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
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.
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.
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
100Hz light modulation) Freq drops to below 200Hz, as light level reduces.
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.
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.
1 meter feet from a kitchen light fixture having a single 13 watt spiral compact fluorescent lamp and a mildly diffusing cover: 1.25 volts. I expect 5 mm narrowbeam GaAlAsP red LEDs to do the same at half this distance.
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.
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.
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
Yeah, I did that for a class project some years ago... 1Mbps was fairly easy with pedestrian op-amps doing the heavy lifting, although I had read Phil Hobb's notes on noise in photodetectors to optimize the design somewhat (they helped so much I decided to go out and buy his book). The original goal was to have a full-duplex system via time-division multiplexing, but I ran out of time to do it (initially synchronizing the two ends when they're first turned on takes a bit of effort), so instead I just had a toggle switch that turned everything around.
Getting way off-topic here:
At the time I argued something along the lines of how RS-232 to fiber optic converters are often used for long distance links in harsh environments (e.g., production floors), and how requiring only one fiber rather than two would save costs. I realize these days that, while, sure, it does save costs, in many plants the labor of installing *any* sort of link completely outweighs the costs of the actual fiber, associated hardware, etc.
I do know of a couple of plants where this wouldn't be the case, though -- a sawmill here in Oregon where the owner has literally dozens of little microcontroller-based projects running the place that he designed himself (most are just soldered together on perfboard because they're all one-offs), and a glass factory in Wisconsin where the entire float glass line is overseen by a single PC... running a DOS program designed some time in the late-'80s! (If you asked someone to design such a system for you, these days the response would probably involve multiple PLCs, multiple PCs, etc...)
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