Red laser detector

Hi all,

The Chinese are knocking out lasers like these at ridiculously cheap prices currenty:

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And I've seen others *even* cheaper than that including postage.

There are a good many applications for these parts that spring to mind, but the ones I'm thinking about require some sort of sensor capable of detecting if the beam is broken. I'm not sure what would fit the bill for this purpose. Light dependent resistors, even if they work well at this single wavelength (typically 650nm IIRC) are probably too sluggish to react and useless for all but the most elementary of applications. I'd like to make something up to measure the RPM of rotating machinery and need something that will provide a speedier and better-defined response. Any ideas for a suitable component?

Reply to
Chris
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A silicon photodiode, reverse-biased, should work fine. For a tach application, just a couple Volts of reverse bias should give fine performance.

Jon

Reply to
Jon Elson

Overpriced!

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Photodiodes: Osram SFH2xx.

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Reply to
John Larkin

Red LEDs make quite respectable photodiodes, as long as you don't need super-fast modulation.

Reply to
Dave Platt

LDRs are pretty slow, and the cadmium in them is too toxic to sit well with RoHS, some suppliers have dropped them altogether - I ordered a couple of packs from China while I still could.

Photo diodes suggested by others are probably best, they're certainly fast enough. Photo transistors are more sensitive, but not as fast as PDs.

Reply to
Ian Field

I'd

For just one................. eBay item number:

252230277757

Dan

Reply to
dcaster

eBay also has laser "collimating/focusing" lens. How would these be used?

Reply to
Bob Engelhardt

At 22 cents each, I wonder how they would last ?

Andy

Reply to
Andy K

Some of these diodes *need* such a lens, because their raw output is ridiculously fanned out, so some form of correction is required. I'm not quite sure of the technical term for it, but it needs to be a lens (or for best results 3 lenses in combination with each other) which focus primarily in one plane.

Reply to
Cursitor Doom

They claim at least 1000 hours. Count on it.

Reply to
John S

Available from Digikey.

Reply to
John S

How much money/time do you plan on investing?

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Mark L. Fergerson

Reply to
Alien8752

I've used a standard Sharp I/R detector and modulate the laser at 38KHz...

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Reply to
TTman

The photo detectors in networking gears should be much faster. We have a bunch of 4 Gb/s fiber channel cards to dismental/experiment with. I think they can do at least 1 GHz.

Reply to
edward.ming.lee

Some of these products are the raw diodes, which diverge a LOT. But others are described as "dot" diodes, implying that they have a collimating lens. And they have a barrel shape that also implies a lens.

Assuming that they are collimated, how good could it be for these prices? That's a real question.

Is it likely that the pieces of a given lot would have the same divergence? I.e., could one do better than average by picking and choosing from a lot of 10?

Thanks, Bob

Reply to
Bob Engelhardt

I was wondering the same thing (I also noticed the "dot laser" qualifier)- suppose they're assembly-line rejects and their "dot" is more of an "oval"? No brand name, manufacturer, or UPC available...

Mark L. Fergerson

Reply to
Alien8752

I just took another old one to bits for something to do. The "lens" is made of some sort of cheap transparent plastic 6mm diameter which does not appear to be aspheric after all. No coating (this was a very low-end pointer) but *very* short focal length (< 5mm). I'm guessing the better quality ones will use aspheric, coated lenses for a better-defined and brighter dot.

Reply to
Cursitor Doom

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