What is Photoresistor: Definition and 14 Discussions

A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The resistance of a photoresistor decreases with increase in incident light intensity; in other words, it exhibits photoconductivity. A photoresistor can be applied in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a resistance semiconductor. In the dark, a photoresistor can have a resistance as high as several megaohms (MΩ), while in the light, a photoresistor can have a resistance as low as a few hundred ohms. If incident light on a photoresistor exceeds a certain frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electrons (and their hole partners) conduct electricity, thereby lowering resistance. The resistance range and sensitivity of a photoresistor can substantially differ among dissimilar devices. Moreover, unique photoresistors may react substantially differently to photons within certain wavelength bands.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, for example, silicon. 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, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (that is, longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

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  1. hugo_faurand

    Need help with MOSFETs and photoresistors

    Hello everyone ! At High School I have to make a little robot which is steerable with light. It has 2 photoresistors, if a photoresistor received light it switches on a motor. That allows us to go leftward or rightward. But, I have a problem. I don't manage to link every components between us...
  2. R

    Selective Incident Headlight Intensity Measurement

    Hey guys, I am trying to come up with an idea to measure incident intensity of headlights from an oncoming car for one of my uni projects. This wasn't specifically assigned project topic, we get to choose what we want to do. It's my Instrument and Measurement unit, and I had this idea thought it...
  3. A

    Working principle of the photodiode and photoresistor

    Hello everyone. I feel myself a little bit confused because I can't understand the difference between the working principle of the photodiode and the photoresistor. The photoresistor changes its resistance under the influence of light while photodiode changes the current across iself. Is this...
  4. T

    Measuring aperture size through Lumen intensity -- Advice.please

    Greetings everyone. I'm working on a project that requires the very accurate measurement of distance varying between 0.500'' and 0.050''. I need to measure the distance within at most 0.010'' tolerance and give the value digitally. The read-rate is once every second. I'm on a very tight budget...
  5. G

    Does a photo-resistor create an AC current?

    I'm a high school physics student. I want to do an experiment to see what happens when a high frequency alternating current goes through a magnet. To create the high frequency alternating current could a photo resistor do that for me? Does the photo-resistor create an alternating current with a...
  6. N

    How to convert Ohms to luminosity?

    So, I recently joined a stargazing club, and for our summer activity, we're supposed to use a telescope, a photoresistor, and a multimeter to measure the luminosity of the moon. I know that luminosity can be calculated with the equation, L = σ AT 4, and that brightness can be measured with the...
  7. W

    Circuit Components-Potential differencd across LDR (photoresistor)

    Homework Statement The following figure shows a potential divider circuit made up of a LDR and a variable resistor R. It is used to activate a buzzer circuit which sounds only at 2.6V and above. The resistance of the variable resistor varies from 100ohm to 10kohm while the resistance of the LDR...
  8. S

    Maximum (cutoff) freq for a photoresistor

    Hi, I am working on a project where I want to use a photoresistor to measure displacement, by reflecting light off of a small component and measuring the light that is reflected. the goal is to measure vibration. I am having a hard time finding information on the cutoff frequencies of...
  9. H

    Using a circuit with a photoresistor

    Hello Wizards, I am using a circuit with a photoresistor that sits under the eave and senses ambient light to turn on a light at night to shine on an American Flag. This works fine for a month or two and then quits working. What I find is the photoresistor goes bad. Ambient light in the...
  10. T

    Robot Laser Tag Photoresistor Circuit

    We're trying to build a circuit that will detect when a laser crosses a photoresistor and light an LED. What we're trying to do is have three different photoresistors, and three diffferent LED's, each of which correspond to one photoresistor. The idea is that when one photoresistor is struck...
  11. R

    Maximum detectable wavelength of a photoresistor

    to increase maximum detectable wavelength of a photoresistor which is equal to planck's constant/ semiconductor energy gap , we need to decrease the energy gap only planck's constant cannot be increased or decreased, right?
  12. P

    Can a photoresistor be reversed to control sound in a circuit?

    I'm a total newbie at this, but I have a photoresistor set up so that it controls the shorting on an audio circuit. When light is on, it shorts and cuts the sound. When it is dark, it closes and allows the circuit to run normally. How can I reverse this so that light will make the sensor...
  13. K

    Solving Photoresistor Problem - Circuit for Motor w/ Battery Pack

    I need a circuit diagram for running a motor using a battery pack (3 volts) that uses a photo-resistor for a switch. Really sorry for bothering you with such simple question but i am new to this thing. Thank-you. The attached was my previous circuit it did not work.
  14. E

    Improve SNR for Raman Spectroscopy with Photoresistor: Tips and Tricks"

    Homework Statement We are using Raman spectroscopy to detect the presence of chemicals from emitted light. We have a light transistor that behaves as a photo-resistor of 50k-ohm resistance. The light signal generates 2 microvolts of 2 kHz signal at normal Raman levels. We want at least...