using a LED as a light sensor

Either way is possible. Perhaps not repeatable or reliable.

Best regards, Spehro Pefhany

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Well I'm not getting how this is done without extra circuits and just firmware, where "extra circuits" is anything other than a LED driver, or IOW only circuits that are strictly output and no inputs. I can see how you could reverse the voltage across the whole array by opening or driving the common and then detecting reverse current, but no detection ckts are allowed.

Software can change most micro digital outputs to digital inputs. You have that and time to play with.

Preset the output to a known level (eg. Vss or Vdd), switch the output drive to an input and measure the time for the input, diode and stray capacitance to charge with the photodiode current from the initial level to the input threshold.

Best regards, Spehro Pefhany

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Yup. That could be interleaved with the digit scan.

John

In art. , John Larkin wrote:

Although I have found the small area to be a big consideration, I have also found some to be better at converting sunlight received by that area into electrical power than same area of monocrystalline solar cell. Most notably with some older tech GaP and GaAsP types as opposed to any more modern ones with optically resonant or semi-resonant structures/layers to favor efficiency in an amission band at the expense of efficiency in an absorption band. Also notably I have not found GaN or InGaN ones to be efficient as solar cells - I suspect emission band is displaced from the absorption band in many of those by the emission being a process with more than one downward jump. Also, many GaN/InGaN LEDs could have absorption bands largely attenuated by being in the UV and attenuated by building materials of the LED, let alone whether the LED chip has any resonant or semi-resonant structures (layer thicknesses) in favor of the emision band and against any absorption band! Keep in mind that GaAs and GaP have been proposed as solar cell materials! Not only have these achieved more power per unit area when illuminated with sunlight (higher voltage outweighs lower current that results from a smaller bandwidth of wavelengths being utilizeable due to longer wavelength infrared not being utilizeable), but also has been proposed solar panels with GaAs or GaP being placed over silicon solar panels, due to some of the infrared not utilizeable by GaAs or more likely GaP passing through to be utilized by a proposed underlying silicon panel!

- Don Klipstein ( snipped-for-privacy@misty.com)

i don't recall the number but we use one that has a center tap on the filament, that way we can move the tap when a filament opens from the outside. :) we never needed to increase the voltage pass spec as far as i know. what i love is when we have an arc on the Pie transformers., makes nice burns in the litz wound pie xformer and some times strikes and arc on the Plexiglas mounting deck for the input side of the diode multiplier stack.

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yes, my mistake, just a type'o .

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hum..

I am doing exactly this with an FPGA but I only see about 0.2% timing difference between dark and light

with some other methods the difference is about 10% Antti

Maybe there's an internal pull-up or pull-down you need to turn off.

Best regards, Spehro Pefhany

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well I specially turned it ON not off

Antti

I'm a bit surprised that LED's are sensitive enough to use a simple input port discharge approach. Microcontroller ports typically spec input leakage in th 1 uA to 10 uA range without pullups. OK, that's over the temperature range. But we are not trying to make a temperature sensor but a light sensor. 1 uA of photocurrent is a lot of current - about 5 x the saturation level of the systems I use with 5x5 mm photodiodes. so, yes, I would surprised if the timing change from dark to light was very great - and swamped by the change with temperature.

Peter

well I was able to make a iopad oscillator that had frequency change

10% based on light intensity, and even that was possible bad solution, eg the change could be even larger.

but getting stable absolute reading is probably more difficult or impossible due to temperature, effects, etc

Antti

There's your problem. The photodiode current is negligible compared to the pullup.

Best regards, Spehro Pefhany

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Typically something like 1nA at room temperature, and say doubling every 10°C.

Yes, that was my WAG that it might be close even under typical figures. With a discrete 5mm SR led I measure about 4nA of photodiode current under normal room illumination with 5V reverse bias. A large-die green 0.56" LED display (decimal point) gives only about half of that current.

Best regards, Spehro Pefhany

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Well, that figures. A LED may have a very small active silicon area. But it's contained in a housing optically designed for the express purpose of getting as much light from that very small area out into the world. Since light paths generally work equally well in both directions, that implies that the same housing is also quite good at

*collecting* light from the world onto that tiny area where the LED is sensitive to light.

--
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Time to call on the Keebler Elf Konsulting Kompany, huh?

Reminds me when I needed a light sensor on the spur of the moment. I took a pair of dykes and peeled off the metal can of a transistor and used that.

P416

was a great metal can housed Ge transistor :) had them as toys at age of 1.5 years (in 1967)

and later on used them with removed hats as light sensor as well.

even better use can find old ceramic packaged DRAMs som guy had them connected to LPT port and used as image sensor some while ago those schematics and software used to be online.

oh well today real image sensor are cheaper than ceramic packaged memories

Antti