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Photoresistor

Photoresistor is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photoresistor rather than just read about it. In short: A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases in resistance as a result of increasing illuminance (light) on its sensitive surface, in other words, it exhibits photoconductivity. A photoresistor can be used in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a semiconductor resistanc…

Photoresistor — main illustration
Photoresistor — illustration

Key takeaways

  • Photoresistor belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Photoresistor to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Photoresistor from memory before moving on to harder problems.

Reference excerpt

A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases in resistance as a result of increasing illuminance (light) on its sensitive surface, in other words, it exhibits photoconductivity. A photoresistor can be used in light-sensitive detector circuits and light-activated and dark-activated switching circuits acting as a semiconductor resistance. In the dark, a photoresistor can have a resistance as high as several megaohms (MΩ), while in the light, it 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 (such as silicon is). In intrinsic devices, most of the 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.

Design considerations

A photoresistor is less light-sensitive than a photodiode or a phototransistor. The photoresistivity of any photoresistor may vary widely depending on ambient temperature, making them unsuitable for applications requiring precise measurement of or sensitivity to light photons. Photoresistors also exhibit a certain degree of latency between exposure to light and the subsequent decrease in resistance, usually around 10 milliseconds. The lag time when going from lit to dark environments is even greater, often as long as one second. This property makes them unsuitable for sensing rapidly flashing lights, but is sometimes used to smooth the response of audio signal compression.

Applications

Photoresistors come in many types. Inexpensive cadmium sulfide (CdS) cells can be found in many consumer items such as camera light meters, clock radios, alarm devices (as the detector for a light beam), nightlights, outdoor clocks, solar street lamps, and solar road studs, etc. Photoresistors can be placed in streetlights to control when the light is on. Ambient light falling on the photoresistor causes the streetlight to turn off. Thus energy is saved by ensuring the light is only on during hours of darkness. Photoresistors are also used in laser-based security systems to detect the change in the light intensity when a person or object passes through the laser beam. Resistive opto-isolators, also known as vactrols, are formed of a photoresistor in conjunction with a light source. The light source may be a small incandescent or neon lamp, or light-emitting diode. Popularity has waned, but they are still used in audio applications, such as dynamic compressors. A common usage of this application can be found in many guitar amplifiers that incorporate an onboard tremolo effect, as the oscillating light patterns control the level of signal running through the amplifier circuit. The use of CdS and CdSe photoresistors is severely restricted in Europe due to the RoHS ban on cadmium. Lead sulfide (PbS) and indium antimonide (InSb) LDRs (light-dependent resistors) are used for the mid-infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

See also Optoelectronics Photodetector

References

External links

Using a photoresistor to track light Connecting a photoresistor to a circuit Limitations of Photoresistor Photoresistor overview - detailing operation, structure and circuit information

Illustrations

Photoresistor illustration
Photoresistor illustration
Photoresistor: Three photoresistors with scale in mm
Three photoresistors with scale in mm
Photoresistor: Large CdS photocell from a street light
Large CdS photocell from a street light
Photoresistor: The internal components of a photoelectric control for a typical USA streetlight. The photoresistor is facing rightwards and controls whether current flows through the heater which opens the main power contacts. At night, the heater cools, closing the power contacts, energizing the street light.
The internal components of a photoelectric control for a typical USA streetlight. The photoresistor is facing rightwards and controls whether current flows through the heater which opens the main power contacts. At night, the heater cools, closing the power contacts, energizing the street light.

Worked examples

Example 1 — a first encounter with Photoresistor

Start with the simplest possible case. Write down what Photoresistor claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photoresistor before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photoresistor ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photoresistor

In research
Photoresistor appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Photoresistor in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Photoresistor is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical devices, Resistive components, Sensors, so understanding it makes those chapters shorter.
In everyday life
Look for Photoresistor outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Photoresistor in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Photoresistor means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Photoresistor out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photoresistor in simple terms?

A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases in resistance as a result of increasing illuminance (light) on its sensitive surface, in other words, it exhibits photoconductivity. A photoresistor can be used in light-s…

Why does Photoresistor matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photoresistor?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photoresistor.

Tags

  • Optical devices
  • Resistive components
  • Sensors

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