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LDR's light resistors Sweep,range,taper,contrast

Discussion in 'General Electronics Chat' started by walters, Dec 12, 2005.

  1. walters

    walters Banned

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    The LDR's are light dependant resistors

    In the Data Sheets how do i find out the Taper,range,sweep,contrast from light to dark or dark to light how the resistance changes?

    The taper from 0 resistance to 100K whats the taper of a LDR's? and how would i find this out in the data sheets?

    Doesn't the Lamp/LED/Light change the taper?

    How does the Lamp change the taper,sweep, contrast?
     
  2. mstechca

    mstechca New Member

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    Why do you care?
    An LDR's package should tell you the resistance it has when the area surrounding the LDR is dark, and when the same area is extremely bright.

    Hook an LDR directly to an ohmmeter and then you have made yourself a meter that indicates the amount of light fallen on the LDR.

    I don't think there are datasheets for LDR's.
     
  3. walters

    walters Banned

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    So how do i tell the light to dark resistance the taper of the LDR?
     
  4. dave

    Dave New Member

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  5. mstechca

    mstechca New Member

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    Why do you care?

    Have you taken my suggestion above?

    I'll give you an example of what you might be looking for:

    If an LDR has a resistance of 100 ohms in the light, and a 1K resistance in the dark, then 900 ohms is what can be altered by the LDR. because the minimum resistance is 100 ohms, the entire resistance the LDR can produce is anything from 100 ohms to 1K.

    A potentiometer is almost the same, except that it doesn't detect light, it has a control, and the range is from 0 to whatever is specified on the potentiometer.
     
  6. walters

    walters Banned

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    Thanks for the information


    But what is the Inbetween light called? its not light or dark its the resistance thats inbetween?

    Can LDR's be Linear or Audio tapered?
     
  7. mstechca

    mstechca New Member

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    Why do you care?
    I don't think there is a specific term for this other than maybe "dim light"?

    I don't understand what you are asking.

    LDR's and Audio do not go hand-in-hand unless it is used in an audio circuit.

    Check the packaging to determine whether an LDR is linear, or logarithmic (correct me if I spelled it wrong). Linear is better.
     
  8. walters

    walters Banned

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    Whats the resistance for the dim light? because the bulb has different contrast for dim light so the LDR's reistances changes

    The light Bulb changes resistance when it gets hot?

    What does the wattage of a light bulb tell or mean?
    the wattage is power so does that mean the light bulb is going to
    be brighter so the wire inside the light bulb it thicker? and has more
    resistance in the wire?

    When the light bulb is light and not dark there is more current when its light than dim light?
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An LDR is a very crude way to switch something with bright light or when it is dark. It isn't linear and doesn't have detailed spec's like a phototransistor.

    Now let's talk about incandescent light bulbs.
    The tungten wire in a lightbulb is a very low resistance when it is cold and increases its resistance about 10 times when it is glowing white-hot at about 2000 degrees C.
    So when you use a 1A transistor to switch on a 1A lightbulb, the cold 10A of current will kill the transistor.

    Yes, therefore its thickness gives it a low resistance and more power.

    No. Thin wire has more resistance than thick wire. Low resistance wire carries more current and since Power equals V times current, then thick wire dissipates more power.

    Yes. A bright incandescent light bulb uses more current than a dim one.
     
  10. walters

    walters Banned

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    Thanks for the help and information

    Whats inside a LDR to change resistance from light rays?
    its photochemical elements or materials

    But using a Squarewave on the lamp to change LDR from light to dark is switching

    Using a triangle waveform on the lamp to change the LDR from light to dark has also the slope angle of the triangle waveform which is the dim light that the LDR has the range of dim light resistance its just not like a switch its variable and linear or tapered with a sweep.

    Were the Carbon Comp LDR's different because of the 10% drift?
    the carbon comp LDR's would never go back to Zero resistance or back from where it started they were more sloppy
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Here's a datasheet of a Cadmium Sulphide LDR:
    http://www.electro-tech-online.com/custompdfs/2005/12/104510.pdf
    Note that its response is very slow to increase its resistance in darkness after being in bright light, it takes up to 15 seconds.
    Thefore if you feed it a square wave or a triangle wave of light brightness, its output won't be much of anything.
    It is used in automatic night-lights that turn-on when it gets dark, and turn off when light is shining around it.
     
