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I completely understand the working principle of light dependent resistor. But there is one thing I am bothered about:

  • Does the LDRs surface has to be equaly illuminated upon all its surface/upon all its photoresistive track? Or it doesn't really matter, if all of its photoresistive track is illuminated or only one part of the photoresistive track is illuminated?

or

  • Are the LDRs values the same, if it gets illuminated upon all of its photoresistive track or if it gets illuminated upon only one part of photoresistive track?

enter image description here

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Try it! Fix the LDR to your desk with tape, connect your multimeter on resistance mode to the LEDs, fix a desk lamp above the LDR and gradually shade it with a piece of card sliding it across the surface of the LDR. You should see the resistance gradually increase.

Why?

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Imagine the LDR as 15 x 1 mm long sections of LDR.

Your photo clearly shows the two silver electrical contacts. The LDR is the brown portion between (and around) the contacts. Judging from your photo and some that I have used the gap is about 0.5 mm and the length of the zig-zag is about 15 mm. You could imagine this being split into 15 x 1 mm LDRs in parallel. Clearly the more that are illuminated the more the resistance will drop.

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  • \$\begingroup\$ That is what I was looking for. One more thing, why are there only zig-zag made LDRs? Would it not be interesting, if there was straight line LDR? Or LDR with only a dot of photoresistive track and all around would be metal - you could experiment with laser on such version of LDR! \$\endgroup\$ – Keno Aug 29 '17 at 12:02
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    \$\begingroup\$ The zig-zag gives the longest length in the smallest area. I agree that a linear one might be useful for some applications. \$\endgroup\$ – Transistor Aug 29 '17 at 12:06
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In the LDR you've pictured, the two electrodes are interdigitated, to give a very wide, very short path across the photoresistor, by eye it's about 40 squares wide, one square long. This puts all of the square elements in parallel. A dot of light illuminating any small element will reduce its resistance, and cause current to flow between the electrodes. If light falls on the whole area, all 40 elements will be conducting in parallel, and you'll get the lowest resistance.

You could imagine a much higher resistance LDR with a long thin photoresistive track between the electrodes. If any part of that was illumninated, the LDR as a whole would not conduct, as it still had high resistance parts in series. In this type, you would have to illuminate the whole surface.

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  • \$\begingroup\$ Is there any LDR alike one you mentioned in the last section? \$\endgroup\$ – Keno Aug 29 '17 at 12:05

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