Coupling LED to LDR

There was a compound we used clear back in the '60's called generically, "greased pig snot" ;-) ...Jim Thompson

I need a current-to-resistance converter that works on millivolts of AC and adds very little distortion and excess noise. None of that other stuff (temperature, memory effect) matters much because it's inside a high-gain control loop.

These things are still being made for a reason, though I guess the old incandescent and neon bulb ones are no longer common.

I think this stuff is still pretty common in some corners of the audio field, maybe Phil Allison can comment?

Thanks, James, looks like my best bet is probably to buy the brightest LED available at my operating current (probably green for maximum response) in a 5mm wide-angle LED.

I'll pick up a few with the next order and get some of John's orange LEDs to compare (sounds like they have a nice old-fashioned neon orange color). Maybe write some code to make them flicker. ;-)

BTW, I tried white and it wasn't better than the green (similar brightness numbers IIRC). It was not a lot worse.

There you go again! :-) Mikek

Spehro Pefhany Inscribed thus:

Try a small dimple in the centre of the lens. A two or three mm dia drill might work. Use a pin vice to hold the drill.

Hi, John:-

Didn't know about (or didn't remember, same thing I guess) those, thanks, very interesting.

Probably too high resistance for this application, but I could see using them. I might add them to the layout to play with them.

Best regards,

--sp

George Herold Inscribed thus:

A very light touch into a flame will polish the end !

That is interesting. Are you sure you mean too a high resistance? It goes down to ~200 ohms or so. I thought LDR's were ~10k.

One bad thing is that it's only (approximately) linear for delta V less than 100mV, or so.

George H.

Thanks Baron, I'll give that a try.

George H.

Depends on how much light you shine on them. ;-)

LDRs are pretty linear if you keep the voltage across them less than a couple hundred mV.

This will depend on the LDR sensitivity to different colors of light; have you considered using a Day-Glo (green or red-orange) phosphor and a UV or blue LED? LEDs with lenses are hard to get uniform light out of, but a flat label-dot with fluorescent paint is a good 2-pi-steradians source, regardless of how the illumination light comes in.

I've recently done some experimenting with 360 nm LEDs and various colors of ebay fluorescent paint.

Into a silicon detector, the orange was best, and the red and pink were close.

A white LED sort of does this for you. Some of the bigger, flat multi-chip parts would be interesting. Some of them are blinding.

I haven't read a spec yet on what you need to detect. Distance ? size? obje ct? speed?

Sharp/Vishay Photo Diodes are the most stable repeatable inexpensive passiv e Sensors. Panasonic makes a nice buffered current source Photo Detector t hat is CIE color corrected with > 7 orders of magnitude sensitivity.

I used to be able to detect a resistor wire passing an LED beam 1 meter acr oss a path to an IRDA AGC Photo Receiver. I recessed both IR LED and Irda R eceiver by >1cm with a 4mm hole to get a narrow aperture to block stray lig ht. I could use multiple paths adjacent to each other using a TDM unique p attern on each path to detect any interruption of light on any path and com puter the direction and speed an object was moving across the lane.

Are you color blind? I bet you have a gray car.

This is mine:

I'm mostly visual. I like shapes, depth, colors, light, patterns, structures, motion, circuits.

Nothing personal John, just teasing you. I took my daughter to a hamfest last year, she bought a little electronics kit for her husband to assemble. I was helping him when I realized he couldn't read some colors on the resistors, I felt bad because I had teased him about reading the colors. I got the meter out and showed him how to use it. Ya got me on the cars though, I have a silver/gray truck, white car and a white cargo van. I did have a Blue Lexus, I let my son take that to college with him. Can I get partial credit for that? :-) I like shapes too, especially a high waist to hip ratio. I'd go with blue as a favorite, like the lighter blue in the Firefox Icon. btw, I had a cherry red 66 Ford Fairlane 500 with chrome wheels, but that was about 40 years ago. Oh, she sounded sweet! Mikek

A weenie car.

I have this one in dark blue...

:-} ...Jim Thompson

Maybe a LM13700 or 13600 configured as a voltage controlled resistor will work for you?

Too late, my lawyer is on the case.

I see whole blocks of cars, all white and black and grey. Booooring. I can always spot my Audi in a parking lot.

I like skinny women. Skinny is a shape!

The price I pay for being so visual is that my auditory processing doesn't work very well. I hear fine, but can't pick out words from background noise ("cocktail party effect") or understand languages or even accents, and I don't care much for music. And I'm not a "people person." My wife is a speech pathologist, exactly the opposite.

On a sunny day (Wed, 30 Oct 2013 15:10:48 -0700) it happened John Larkin wrote in :

Nice round spot. Phil Hobbs nanowatt photoreceiver! ;-)

Original artistic signature... Must be expensive...