using a LED as a light sensor

Hey, the sun came out! Weird.

Random yellow T-1 LED, -9 volt bias, full sunlight, 2 uA. As a pv, it makes 1.5 volts into a 10 meg fluke, pointed at the sun, but only about 7 mv in office/room light.

John

I got out the old LED tray a couple of months ago and was basically testing LED's and photodiodes for light sensitivity. The photodiodes were easily recognizable compared to LED's and some LED's have very little light sensitivity, but I still need to finish that batch. I built a probe for my portable oscilloscope so I can measure light noise from overhead lighting.

greg

I think I can see a way to work this, in a CPLD; will test as time permits.

-jg

Results :

Circuit 1: Connect LED as photo-Diode, into a CPLD, wired as Open Collector Schmitt Single Pin oscillator : LED + 1 CPLD pin. No Caps, No resistors !

Freqency is ~ 7-9KHz with incandescent bulb close (Freq variation due to

Circuit 2: Drive LED, with simple waveform 25% ON (L), then 25% OFF(H), then TriState the LED pin. The Photo current will drive the LED pin down, and that time does vary with light level. So yes, you can use the same LED to sense ambient light.

The practical problem, is the conflicting nature of the drive and bias. The LED Diode starts to clamp the photo current, giving something in region of ~1150mV of available swing, and that has to cross the voltage threshold in order to be timed. Expected wide LOG range of Sense current is observed.

Best results with Digital levels, seem to be from a Non Red LED (Amber, or Green), and usable LED Light output comes from Vcc >= 2.5V. With this, see appx 300mV of Sense-Overdrive margin.

If more margin is required, options are to raise the LED voltage during sense, or lower the threshold point.

-jg

Out of curiosity I just stuck a red LED on a volt meter to see the voltage swing. In bright room light, the LED generated about 20millivolts max. That level is in the noise floor of most microcontrollers.

I just stuck a Radio Shack 276-086A red LED onto a digital voltmeter. This is a 10 mm narrowbeam one with a beam only a few degrees wide, but I expect other GaAlAsP red LEDs to perform similarly when directionality is not an issue.

I did not try this tonight with other LEDs out of belief that I knew a winner to pick.

Point-blank-range at surface of compact fluorescent lamp tubing: 1.4 volts.

Obviously, for getting waveforms of light intensity, the LED (or photodiode) needs a resistor to load down the voltage to much less than its open circuit voltage, or better still reverse-bias it.

Point blank range on my Dell 17 inch CRT monitor: .27 volt.

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

This would suggest that only LEDs in a non-colored case would be usable for photovoltaic applications ?

A red LED in a red case would receive only very little green or blue light and thus produce very little current.

Paul

"BrunoG" wrote in message news:445d31a5$0$19698$ snipped-for-privacy@news.wanadoo.fr...

reminds me of this led sensor

the led backlight in the 40*2 character lcd module on my bench seems to make a respectable pv sensor

That's a very ugly methodology, leave it to Larkin to use such a thing.

I agree, completely. I think this is the reason you see so many reports of poor performance of LEDs as light sensors. It is also a possible reason that blue LEDs are so variable, type to type. Some of the epoxy cases may be more transparent to UV than others, but this difference is not evident to the eye.

The operative mnemonic is Roy B. Giv in so far as the wavelength hierarchy of efficient light power to current conversion. Any LED emitter will efficiently convert wavelengths to its left in the sequence, red-orange-yellow- etc...

I'll use anything that's reliable and profitable. And if it's clever and fun, even better. The 7-seg as its own light sensor falls into the clever category, but it may not be reliable, since we'd have no good control over the photosensitivity of the displays on a production basis. A proper photodiode, connected directly to a uC port pin and doing the tristate pump/discharge thing, would be perfectly good in a lot of applications, providing "analog" light measurement ability.

It was an idea to play with. I know you don't approve of ideas, or of playing.

John

ITYM Roy G. Biv. Green is longer than blue.

Regards, -=Dave

Connected to a port pin, it would be more reasonable to use it in reversed-biased photodiode mode, where volts of swing are easily available.

John

hi thats a good thought i havent heared it before

Same curiosity:very clear uncolored led:150 millivolts in 6 oclock pm daylight. ( a pity leds do not show any type mark , could have been a IR led)

What's a good thought? Include context. See below.

-- "I'm a war president. I make decisions here in the Oval Office in foreign policy matters with war on my mind." - GWB 2004-2-8 "If I knew then what I know today, I would still have invaded Iraq. It was the right decision" - G.W. Bush, 2004-08-02 "This notion that the United States is getting ready to attack Iran is simply ridiculous. And having said that, all options are on the table." - George W. Bush, Brussels, 2005-02-22

Seems one could take advantage of opto ability of LEDS as a bi-directional optical data coupler.

Point the LEDs at one another. Digital data should easily be transferred half-duplex in both directions.

Cool.

Invented, I think, by Forrest Mims about 25 years ago, and the subject of a patentability dispute, or something. Google it.