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Old 01-02-2013, 04:47 PM   #67
geekmaster
Всё гениальное просто.
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Quote:
Originally Posted by mike.s View Post
Strange, because when I search at Mouser.com for white, surface mount LEDs with a Vf of 2.8-3.2, they show 47 matches, from 13 different manufacturers.
By "commonly available" I was referring to inexpensive common LEDs, which use the voltage ranges I claimed. There are other LED technologies (such as super-bright blue) that do use about 3V, as I also mentioned. What concerns us here though is the specific type of LEDs that are used in the PW. It would be an easy task to measure their voltage drop while they are lit. Please let us know what you find when you measure them. Thanks.

It is certainly possible that Amazon did use super-bright LEDs in the PW even though they cost more and require higher power driver circuitry, especially because they used only four of them and their effect is noticeable even in brightly lit rooms. Fewer LEDS may also simplify the design of the light guide.

But arguing about forward voltage drop is really distracting us from the original suggestion to use a reverse bias to make the LEDs darker than no power to them at all. Reverse biased LEDs do not actually CONSUME light (other that to discharge any internal capacitive charge, which can be useful when using them as photo cells to detect light).

EDIT: Using inexpensive LEDs as optical detectors:
http://www.sensorsynergy.com/helpfulhints.htm
Quote:
Although LEDs are not intended to experience large reverse bias voltages, most can be reverse biased by a few volts (3v to 7v) and operate in the photodiode mode. Make sure you limit the magnitude of the reverse current so that you do not damage the LED.
Another useful reason to reverse bias LEDs is for Charlieplexing them:
http://www.instructables.com/id/Char...s--The-theory/
Quote:
What is 'Charlieplexing'? It is driving lots of LEDs with only a few pins. In case you're wondering Charlieplexing is named after Charles Allen at Maxim who developed the technique.
It seems that Don Lancaster published the details of "Charlieplexing" years before Chares Allen got credit for it. There is an excellent schematic diagram (figure 3) here (and the article preceding it in the PDF is quite illuminating too):
http://www.tinaja.com/glib/muse152.pdf
Quote:
Fig. 3 – 56 LEDS can be driven from one [8-bit] microcomputer port by using this n-connectedness trick. No external decoding is needed!

Last edited by geekmaster; 01-02-2013 at 05:35 PM.
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