Electronics notes/Light sensing

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This is for beginners and very much by a beginner / hobbyist.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.


Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors and transformers · ground

Slightly less basic: amplifier notes · varistors · changing voltage · baluns · frequency generation · Transmission lines · skin effect


And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes


Platform specific

Arduino and AVR notes · (Ethernet)
Microcontroller and computer platforms ··· ESP series notes · STM32 series notes


Less sorted: Ground · device voltage and impedance (+ audio-specific) · electricity and humans · power supply considerations · Common terms, useful basics, soldering · landline phones · pulse modulation · signal reflection · Project boxes · resource metering · SDR · PLL · vacuum tubes · Multimeter notes Unsorted stuff

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

See also Category:Electronics.


Photo-somethings

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

photocell, or photoconductive cell, photosensor, and photodetector [1] are more descriptions more than a specific implementation, though they tend to refer to one of the first three:


In comparison:

  • photoresistor, a.k.a. light dependent resistor (LDR)
Response: slowest - ~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)
regularly 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.


  • photodiode
Response: fastest of the three - microseconds
less sensitive than phototransistor (and photoresistor)
response curve more linear (verify)
(also note you can use LEDs as photodiodes, see below, though not a very good one)


  • phototransistor
Response: tens of microseconds. slower than photodiodes but fast enough for a lot of things
higher sensitivity than photodiodes


Also:

  • 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)


http://www.electronicshub.org/light-sensors/


Photoresistors

Photoresistor

Photoresistors (a.k.a. Light Dependent Resistor, LDR) are passive semiconductors. It is photosensitive because it has a (large(verify)) n-p junction, that is intentionally exposed to the outside world.


Not to be confused with phototransistors or photodiodes


Historically best known for the CdS type, though it's now less used because the Cadmium in CdS (and CdSe) is considered toxic by RoHS so can't really be sold in Europe.

Resistance in the dark tends to be on the order of 0.1MΩ to 2MΩ,

In the light (e.g. 10 lux) resistance is usually somewhere in the range of a few kΩ to a few dozen kΩ.

The curve is nonlinear, rising faster at the light end.


Less sensitive than photodiodes and phototransistors.

Also slower than them - photoresistors may take

e.g. 10-30ms dark to light
30ms to maybe a second from light to dark



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, and can be used as crude light sensors. 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.



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

Phototransistors

Image sensors

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

See also