Photoresistor polarity

I received my particle photon kit last week. Was doing the demo with the photoresistor but there seems to be an issue related to the polarity of the photoresistor. If I use the demo app “Function Variable”, with the photoresistor in one direction I get readings of around 15. If I flip the photoresistor I get readings of 0 or an occasional 1. I don’t get it…

There is no polarity with a photoresistor, it is symmetrical. It could be, you have a bad one, or you might be making bad contact with the breadboard. Try other holes, or crimp the wire over to double its thickness. I do that sometimes with resistors that have such fine leads.

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The photoresistor has to be bad. I’ve tested with a multimeter both by sticking the photoresistor into the breadboard just to hold the ends steady for me to touch with the meter and also by doing the same thing by just sticking it through a piece of cardboard. One direction I get .750 mohms and the other way my meter just shows 0F which means out of range. I have range set to auto so it should figure it out. Even if I lock the range on .xxx mohms (the range the first direction reads on) it still doesn’t read anything.

Is that the proper reading I should be getting? I know my amount of light will determine the actual reading but is it at least within spec?

I don’t have this kit so I don’t know exactly what part you have. Those value seem high compared to other photoresistor that I’ve used.

Not to beat a dead horse here but the other odd thing is this… I stick the photoresistor into my meter in one direction and I get 114 kohms. Under the same light I flip it and my readings are in the mohms and that was me needing to make sure if didn’t cast the slightest shadow on it otherwise I wasn’t even getting a reading. In both cases as I move closer to the light the resistance drops so it seems to “work” it’s just that one direction seems to essentially give a totally different range of resistance. Can anyone confirm if this is somehow normal or definitely a bad photoresistor or at I doing something else wrong? Sorry, I’m new to this and trying to learn but when strange things like this happen it makes you wonder if you’re just not comprehending what’s happening or something is truly defective.

Thats not a photoresistor , its a LED

I’m humoring your statement but i’m not convinced… For starts it doesn’t light up when I apply voltage and gnd. Tried both polarities… Next, I hooked it in series with the actual LED from my kit. (Keep in mind the kit comes with 1 LED & 1 Photoresistor, so unless they screwed up in packing, one should be an LED and the other is a Photoresistor and I know for sure one is an LED because it lights up). When its in series with the actual LED, the more light the “photoresistor” picks up the brighter the LED shines… But again, if I flip the “photoresistor” to the other polarity, the LED barely shines at all, if I aim the “photoresistor” at my light source the LED brightens ever so slightly.

It does not look like a photoresistor to me either. I could be an LED or more likely it could be a photodiode (which would not light up). If the part is a photoresists and is 114 Mohm one direction and 37 Mohm the other, maybe it is just broken.

Try putting your meter is diode test mode (see the little diode icon above the ohm?) with the select button and see if that changes your reading. In diode test mode you ought to be able to cover it and illuminate it and change the readings.

so in diode test mode, one direction does not show anything, the other direction starts just under 2 and the number drops as I get closer to the light source… The lowest number I can get to display is about .3 when I am the closest to the light…

Take a couple closer photos. One from the side and the other looking straight at it.
Just because it does not light up does not mean you didn’t blow it up already.

ya, thats a led.

But why does resistance change based on light?

Cannot answer that.

LED are bad photodiodes and people have used them for detectors at times. It is definitely a diode of some kind and the behavior you saw with the meter in diode mode is consistent with that.

As to why diodes are photodetectors, that would be due to photoelectric effect which Einstein explained pretty well:

BKO, then do you agree with mikemoy that this was/is an LED? Or do you think its a photodiode?? Its just strange that it acts like a photoresistor, and now like i said the difference in polarity essentially seems to be the range that its reading in… kohms for one polarity and mohms for the other… Obviously not a photoresistor but an LED??? Anyway I guess its time to just get some more. Was just boggled.

along the photodiode idea: how much current does it generate under no light vs bright light?

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It looks like a photodiode to me but it is hard to tell from the pictures. If it is an LED, it might not be a visible light LED if you can’t see it light up. Was one of the leads longer than the other? Normally on an LED, one lead, the anode, is longer.

Your meter does not generate enough voltage in ohm mode to make a diode saturate and “turn-on” so when you are not in diode mode, you are not getting true readings.

@BulldogLowell experiment is a good one-measure the dark and light current with your meter on the lowest current (amps) scale.

Edit: Just one more point: It is definitely not a photoresistor.

getting 0 amps, tried on both the mA setting with the red cable on the right terminal and the A setting with the red cable on the left terminal… Tried reversing polarity and held right up to a light source…