Electronics notes/Light sensing

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This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

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See also Category:Electronics.


This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

There are a bunch of similar devices, some general names and some not, and some specific implementations, and some more sensitive than others. Roughly from more general to more specific:

  • photocell, or photoconductive cell, photosensor, and photodetector [1]
are generally desscriptive of the function rather than a specific implementation
  • photoresistor, light dependent resistor (LDR)
cadmium sulfide (CdS) cell, cadmium Selenoum (CdSe) cell have a good response in the visible spectrum
PbS, PbSe, InSb, and GeCu are mainly sensitive in near, mid, and far infrared
note that cadmium and lead are not not RoHS compliant. shouldn't matter for DIY, may matter when building products.
  • phototransistors
  • photodiodes
also note you can use LEDs as photodiodes, see below
  • avalance photodiode (APD) [2] use a high reverse bias voltage to create an effect much like
  • photomultiplier - a vacuum tube design, rarely seen now (the nearest semiconductor equivalent is the APD)



Photoresistors are passive semiconductors. It is photosensitive because it has an np junction - that is exposed on purpose.

In the dark they may be 1MΩ, in bright light they may go down to a few hundred Ω or less. Both order of magnitude, can be varied (higher) intentionally.

Regularly hooked up in a voltage-dividing sort of way, see e.g. some arduino examples.

Passive, and slow (e.g. 10ms dark to light, possibly hundreds light to dark) compared to photodiones or phototransistors (also see below).

This sluggishness is sometimes an interesting quality, e.g. in analog sound processing (e.g. slow response is potentially nice in compressors, for smoothness in optical theremins and such) and when switching certain other devices using mostly analog circuitry, there are other situations (such as optical communication and anything digital that wants hard edges) where you would always want a photodiode or phototransistor instead.

See also:

Photodiode and phototransistor

Again, all PN junctions are light sensitive, so if exposed, you create a photosensitive thing.

(This is also why LEDs work as photodiodes - they are in essence the same device, it's just that LEDs have been optimized for emission and aren't as sensitive used as photodiodes. See e.g. [3])

Photodiodes seem to typically be typically Si-Ge (Silicon-Germanium).

Phototransistors react slower (below a few dozen kHz) but can be ~100 times more sensitive than photodiodes (because they have built in gain(verify)), but also easily 10x slower response (still microseconds, but it can matter)

Photodarlingtons are a variant of phototransistor with higher gain yet.

Phototransistors may not expose their Base, as it's not necessary. It may still be there, letting you use it as a regular and phototransistor. (verify)

Sensitivity is in part about gain - which in a circuit you can always control, though in a single component phototransistors are better at, which also makes it more about noise, because it becomes the question of "what sort of signal can I deal with cleanly".

Also the fact that photoresistors are nonlinear can make a big practical difference.

In comparison:

  • photoresistor:
slowest response (~tens of milliseconds for rise, and noticeably slower for fall)
medium sensitivity
easier to produce for certain frequencies (like IR)
response curve more logarithmic (which can be useful for some purposes)
  • photodiode
fastest response of the three (microseconds)
less sensitive than phototransistor (and photoresistor)
response curve more linear (verify)
  • phototransistor:
slower response than photodiodes (tens of microseconds)
higher sensitivity than photodiodes

See also:

Also related

Optocouplers (a.k.a. photocoupler, opto-isolator) - since the point is electric isolation, they are typically inside the same IC, nothing optical you can do with it


(Compare photoresistors)

Acts like a transistor with light triggering its base.

Faster response than photoresistors, but can be seen as having a large gain so it's hard to use it to measure amount of light.

See also:


Image sensors

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

See also