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Topic: Increasing the coverage size of an LDR (Read 1 time) previous topic - next topic


I'm trying to build some kind of laser detector, to know if the laser as been obstructed. I know it can easily be done by pointing the laser beam directly at the LDR.
The laser module has to be separated from the LDR module (in this case by about 3 meters or 10 feet).
Each modules will be made in wood.

The problem is that my LDR is 3mm in diameter and I don't want to fine tune it's position every time I move my device.

Is there a way to increase the size of my LDR coverage, say, to about ½ inch or more? (Already tried biconvex lens, but it won't work efficiently with lasers apparently)

Thank you.


Lens should work with laser - light still refracts the same whether its coherent or not.  Just focus the lens at infinity for best results.  So long as beam falls somewhere on lens it will steer it to the sensor...
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Feb 02, 2012, 02:34 am Last Edit: Feb 02, 2012, 04:03 am by pcpc9 Reason: 1
That's what I thought, but I tried and it didn't work (laser entering the lens at a right angle). Very strange, maybe I wasn't holding my indicator at the focal point (LDR should be at focal point right?). I'll try another time when I get some lenses, but for the moment I'm stuck. Also if I can find a cheaper way then lenses, i'd like it.

BTW, does anyone know where to buy cheap lenses with small focal length?



Its crucial the LDR is at the focal point...  Can you not ensure this?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


I feel stupid now, I tried with some cheap lens (magnifiers from the dollar store) and it turns out it works perfectly. I knew something was wrong.

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