  12. walters

    walters Banned

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    thanks

    How do i study LDRs components?
    What are they made out of?
    What causes the timing of the LDR?

    There is so many different kinds of LDR's and they are more complex then a regular resistor because of the light and dark elements and "timing" of a LDR gets me really confussed because each LDR is different for the application

    what are the different types of LDR's?
     
  13. walters

    walters Banned

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    Illuminance VS Resistance graphs
    this is really good graph because it shows the Illuminance VS resistance
    of a LDR

    Spectral Sensitivity graph= Spectral Response curve

    a photoconductive cell or light dependent resistor. (LDR) A device whose resistance decreases with exposure to light. Is an electronic component whose resistance decreases with increasing incident light intensity. It is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
    A photoelectric device can be either intrinsic or extrinsic. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band - since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device.
    Inexpensive cadmium sulfide (CdS) ones
    scale, Ge:Cu photoconductors
    Photoresistors are light sensitive resistors whose resistance decreases with the amount of light falling on them. You can vary a voltage using them by a simple voltage divider circuit.

    How do photoresistors work?
    It's a quantum thing
    range of light intensity.

    Use an ohmmeter to measure the resistance range of your photoresistor for your range of light intensities

    varying resistance depending on hte amount of light reaching the device

    When the light shining on them increases in intensity, their resistance is lowered. Working photoresistors into your circuits will allow you to detect changes in lighting.

    Their resistance is very high when no light is present (up to millions of Ohms), and significantly lower when they are illuminated (hundreds of Ohms). These are also often called Light-dependent Resistors (LDRs) and Cadmium-Sulfide (CDS) cells.

    Optoelectronics

    Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically
     
  14. mstechca

    mstechca New Member

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    Figures. Just like many potentiometers are not linear.

    Walters, Did you remember the parallel resistance equation?
    Thin wire has more resistance than thick wire because thick wire is like many thin wires touching each other at every single point.

    When you think about it equation wise, think this:

    1 / ((1 / x) + (1 / x) + (1 / x)...)

    x is the resistance of each strand of wire.
     
  15. walters

    walters Banned

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    thanks for the help

    Most pots are 270 degrees of shaft rotation
    so the graph would be resistance VS degrees of shaft rotation

    If the resistance is 100K and the shaft degrees is 310 what is the Taper?
    If the resistance is 100K and the shaft degrees is 270 what is the Taper?

    Whats a Semi Logarithmic Taper? how would it look on a graph?

    How do i get a radical "S" taper?

    The taper of a pot refers to the way its resistance changes as the shaft is rotated. A linear taper pot is exactly what you would guess it to be, a graph of resistance (from one end to center) versus shaft rotation would be a straight line
     
  16. mstechca

    mstechca New Member

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    Why do you care?
    If the pot is linear, you could easily calculate the resistance like this:

    resistance = (# of degrees turned)/(total degrees) * (total resistance)

    But if the pot is logarithmic, then complex calculations are required. This is beyond me, so I can't help you on that.

    To find the maximum resistance of the pot, connect an ohmmeter to the two outside legs of a 3-legged pot.

    To make a decent variable resistor, connect the inside leg to any outside leg (of a 3-legged pot) and connect that to an ohmmeter. Connect the other remaining leg to the same ohmmeter. Watch the ohmmeter as you turn the knob.

    Maybe you can make a graph that way. Pick an angle, and turn the knob to that angle, and record the resistance. You can do an angle Vs resistance graph.
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If a pot is log taper (audio taper), the resistance from its slider to its lower terminal is 1/10th the total resistance, when it is set to halfway.

    A linear pot has the resistance from its slider to either end at half its total resistance, when it is set to halfway.
     
  18. walters

    walters Banned

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    Thanks for the information and help

    Whats a Semi Logarithmic Taper?

    Where can i find "S" taper pots?
     
  19. walters

    walters Banned

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    What is taper? It's just the ratio of the resistance already passed as the pot turns to the total resistance of the pot, described as a curve

    Percentage or ratio of Rotations VS percentage or ratio of resistance

    tapering resistors-connect a "tapering resistor" across the wiper and CCW lug,

    What are some tapering resistors with other tapering components across the wah pot can make different curves or sweeps?

    A Linear pot that has a "tapering inductor" across the wiper and CCW lug to give a different sweep what would this do?
     

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