Being able to detect light is very important in many electrical systems - for example, many phones have a light sensor on the front to automatically dim the screen.

Today I'll be discussing the Photoresistor, a really neat electrical component that can easily detect light. I hope you find it interesting!

A large photoresistor
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Detecting Light: Phototube and Alternatives

Decades ago, devices called phototubes were used to detect light (see my post on this here). Visible photons eject electrons from a filament cathode via the Photoelectric Effect, causing current to flow across the tube, carried by the accelerating electrons.

Gas-filled Phototube - The big metal plate is a low work function cathode that ejects electrons when certain color lights strikes it.
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Of course, vacuum/gas tubes have their downsides. Notably, they are large, expensive, and prone to failure (it's somewhat easy to breach a vacuum). While specialized phototubes are still used today to detect gamma ray strikes (namely photomultipliers), they have mostly become obsolete. The reason? Solid state light detectors.

The two big solid state light detector are photodiodes/cells and photoresistors. The former produces electrical current when light strikes a PN semiconductor junction. The latter changes resistance based on incoming light, and is the subject of today's post (I'll be covering the photodiode at a later date).

Band Gaps and Semiconductors

Let's briefly cover energy band gaps before moving on. Solid semiconductors do not ordinarily conduct electricity very well at room temperature. This is because the energy levels of the atoms in the solid form two "bands": A lower energy band, with almost every state filled with electrons, and a higher energy band with very few electrons in it. Current can only flow when electrons can move to other states in the solid, but since electrons have spin-1/2 and as such as known as fermions, two electrons cannot occupy the same quantum state at the same time (this is known as the famous Pauli Exclusion Principle).

Because of this, electrons in the lower band have nowhere to go. As such, not much current flows when you connect a battery across the semiconductor. These electrons could move up to the higher band (known as the Conduction Band), but they need to be supplied enough energy to cross the energy difference between the bands, known as the band gap. In the conduction band, there are many empty states and electrons can freely move between states in response to an electric field, conducting current.

An illustration of the two bands and the band gap. Note that at temperatures greater than absolute zero, random thermal activity causes there to be a select few electrons in the conduction band at any given time - this means that semiconductors typically get more conductive as you heat them up.
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What sets semiconductors apart from most other insulators is that their band gaps are somewhat small - but still not small enough to have excellent conductivity at room temperature.

Difference between insulators, semiconductors, and conductors.
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In short, if we want a semiconductor to better conduct electrical current, we need a way to apply energy to the electrons and allow them to move up across the band gap to the conduction band. I already mentioned heating up the semiconductor as a way to do this.

Photons of light can have energies greater than typical band gaps. What if we used photons?

Photoconductivity

Imagine we take a strip of some semiconductor and measure its electrical resistance. Since semiconductors typically don't conduct current very well at room temperature, the conductivity will be low and the resistance will be high - this chunk of semiconductor will act just like a high value resistor.

Now let's bombard the semiconductor with some photons. If they are too low energy (low-infrared, microwaves, radio waves), they will have no effect: The semiconductor will keep on acting like a high value resistor. But fire photons at the solid with energy greater than the band gap (typically on the order of ~1 electronvolt) and things change. If the photons hitting the solid have sufficient energy, they can cause electrons to move up out of the lower band and enter states within the conduction band. Now, these electrons have other empty states to move into when an electric field is applied, and they can carry current across the solid. So if we measure the resistance now, it will be much lower than before.

Increase the brightness of the light source and the resistance will drop even more. This change can be very substantial - Wikipedia quotes the maximum change as ranging from several million Ohms in the dark to as little as 100 Ohms in bright light for typical photoresistors.

This effect is called Photoconductivity, and this is essentially how the photoresistor device works.

Photoresistors!

Now we have everything we need to discuss photoresistors themselves! These are small devices that operate using photoconductivity. Being solid state devices, there are no vacuum chambers or gas envelopes to worry about, and they can take up far less space than a phototube light sensor.

On top of being portable, photoresistors are dirt cheap: Look on Ebay or Digikey and you'll find them for pennies each.

In the dark, the photoresistor has very high resistance and not much current can flow across it. In the light, the photoresistor's resistance drops dramatically and lots of current can flow. It technically wasn't necessary to get into the details above to show how these work, but I hope it clarified something for you.

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On top of all this, photoresistors are completely passive: They work with no external power source needed (other than the light, of course).

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Photoresistor Uses and Wrap-Up

You can use a photoresistor anywhere you want to detect light. This makes them abundantly useful.

Want to make an alarm clock that automatically turns on when the sun comes up? A photoresistor provides an easy way to spot the sunrise. Need a porch light that lights up after sunset? You can use a photoresistor for that, too. Laser tripwires for nighttime use? Automatic morning toaster (hey I'm sure someone's tried this...)? Headlamp that turns on at night? The photoresistor can help.

Photoresistors are basically just cheap, reliable light sensors. I haven't used them in too many projects yet, but if you are into DIY electronics you should definitely have some of these on hand - they're absolutely worth the few Dogecoins they cost.

Now you know how a photoresistor works! Hopefully you learned something new here. If you have any questions, comments, or corrections, don't hesitate to let me know and I'll do my best to respond.

While writing this I of course starting thinking about ionizing radiation and wondered if you could produce noticeable resistance change on photoresistors by irradiating them with gamma rays. The radiation definitely has the energy to boost electrons to the conduction band but may cause complications due to this energy being many times the bandgap, and the amount of radiation being small. If I have any luck I'll make a post about it and call it a radioresistor. If you've heard of this being done before, please let me know as I'd love to learn more.

Thanks for reading!

Additional source:
Solid State Physics, an introduction - Phillip Hofmann