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Photocell tutorial!

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Photocells a.k.a CdS cells, photoresistors, LDR (light dependent resistor)...

What is a photocell?

Photocells are sensors that allow you to detect light. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they often appear in toys, gadgets and appliances. They are are often referred to a CdS cells (they are made of Cadmium-Sulfide), light-dependent resistors (LDR), and photoresistors.

A Photocell is basically a resistor that changes its resistive value (in ohms) depending on how much light is shining onto the squiggly face. They are very low cost, easy to get in many sizes and specifications, but are very inaccurate. Each photocell sensor will act a little differently than the other, even if they are from the same batch. The variations can be really large, 50% or higher! For this reason, they shouldn't be used to try to determine precise light levels in lux or millicandela. Instead, you can expect to only be able to determine basic light changes

For most light-sensitive applications like "is it light or dark out", "is there something in front of the sensor (that would block light)", "is there something interrupting a laser beam" (break-beam sensors), or "which of multiple sensors has the most light hitting it", photocells can be a good choice!

Some basic stats

These stats are for the photocell in the Adafruit shop which is very much like the PDV-P8001. Nearly all photocells will have slightly different specifications, although they all pretty much work the same. If there's a datasheet, you'll want to refer to it

  • Size: Round, 5mm (0.2") diameter. (Other photocells can get up to 11mm/0.4" diameter!)
  • Price $1.50 at the Adafruit shop
  • Resistance range: 200K ohm (dark) to 10K ohm (10 lux brightness)
  • Sensitivity range: CdS cells respond to light between 400nm (violet) and 600nm (orange) wavelengths, peaking at about 520nm (green).
  • Power supply: pretty much anything up to 100V, uses less than 1mA of current on average (depends on power supply voltage)
  • Datasheet and another Datasheet
  • Two application notes on using and selecting photocells where nearly all of these graphs are taken from
 
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BookLover71511 months ago
Is it possible to get photo sensors that are sensitive to other colors, like blue/UV or IR?
Really nice tutorial. There is another photoresistor setup, that's worth mentioning. Instead of having a variable resistor (in this case a photoresistor) connected to the positive and a fixed resistor connected to ground, you could use two photoresistors (schematic shown below). this is called a differential light sensor. This setup can be used to measure the difference between the two sensors with one analog input. If the two sensors are exposed to the same amount of light, the readout on the ADC pin should be around 512, if the microcontroller has a 10 bit resolution. If the photoresistor connected to ground is exposed to more light than the other one, the analog voltage would drop, and if the photoresistor connected to positive is exposed to more light, the voltage would rise.
Sorry. The schematic, I uploaded somehow couldn't be added to the comment.
If both light sensors were receives bright light (low resistance) wouldn't there be a short circuit?
I don't think they wld go all the way to 0, but you cld include 2 equal resistors in series with each LDR
ReCreate4 years ago
that is why they call it photoresistors...I always thought that they either went ON or OFF...
drock2674 years ago
Great 'ible. I just finished my senior project using photoresistors to make an interactive LED table. It also involved some PLC programming for the I/O behaviors. I am entered in a competition tomorrow against my fellow graduating classmates. Here's hoping I win some $$ and get an interview or ten. I learned a great deal about photo cells from personal research and this 'ible explains it pretty clear.
alessiof764 years ago
good tutorial!
Did you know that the LED can be used as a photocell?
25 mV with 60 W of lighting small green led
i test it: http://www.youtube.com/watch?v=L3-0aCJOedo (only in italian sorry)
balam4 years ago
Thank you for this tutorial very informative
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