using green LED instead of photoresistor


I made a simple project using photoresistor to read light intensity and send it to pc and it work well.

I wonder if i can replace the photoresistor in my circuit (including 10 kohm and send the reading of light intensity to computer) by green LED after remove its dome to act as sensor?

i don't know if i need to use other resistors or components to do this!

I doubt it can be used as a direct replacement. It should be easy enough to try. I'm not sure how much more sensitivity you'll get (if any) by removing the dome.

Usually, photodiodes are used to generate current rather than control it like light dependent resistor. And, usually there is an op-amp to boost the signal.

Photodiodes (and I guess LEDs too) can apparently be used in the photoconductive mode (like a photoresistor) but I don't know anything about it. I found [u]this page[/u].

Yes you can if you drive it right. No need to remove the dome

When using an LED as a photodiode, you need to be aware that in comparison to LDRs and standard photodiodes, LEDs are very wavelength sensitive. They have significance response to light only in a rather narrow range of colors that are a bit “bluer” than the emission.

See the attached response curves for IR LEDs used as photodiodes. Similar curves have been published for red, green, blue and UV LEDs, showing that the wavelength of the response maximum is usually shifted 30 - 60 nm (towards shorter wavelengths) from the emission maximum.

The attached graph was copied from a publication by Forrest Mims, Jr.


The only sense I can discern in "removing the dome" it to provide a flatter surface and make it easier to focus the light on the sensor element.

You would not want to be using a LED with coloured encapsulation anyway as that necessarily attenuates the light, even if you remove most of it. What you do have to be careful about if you pare off the encapsulation, is that you do not in the process remove the bond wire to the LED chip.

Not to be confused with the "old days" when we made photo-transistors by scraping the paint off OC71s or sawing open metal cans (and blowing out the beryllium oxide powder).