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LDR / photo resistor - increasing resistance with increasing light???

Discussion in 'Electronic Basics' started by mike, Oct 19, 2004.

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  1. mike

    mike Guest

    Is there such a thing as a LDR that increases resistance with
    increasing light?

    Or, is there a simple circuit that will invert the resistance of a
    common LDR?

    Disclaimer: I have very little experience with electronic/circuits.
    Please be gentile.

    Any help is appreciated. Thanks!
  2. What is the objective? If you want to control a circuit using an LDR,
    you would normally arrange the LDR with a normal resistor (or pot or
    preset) as a voltage divider pair. You'd then use the output voltage
    from that as input to a subsequent (analog or digital) circuit
    section. So, simply reversing the position of the LDR and the other
    resistor would reverse the operation of the circuit.

    As an example, see the first section of my garden lamp circuit here:

    In the unlikely event that I wanted my lights to go on at dawn rather
    than dusk, I could achieve that by swapping the positions of the LDR
    and the resistors.
  3. I don't know of any.
    This is fairly weary in some cases.
    Tell us all the details you can think of about what you are trying to
    accomplish. This will save us a lot of useless questions.
  4. mike

    mike Guest

    wow, thanks for the response. here is exactly what i want to do:

    i live in a basement w/o windows, and i want to rig inside light so it
    coincides with the outside light. i was figuring i could run an LDR
    placed outside, to a dimmable ballast in my basement apt.

    i have an electronic dimming ballast, made by the now defunct JRS
    Technology (#C232120RS501). it has a dimming control by way of a 0 to
    10v class 2 circuit, 0.5mA output. it is 0-10v out, and by connecting
    a 0-100k ohm potentiometer to this circuit i can dim the connected
    flourescent bulbs no problem-- the greater the resistance, the greater
    the V, the greater the resulting brightness.

    however, what i want to do is connect an LDR in such a way that a
    greater amount light hitting the LDR makes the bulbs brighter. this
    means that in order to get brighter bulbs, i need a greater voltage
    accross this circuit, which means i need greater resistance connected
    to this circuit (i think, right?)

    i have an LDR (from a radioshack multi-pack) connected in series (with
    a 0-100k trimmer/potentiometer) to this 0-10v circuit, and it does
    what i want it to do, only in reverse. that is, as lighting
    conditions on the LDR get brighter, the dimmer makes the bulbs get

    this makes sense since an LDR decreases resistance as more light hits
    it. what i need is increasing resistance with more light. or a
    circuit that simulates this.

    if this looks familiar, i posted 6 weeks ago before i had any idea
    what i was talking about (thanks to those who offered suggestions):
  5. KM

    KM Guest

  6. rayjking

    rayjking Guest


    If you add a cad photocell in series with a 10k resistor that is connected
    to a low current ( 10ma or more ) 10/12 volt dc power supply the signal you
    wish will appear across the 10k resistor. The better circuit would
    substitute the 10k for a 100k pot and a series 470 ohm ( safety resistor )
    to allow adjustments.

  7. You can make a voltage divider with a light dependent resistor in
    series with a fixed resistor that has the LDR in either the pull up or
    pull down side of the divider. If you have a 10 volt supply available
    (a wall wart, perhaps) or if this voltage is available from the
    ballast, you can experiment with the LDR and fixed resistor replacing
    the potentiometer. Varying the value of the fixed resistor shifts the
    light level that produces about half light output.
  8. CFoley1064

    CFoley1064 Guest

    Subject: Re: LDR / photo resistor - increasing resistance with increasing
    Hi, Mike. I'm hearing you say you tried the RS photoresistor, but it operated
    in a bass-ackwards fashion. You want resistance to increase as light

    You might want to try something like this (view in fixed font or M$ notepad):

    | VCC
    .-. 1/2 LM358 +
    | | VCC |
    1K| | |\| |
    '-' .---|-\ ___ |/
    | | | >-|___|--|2N3904
    .----o-----|---|+/ 100 |>
    | | | |/| |
    | | | GND | ___ 1 H11F1 6
    ~~.-. .-. '-----------------o-|___|---o--. .---o--->
    ~~| | | | 150 ohms | |
    LDR| |1K| | V ~~ ||-+
    '-' '-' - ~~ || To Ballast
    | | 2 | ||-+
    | | .---------o--' | 4
    | | | '---o--->
    | | |
    V | |
    - | ===
    | | GND
    === ===
    GND GND .------------.
    | |
    1N4001 | |\ |
    '---|-\ |
    | >-----'
    | |/
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    There are a couple of components here you'll have to purchase, and I'm afraid
    they're special order at RS. You can try for better prices and good
    service for the hobbyist. You also might want to pick up a 9VDC wall wart for
    the Vcc in the diagram.

