From: Jonathan Kirwan
Subject: Re: LEDs For Dummies? Pointers needed.
X-Newsreader: Forte Agent 1.92/32.572
NNTP-Posting-Date: Mon, 09 Dec 2002 22:58:44 GMT
Organization: AT&T Broadband
Date: Mon, 09 Dec 2002 22:58:44 GMT
On Mon, 09 Dec 2002 14:04:44 -0600, Greg wrote:
>I am starting a simple project where I would like to control an LED's
>intensity from a computer. I have experience programming and with data
>acquisition cards. However, I am a newbie with LED control.
>Any pointers for good books or web sources for information on this topic
>to get me started?
Changing the intensity is mostly a matter of adjusting the
current through the LED. And if you don't really care about a
great deal of precision or repeatability, then you can ignore
temperature effects and time effects and simply work on
controlling the current.
A rather simple model of a high efficiency red LED is:
V (forward) = 1.55V + I (forward) * 21 ohms
So the voltage across the LED will change some with current.
This is true for LEDs other than the high efficiency reds, as
well. But the model will be similar, just with different
You need to make sure that your power source can handle the
variations in voltage, while supplying the current. This isn't
usually a problem, though. It's unlikely you'll be operating
with more than 30mA and many common methods of driving an LED
can handle the 2.2V compliance indicated by the above equation.
Other LED types may require more compliance, though. Perhaps
twice as much.
You can also use pulse-width modulation to control the average
current through an LED. It's more efficient, that way, since
the LED dissipates more of the total power. You'll need to use
a basic frequency above say 45 Hz or so (depends on who's
looking and whether the LED or observer is in motion) to keep
most of the flicker out.
So, is this a microcontroller? Or a large mega-logic PC type
thing? Analog outputs being used? Or control through PWM?
What's going on?