Light Sensors

[Electroenthusiast]

How exactly does Photodiode look?
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The Light that i need to ON will be just near the Photodiode. (Light slightly falls on Photo Diode)
What would happen?
Also saw flattened top in one LED. Why is it so?

What does highlighted portion tell?
X k / y k ??

 

[cr0sh]

Well, for one thing you are asking about a photodiode, and showing an image of a photocell (same thing as a solar cell, though usually much smaller); strangely, though, the highlighted portion in the schematic seems to be indicating resistance of the photocell (dark vs. light resistance, or vice-versa) - which I usually associate with cadmium sulphide cells/photoresistors/LDRs; but I suppose a photocell can have a resistance, too.

As far as what a photodiode looks like - I have seen all kinds; some are nearly "bare", some look like LEDs, some look like transistors (ie, in a TO-92 case, sometimes a clear case, etc). If you have a device and don't know what it is, I am not sure how you can tell. I mean, a regular transistor, and a photodiode/phototransistor in a similar case are easy to tell apart, because the light sensitive version will have a clear window or plastic to admit light to the junction.

But then again, I have in one of my drawers a device that was mixed up in with a bunch of phototransistors that looks like a very small (size of a grain of sand, actually) axial lead LED! I have no clue what it actually is, although the fact that it was in a small plastic box with a bunch of old phototransistors indicates to me that it to, is either a phototransistor or a photodiode of some sort. I guess I will have to test it as such first, then if that doesn't work, try it as an LED. If the smoke comes out, oh well - a 5 cent part down the tubes!

To make matters worse, though, an LED can be used (in reversed-bias mode) as a crude photodetector; its not a very good one, but it can work.

Finally - about that flat-topped LED - this is usually done to make for a much wider dispersion of the light the LED produces, generally for a panel light. Some LEDs have narrow ranges at which they are brightly visible; by flattening the LED, the range is increased (at the expense of brightness). Also, on some panels they are used for aesthetic or design reasons (and depending on whether the panel will be used at an angle, etc - instead of being looked at straight on).

Hope the above helps!

 

[Sceadwian]

The small section of the schematic shown explains nothing without the rest of the circuit as a reference, the text in the box could be related to another portion of the circuit even being near the symbol of the photocell. As it stands the highlighted portion doesn't make sense.

Well, for one thing you are asking about a photodiode, and showing an image of a photocell (same thing as a solar cell, though usually much smaller)

All photodiodes are photocells. What some people call photocells are not photodiodes. The symbol used is polarized so it can't be a Cadmium Sulfide cell, it could be a photocell array or any number of a thousand other type of devices, without refrence there is not way to tell.

The pictures posted above of essentially LED looking devices is a common package for photodiodes but they come in many others as well, so it's not always easy to spot one, and you could mistake a photodiode for a phototransistor easily.

 

[Electroenthusiast]

Well, for one thing you are asking about a photodiode, and showing an image of a photocell (same thing as a solar cell, though usually much smaller)....

....because the light sensitive version will have a clear window or plastic to admit light to the junction.

To make matters worse, though, an LED can be used (in reversed-bias mode) as a crude photodetector; its not a very good one, but it can work.

....that flat-topped LED - ....used for aesthetic or design reasons (and depending on whether the panel will be used at an angle,....

the above really benefited!
Can i make it flat? / Can i get it like that?

....stands the highlighted portion doesn't make sense.

Highlighted portion is for Phototcell(i.e, Photo-resistance:LDR)

 

[cr0sh]

the above really benefited!
Can i make it flat? / Can i get it like that?

You can buy them with flat ends, otherwise you can use sandpaper and/or a file; be careful not to take so much off you reach the junction.

the above really benefited!
Highlighted portion is for Phototcell(i.e, Photo-resistance:LDR)

An LDR doesn't look like either of the device images you posted, they typically look something like this:

http://202.44.68.33/files/u21019/Cds_Photoconductive_Cell_Photoresistor_LDR.jpg

...in various sizes, of course.

 

[Sceadwian]

That is not the symbol for a photo resistor. The symbol is a bit simple but it is obviously polarized, it looks almost like a capacitor symbol so it could be a reverse biased photodiode. The rest of the schematic is incomplete, so it's anyone's guess.

r.vitt, don't forget the housing of most LED's are gigantic in comparison to the actual LED element, look at a clear one with a magnifying glass, you'll see a tiny little foil cup with two wires coming off of it, that's it, the rest if just effectively an optical pipe. Sandpaper does wonders to diffuse the otherwise sharp glow of an LED into a softer more spread out light, you lose a bit of brightness but not much. If you want as close as you can get to a raw LED element with no package get yourself an SMD LED, you could swallow a hundred of them and it probably wouldn't hurt you, and they're every bit as bright as the full sized units.