    The two components you need are an LM358 (dual op amp) and a H11F1 (an opto
    analog output FET). The distinguishing characteristic of the H11F1 is that, as
    current through the LED increases, the resistance of the FET decreases. You
    can use that to advantage with your CdS photoresistor by seting up a voltage
    divider, and using the CdS to shunt the lower resistor. That voltage is picked
    up by the op amp, and used to drive the LED.

    Notice that, as ambient light increases, the voltage decreses, so the current
    througfh the LED decreases, causing the resistance the ballast sees as
    increasing. Also note that this assumes your CdS LDR has less than a couple
    hundred ohms "ON" resistance. Since there's such a wide variety in the RS CdS
    parts, you'll probably have to tweak this circuit for your part to make it

    I hope this is of value, even though it's a little on the complex side. If you
    need more help, feel free to email. Please put "LDR" in the heading to avoid
    having the spam filter dump you.

    Good luck
  9. Rich Grise

    Rich Grise Guest

    I seem to remember this from all those months ago, and it seems like
    someone had suggested another LDR inside, with a comparator/difference
    amp, to make the inside lights _really_ track the outside. Then, as
    long as you get it scaled right, you won't have to worry about polarity,
    or even nonlinearity. :)

    Of course, on cloudy days, it could be a little dim. )-;

    Have Fun!
  10. But eclipses and airplanes would be fun.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  11. mike

    mike Guest

    ha! actually it *would* be great if the lights dimmed a little when a
    plane flew overhead! i'm thinking simulated shadows?

    you guys may think i'm a little crazy for wanting my lights this way--
    ok, i may be crazy, but for entirely different reasons.

    here's a little more background, hopefully to put things in better
    perspective: i have regular lights/lamps in the room, apart from this
    project. for this project, dimmable flourescents will be mounted in a
    "light box". this light box will actually be an old window i'll be
    hanging on the wall, with a reflective box framed behind it (open to
    the glass in front). the lights will be mounted behind the framing of
    the window, the plan being that you won't see the bulbs, but only the
    light. think "hangable" windows, with real light shining through. i'm
    using flourescents because the natural-sunlight models are much closer
    (in color) to real sunlight.

    now back to the gritty. i've been reading up on voltage dividers and
    op amps, but they only seem to apply when you want to change or divy
    up the voltage of a known voltage source. the dimming control of the
    ballast i'm using has 2 wires out for a 0-10V, .5mA, class 2(DC)
    circuit. this is a voltage source built into the ballast. i can dim
    the lights by adding a 100k trimmer to these 2 wires. i was measuring
    today and found response on the lights came from a resistance between
    15k and 75k ohm (the higher the resistance, the brighter the light).
    this didn't make sense to me because when I did my equations (V=IR) I
    thought response should have been when R was between 0-20k ohm. i
    figure i didin't consider dynamics in the ballast or internal
    resistance or one of a million other concepts i've never heard of.

    the LDR, hooked in series with a trimmer on these 2 control wires,
    makes the flourescents go dimmer as light hitting the LDR gets
    brighter-- i need it the other way around.

    i really like the idea of having an inside LDR and an outside LDR
    balance each other out, because (1) i don't think i'd ever get a
    linear response otherwise and (2) this will eliminate problems with
    the resistance of the outside LDR changing when it get's wicked cold
    out. (i understand other inside lighting may interfere, but creative
    placement of the LDR should fix that.)

    BUT... it there a way to do this not with voltage, but with
    resistance in to the ballast control? from your suggestions maybe the
    ballast 0-10v controls hooked up to one side of an op amp, a similar
    but steady vcc hooked up to the other side, both sides with LDRs in
    series and whatever necessary trimmers/resistors-- but how would this
    raise or lower the voltage of the ballast control wires?

    sorry, i'm a little dense. but i'm slowly learning :)
  12. CFoley1064

    CFoley1064 Guest

    Subject: Re: LDR / photo resistor - increasing resistance with increasing
    Again, take a look at the H11F1. Its output is a bilateral FET which is seen
    by your ballast as a plain old resistor, as long as the voltage applied across
    it is less than 30V or so. Depending on the amount of LED current, it runs
    from 300 Megohms down to a couple of hundred ohms -- ideal for your
    application. The op amp circuit I provided is definitely a fiirst cut -- it
    can definitely be improved. With a little more effort and a trimmer with an RC
    filter instead of a straight voltage divider to set the current servo point,
    you should have a good solution. But that shows the way.

    Good luck
  13. mike

    mike Guest

    thanks for the help chris-- i have the H11F1 on order. i'll let you
    know how it goes.
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