I'm getting into a little more obscure components now, but I think that those interested in electronics will find this interesting.

The Photodiode (along with the corresponding Phototransistor) are semiconductor devices that can be used to detect or harvest energy from light. Completely solid state, these devices have no moving parts and have found uses in tons of circuits, ranging from sensors that detect the sun to radiation detectors to power generators. The best known type of photodiode is the Solar Cell, which is quite literally just an enormous photodiode.

A generic photodiode
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Photodiodes

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Regular diodes (read about them here) use a semiconductor PN junction to only allow current to pass in one direction. Hook them up to a battery one way and current flows across them. Reverse the diode and nothing happens, with no current.

This PN junction (semiconductor with excess positive charge on on side, semiconductor with excess negative charge on the other) has a special property. This junction is formed by applying different impurities to the semiconductor lattice. Shine line on it, and the photons that make up the light will cause electrons to jump energy levels. This produces a current and voltage across the diode.

Sketch of a PN Junction. If light hits the junction itself, current will flow across the diode.
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If you build a circuit to detect this current, you can now know when there is light on your circuit. If you connect the photodiode to a battery charging circuit, you can charge the battery (this is what a solar cell is).

Photodiode Schematic Symbol
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Interestingly, the regular diode is actually a special case of the more general photodiode. Every diode is in fact a photodiode, and every PN junction will generate electric current when exposed to light. If you could scrape off the epoxy coating on a diode, it would become a photodiode, generating a current in the sunlight. LEDs, with their transparent coatings, also in fact act as photodiodes - if you put a LED out in the sun, you can actually produce a voltage across it and even power other (very low power) circuits. Some diodes even come in clear casings, in which case special care must be taken to make sure that they don't act like photodiodes when they shouldn't.

The common, ubiquitous LED can in fact be used as a weak photodiode. If you have a voltmeter, you can shine a flashlight on a LED to witness the voltage produced by it.
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Of course, devices actually made as photodiodes have special characteristics to set them apart from other diodes. Photodiodes typically have a very large "active area" where light can produce current. They often appears as large blue squares of semiconductor material. Photodiodes are of course always housed in a transparent material to allow light to enter.

That material doesn't have to necessarily be transparent to us, however. Some photodiodes work on detecting infrared light too low frequency for our eyes to see. These may have covers that appear dark and opaque to us, but are perfectly transparent to high infrared light. The infrared receiver on your television that lets you use your remote control is likely one of these (which may be a modified infrared LED, or IRED).

Silicon photodiode. Notice the large, dark active area.
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Different semiconductors can be used to detect different colors (frequencies) of light, ranging from visible to ultraviolet to infrared. The most common material is regular silicon, found in sand and glass. Silicon is used to produce most solar cells and many common diodes along with smaller photodiodes.

Once you chose a photodiode, it is only a matter of figuring out how to read the current to build a device that can sense when it is in the sun or when the lights turn on.

Phototransistor

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A more uncommon but quite useful device is the Phototransistor, which builds upon the technology of the Photodiode.

Phototransistor Schematic Symbol
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It's right around now that I am realizing I really should have done a transistor post before this, but here goes. Phototransistors are essentially regular bipolar junction transistors (BJTs) with a clear casing.

BJTs act as electrical switches. Each transistor has three pins: Base, Collector, and Emitter. When you allow a small electrical current to flow across the path between Base and Emitter, a much larger current can flow across the Collector to Emitter path (if you have a power source attached). This lets a transistor not only switch things on and off, but act as an amplifier, turning tiny signals into much larger ones.

The important thing for the Phototransistor is that the Base-Emitter path mentioned above is essentially a diode. It's a PN junction as usual, and controls the flow of current across the other path (Collector-Emitter).

So, perhaps you see where this is going. By making the transistor case clear and transparent, light can reach the PN junction of the Base-Emitter path. This generates a current across this path. But this is a transistor, so now the other path (Collector-Emitter) opens!

A Germanium Phototransistor
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This makes the phototransistor a useful device for generating large signals from small amounts of lights. By hooking up a battery across the phototransistor collector and emitter, the transistor will draw large amounts of current from the battery when a little bit of light hits the transistor. Remember that you still need a power source across the collector-emitter path to do anything useful, otherwise we just have a regular photodiode. You could easily make this into a light detector circuit by feeding the large currents triggered by the light on the Base-Emitter path into a detector circuit.

Conclusion

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Photodiodes can also be used for more indirect uses, like detecting ionizing radiation (which produces a current spike across the diode) and serving as an optoisolator (which uses a LED and photodiode to isolate parts of a circuit). If you include solar panels, photodiodes probably serve a somewhat large role in the electronics you use.

Let me know if you have questions, comments, or corrections. Also, please let me know if this post was too technical (or not technical enough), as I'm not entirely sure if this is accessible enough to people not familiar with electronics. As always, I hope you learned something new and useful.

Thanks for Reading!

Additional Sources:
Photodiode Wikipedia Entry
Photodiodes: Physics and Radio Electronics
What is a Phototransistor?
How the Phototransistor Circuit works