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Light Sensor identification? Answered

Hello all,
I've just started a project and found a spare light sensor in a Megabrite LED Color Changing Night Light. However I am a complete novice and do not know how I use this specific light sensor.
So here's the question: Are all light sensors the same? Can i just use any lightsensor 'ible or do I need specifics for this sensor? And if so, does anyone know how I can figure out what the light sensor is?
Thanks for any help.

3 Replies

Jack A Lopez (author)2011-12-24

Not all light sensors are the same.  So part of this game is figuring out what kind of light sensor you've got there.

I don't know for sure, but I am going to guess that this sensor is a photoresistor. Sometimes this is called a CdS cell, where Cd-S stands for cadmium sulfide, a material whose electrical resistance changes in response to light. 

This type of light sensor has a certain appearance, with wire traces that look kind of like two combs stuck together, like the one in the picture here:

If your light sensor looks like that one, then it's probably a photoresistor, aka CdS cell, aka light dependent resistor (LDR).

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Renagade (author)Jack A Lopez2012-01-02

Oh wow, that's fantastic! Thank you.
So next question is, is there any specific way i should handle this? How would i know how much resistance i should put to powering it? etc

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Jack A Lopez (author)Renagade2012-01-03

Try connecting it to a multimeter on the resistance setting, i.e. as an ohmmeter, and then measuring the resistance through it, in response to different amounts of light.

I think these things typically have a resistance of around a few Kohm, in light bright enough to see by, and around a few Mohm in darkness.  Some pictures of this are attached. I was using the cap from a Sharpie(r) pen to block out light for the darkness-test.  For that picture, I do not have the cap completely on the LDR, so I'm letting a little bit of light in, but that's because I wanted the meter to say something.  The highest resistance it can measure is 2000 Kohm = 2 Mohm.  When the cap was completely on, the LDR was offering resistance higher than 2 Mohm; i.e. out of range.

Assuming you want to connect device to an external circuit, one way to do this...

...is to make this LDR be one side of a voltage divider. You pick the other resistor in the voltage divider so that it has a resistance in the middle of the range in which you are interested. As a guess, maybe this other resistor would have a value of about 10K.  The point of doing this is so that the output from the divider is a voltage signal.  Then you have some other circuit that watches that voltage, and I sort of drew a circuit diagram of how I imagine this might work.

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