From: email@example.com (David A. Johnson)
Subject: Re: LED Pulsing - Apparent Brightness?
Date: 7 Jan 2003 14:18:44 -0800
NNTP-Posting-Date: 7 Jan 2003 22:18:44 GMT
Mark Klinger wrote in message news:...
> I am experimenting with wireless communication via LEDs, and have a
> simple question of apparent brightness vs. actual light output.
> Keeping in mind that my receiver will be a photo detector (Fairchild
> L14N2) I'm trying to understand brightness.
> At DC, 20mA is the max drive for my LEDs and they are extremely
> bright. If, however, I put them in my circuit and pulse them with a
> width of around 1mS, they are barely even illuminated.
> Will the distance of the transmission suffer since they are
> "apparently" not as bright, or are they actually the same brightness
> as at DC to the photodetector, but my eyes just aren't seeing that?
> Ultimately what I am wondering is if what the photodetector sees is
> effected by the short pulse duration, or is it equally as bright at
> 1mS as it is at DC?
The visual brightness of the LED is proportional to the average LED
current. The average current is the peak current times the duty
cycle. The duty cycle of the LED modulation is equal to 100% times the
LED on time, divided by the total cycle time, which is the LED on time
plus the off time. For a square wave type modulation, the duty cycle
would be 50%.
You can increase the communications range by pulsing the LED at higher
currents, even at low duty cycles. I have hit standard infrared LEDs
with 10 amp pulses with no ill affect. However, to insure you don't
burn up the LED, the duty cycle should be kept to below 0.1%, if you
use such a high current. A couple amps of peak LED current with a 1%
duty cycle might be a good compromise.
The important factor in dealing with light pulse detection is the
dI/dt factor, where "I" is the light intensity. An inductive network
in the light detector load impedance circuit makes a dandy light pulse
receiver, since it produces a nice voltage pulse in response to the
fast rise time light pulse. An inductive network especially works
well with xenon lamp flashes, which can provide a 100 mile or more
range. The inductor also provides very high ambient light rejection.
Mother nature just doesn't generate fast light flashes. Even
lightning flashes are rather long when compared to an LED or xenon
For more information on this subject, I have a whole section on
optical communications at my Imagineeringezine website.
David A. Johnson, P.E. --- Consulting Engineer
http://www.imagineeringezine.com Home of the Imagineering on-line
Also, http://www.discovercircuits.com A collection of over 6,000