Coupling LED to LDR

What's the LDR package?

Could you surface-mount a couple of LEDs and mount the LDR flat, above them?

The trick will be to get reasonably uniform light across the sensitive area of the LDR.

It's one of those old-school ceramic disk things, about 5mm diameter, flat front (no lens), leads coming out of the back.

Interesting idea, Invert the LDR so it looks down onto the board. That might work if the SMT LEDs are actually bright enough.

Yes, I think I'm illuminating the heck out of the middle part, but only getting bounce on the outside edges.

Best regards, Spehro Pefhany

Newark has almost 3000 of these for a few bucks:

Cheers

Phil Hobbs

If it's a serpentine pattern, and you only light up some of it, the dark parts will stay high resistance, in series with the low ohm segments. Bad, bad.

How about *four* surfmount leds? I like the super-bright Osrams

Orange is my favorite. Beautiful pure color.

A ways back I did all sorts of experimentation.

Yes, it'll help a lot. Even a light sanding with a fine grit will spread the light enormously, even leaving the radius unchanged.

Cheers, James Arthur

P.S. A flat, polished end will produce a bulls-eye pattern. For a more uniform spread, an unpolished end and a gap would be helpful, even if you have to trim the LED to get it. Absent that, interposing a diffuser might help.

Sphero might have a small bench-top sandblaster in his workshop - great for creating a consistent diffuser finish on the LED

Now that's a blast from the past,.

Flattening the end didn't help, in fact it made it a bit worse.

Best regards, Spehro Pefhany

Thanks, Phil, that one is about as good as I'm getting with the homemade coupler, and it's about 2:1 high in resistance.

Newark does lightly stock some from Silonex that are okay.

--sp

Best regards, Spehro Pefhany

I used to have the ideal thing for that- a little doodad for sandblasting individual sparkplugs. I don't think anybody does that much anymore.

Best regards, Spehro Pefhany

Ha! You must be of my vintage - I remember the spark plug cleaner.

I wrote a spreadsheet for designing LED patterns a ways back. The optics are complicated. Back then I was trying for a uniform far field, with a tight divergence angle.

The LED chip in the die-cup throws a cone, and the sides of the casing capture side-emissions, forming a TIR optic.

If you just chop off the lens completely you get a center spot from the die reflector, surrounded by a ring from the TIR. . ---- . .' '. . . | | | | | | | | | | | \ __ / | | \|__|/ | .--------------. '--------------' || || || || || ||

I just tried modding a few LEDs in the shop.

Wide-divergence LEDs put a clean 5mm spot onto a piece of paper in contact with the tip, while the tight-angle LEDs make a bull's eye.

I assume you've got a tight-focused 5mm, otherwise you wouldn't be having this trouble.

Sanding a tight unit's tip blurred the bull's eye a little, but only a little.

Hacking the lens off entirely is worse--very pronounced bull's eye.

The best pattern was from flattening only the very tip, letting the central cone from the die-cup reflector pass, but leaving the shoulder of the lens to re-direct the TIR portion. It was a little fiddly, but not too bad. The result was a bright, clear, even, 5mm spot.

In order of ease, I'd try a wide-angle slightly diffused unit, then a wide clear; last would be modding a tight, clear LED.

A wide white might be even better--I didn't try it. The wavelength is probably pretty good for the LDR, and the phosphor diffuses the pattern a bit.

Cheers, James Arthur

On a sunny day (Tue, 29 Oct 2013 19:32:31 -0400) it happened Spehro Pefhany wrote in :

Phil Hobbs told me LDRs have hysteresis... What will u use it 4?

The you have limited lifetime too, I know that, were used in the old ages as volume control in studio equipment. I remember some tech was replacing the bulbs in that just before that place burned down...

"Spehro Pefhany" schreef in bericht news: snipped-for-privacy@4ax.com...

Why on earth are you using an LDR for prototyping an optocoupler these days? The (CdS) LDRs I ever used were unstable, unreliable, temperature sensitive and died within some months while 24/7 active. Some said it was due to the DC current though I never checked but got an ordinary photo transistor that worked for years.

petrus bitbyter

Grin... It's cheap enough to find out. I filed down the nose of an LED (as light sensor) and got better green photon efficiency through the transluce nt 'red' led. But it made the sensitivity to yellow led light worse... pres umably because I was losing the focusing effect of the 'lens'. The surface was a bit rough, but a bit of spit* on it helped that. (obviously spit is not a permanent solution but I'm sure there's some other optical 'goop' th at would help.)

Are you making an AGC circuit?

George H.

com

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as light sensor) and got better green photon efficiency through the translu cent 'red' led. But it made the sensitivity to yellow led light worse... pr esumably because I was losing the focusing effect of the 'lens'.

Did you try lighting up the yellow LED to see what kind of pattern it put out at its tip?

As a sensor, it might not quite match Spehro's situation because of the LED's directional, funky-pattern. His LDR's element is evenly spread.

The several wide-angle 5mm LEDs I tried last night all made pretty even 5mm spots at their tips, which ought to be great for a 4x2mm LDR.

ously spit is not a permanent solution but I'm sure there's some other opti cal 'goop' that would help.)

Spit is also good for temporarily attaching reflective glass beads to an LED tip. Unfortunately, the beads weren't much help :-)

Excellent thermal matching to icy flagpoles...

Cheers, James Arthur

It's inferior to nose grease for fixing scratches on CDs. ;)

Cheers

Phil Hobbs