pins on the connector, for the exact purpose of making
    it easier to drive it separately from the rest of the module. You'd
    need to trace it throught to wherever it's getting its power now.
    (Most likely straight to the +5V connection via a small resistor on
    one wire, & direct to ground on the other.) Alternatively, you could
    phone or email the makers, & ask them for the info. If you tell them
    that you need to isolate the backlight so you can drive it separately,
    they should know what exactly what data you need.

    Having rewired the backlight, wire it in place of the LED in "Circuit
    A" on your diagram, replace the 1K resistor with a 120R resistor, &
    give it a test. If it works, but is too dim, use smaller resistors
    until you're happy. If shorting out the resistor isn't enough, you'll
    need to add a PNP transistor to boost the currnt through the light.
  7. Jan Nielsen

    Jan Nielsen Guest

    Lionel skrev:
    Im using LM331, but guess its the same for that one.
    Right now the comparator works, but by working on the ground level, so
    it cant turn off the lcd, which gets ground from the data cable?
    I had no idea switching a simple 100mA 5V LCD would be this difficult.
    I will look into changeing the backlight instead.

  8. Look through an encyclopedia of electronic circuits in your local library.

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