Subject:
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RE: Home-made Light Bricks
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 27 Jan 2000 23:34:51 GMT
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Original-From:
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Mark Geddes <mark@AudeSi.com>
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Viewed:
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846 times
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I would avoid exceeding the current rating on a LED, even for a fast pulse.
It's much easier to design a circuit with the proper current limiting
resistor, then to experiment with pulse widths. A wrong pulse width, with no
current limiting, will fry a LED instantly.
The calculations for the proper resistor are easy. Remember Ohm's law?
Voltage = Current x Resistance
example:
8V power source
2V voltage drop across LED
10mA typical current to operate LED
8V - 2V = 6V across resistor
6V/10mA = 600 Ohms
The voltage & current rating for LEDs varies greatly, so check the
datasheet. (I'd assume 2V & 10mA if you don't know - the worst you can do is
burn one out.)
As long as you don't exceed the maximum voltage or current limit your LED
should almost last forever. Most designs pulse the LEDs to avoid wasting
power, not to avoid using a current limiting resistor. A few un-pulsed 10mA
LEDs in a circuit can kill your batteries very quickly.
MG
> drive multiple LEDs from a single output.
>
> LEDs work quite well with pulse power. If pulsed, the current
> during the on
> period can exceed the current rating of the LED. So instead
> of simply turning on
> the LED, you can generate a series of bright flashes. That's
> how your digital
> clock with LED display works - the different segments of the
> digits are actually
> never on at the same time. Each segment just flashes very
> brightly for a short
> time. Your eyes are too slow to notice that, and it appears
> as if the LED is on
> constantly.
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Message has 1 Reply:
 | | RE: Home-made Light Bricks
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| I brought up the pulsing in connection with a de-multiplexer, not to avoid the resistor. If you just want to hook up a pair of LEDs to the RCX motor output, by all means, use a proper resistor, and use forward/stop/reverse settings only. Not using (...) (25 years ago, 28-Jan-00, to lugnet.robotics)
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