Hello Intelligent people,

I have successfully made Light detector using simple reversed biased clear package 5 mm RED LED and darlington transistors (attached circuit and Datasheet of detector LED)
I shine white/green LED light on that RED LED detector and it detects light.

Then I tried to use SMD LED instead of 5 mm RED LED I scavenged from somewhere. (attached pic)
SMD LED measures 3 mm by 3 mm and is of bright yellow color.
I actually don't know the part number of SMD LED.
Forward voltage of SMD is 3.4 V with 48 ohms of resistance.

I shined every color of light on it and It doesn't work, any guess why ?

 

I shined every color of light on it and It doesn't work, any guess why ?

Your bright yellow SMD LED is phosphor based LED.
It contains blue LED (3.4V forward voltage), coated by phosphor+epoxy.
Blue LED is sensible to UV light, but phosphor coating shields LED from external UV.

 

Your bright yellow SMD LED is phosphor based LED.
It contains blue LED (3.4V forward voltage), coated by phosphor+epoxy.
Blue LED is sensible to UV light, but phosphor coating shields LED from external UV.

oh man, i would have never known that. any idea how to remove that phosphor coating

 

oh man, i would have never known that. any idea how to remove that phosphor coating

You can't.

 

oh man, i would have never known that. any idea how to remove that phosphor coating

You obtain a different LED

 

White is also out, might just stick with red.

 

I shine white/green LED light on that RED LED detector and it detects light.

LEDs used as detectors have peak response according to their emitted spectrum:


A red LED won't be very sensitive to green light.

 

That graph is showing light output, not sensitivity as a detector.

I think the LED will be sensitive to any wavelength shorter than what it emits.

Bob

 

I think the LED will be sensitive to any wavelength shorter than what it emits

I haven't checked, but you'd have to test that theory by using only LEDs with a colourless housing.

 

I haven't checked, but you'd have to test that theory by using only LEDs with a colourless housing.

I have checked it and he is right, RED is sensitive to both green and blue, while blue one is only sensitive to Blue

 

I have checked it and he is right, RED is sensitive to both green and blue, while blue one is only sensitive to Blue

These are subjective statements. LEDs as detectors will have a peak sensitivity near their emitted peak and will decrease at other wavelengths. From what I've read, the spectral response is narrow compared to photo diodes.

 

Hello,

Perhaps this article might shine some light on the subject:
https://makezine.com/projects/make-36-boards/how-to-use-leds-to-detect-light/

Bertus

 

Perhaps this article might shine some light on the subject:

I wonder why Mims didn't include that in his book on Optoelectronics circuits.

 

I wonder why Mims didn't include that in his book on Optoelectronics circuits.

I think it's in "Sensors". Or one of his "Circuit Scrapbook".

 

I have experimented with using LEDs as light sensors and some of them work well, some work poorly, ans many work not at all. Some green work and some red ones barely work. I have not determined how to predict what will work well and what will not work.