The iris of the eye is the part of the eye that controls how much light
get into the eye itself. It reacts rapidly to light, but takes longer
to open up in the dark. I think that this is what Andy is alluding to.
While this is all correct, I don't think it enters in to why the eye
is more responsive to a pulsed light source that is being pulsed
more than 60 times per second. The flicker time varies for each
individual, but the lower limit is usually around 24 pulses per second
as far as image retention goes. That is why movie cameras were made
with 24 frames per second minimum.
The human eye is responsive to the peak value of illumination. An LED
pulsed with 200% current for 50% of the time will actually appear
*brighter* than the same LED run steady with 100% current for 100%
of the time. It is assumed here that the pulses have a repitition
rate greater than 24 pulses per second. Otherwise, as Andy pointed out
in an earlier post, the perceived intensity is also a function of
the repetition rate. Above about 40 pulses per second this perceived
increase in brightness levels out and is no longer a factor.
As the number of multiplexed LEDs increases, the percentage of *on* time
per LED becomes progressively less and less. Eventually you reach a
point where you can't get the necessary current into the device in the
required time without damaging the device. You can generally have about
eight LEDs or displays being multiplexed before you begin to experience
problems. For larger numbers of displays you break them down into
groups of 8 and multiplex the groups in parallel.
Besides the applicability of pulsing due to the human eye's response to it,
pulsing is also very useful when using infra-red LEDs. Remote controls
are almost always pulsed. In fact, if you want to get more than a couple
of feet covered, you almost *have* to pulse the IR LEDs. Note that
since current in a series circuit is the same through all the series
elements (non-reactive), you can string several LEDs in series and pulse
the whole string using the same amount of total energy that you would
have used to pulse just one. Instead of wasting the energy as heat in a
resistor, you recover it as more usefil light from the LEDs. Multiple
LEDs give you better 'coverage' than a single LED.
When pulsing LEDs it is important that the LEDs not be accidentally
left ON without pulsing them. If left statically ON for too long
you can damage them permanently. The digit strobes need to be turned
off explicitly early in the start-up sequence. It is also useful
to sometimes explicitly turn off all strobes at certain places, such as
at the beginning of your main programming loop.
Note also that if you are providing unequal timing to the digit selects,
then one or more of the digits will appear MUCH brighter than the others.
(This bug can be turned into a 'feature' if you need to highlight
a particular digit.)
You can't pulse LEDs with large currents by just connecting directly to the
port pins of a PIC, since the PIC prt pins will only source/sink a few
tens of milliamps. Use a transistor to activate the LEDs, and make sure
you use sufficient base drive by keeping the base resistor something
like 330 ohms instead of several thousands of ohms. A small capacitor
in parallel with the base resistor can sometimes help to speed up the
turn-on and turn-off of the transistor. Your Mileage May Vary.
Fr. Tom McGahee
----------
{Quote hidden}> From: Andy Kunz <
@spam@supportKILLspam
MONTANADESIGN.COM>
> To:
KILLspamPICLISTKILLspamMITVMA.MIT.EDU
> Subject: Re: PIC pulsed LED's
> Date: Thursday, June 17, 1999 10:28 AM
>
> > Is the attack time significantly shorter than the release (or decay)
> >time, making the eye tend to be a peak sensing device? Or are these
> >times equal, making it more of an RMS detector?
>
> Surprised you didn't get that in Drivers Ed.
>
> The eye responds very quickly to light, slowly to dark. This protects the
> eye itself, though not your big toe very well <G>.
>
> Peak sensing. The decay is pretty slow, and depending upon the contrast
> with surrounding images, may take MINUTES to be "normal" again.
>
> Andy
>
> ==================================================================
> Andy Kunz -
http://www.montanadesign.com -
RemoveMEandyTakeThisOuTmontanadesign.com
> Life is what we do to prepare for Eternity
> ==================================================================
