Red laser detector

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

Jon

Overpriced!

Photodiodes: Osram SFH2xx.

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

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.

I'd

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

Dan

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

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

Andy

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.

They claim at least 1000 hours. Count on it.

Available from Digikey.

How much money/time do you plan on investing?

(Not Affiliated)

Mark L. Fergerson

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

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.

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

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

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.