From: kjell@elixir.e.kth.se (Kjell Rilbe)
Subject: Re: Multiplex displays (Was Re: You know you've been hacking too long...)
Organization: Dept. of EE, Royal Institute of Technology, Stockholm, Sweden
Date: Fri, 14 Aug 1992 06:33:01 GMT

In article <13AUG199210483380@erin.caltech.edu> shoppa@erin.caltech.edu (TIM SHOPPA) writes:
>In article <1992Aug13.143452.18092@kth.se>, kjell@elixir.e.kth.se (Kjell Rilbe) writes...
>>In article <BswnIE.Dpt@world.std.com> bmarcum@world.std.com (Bill Marcum) writes:
>>[Referenced articles deleted]
>>>Speaking of screen color, I've noticed somthing interesting on a computer
>>>where I work.  The screen display has alternating lines of red and light-blue
>>>text on a dark-blue background.  I wear very thick glasses, and as I move my
>>>head, the red letters seem to move around while the light blue letters stand
>>>still.
>> 
>>This has got nothing to do with computer folklore, but I just have to tell
>>you all about this. The other night me and some friends were out to watch
>>the fireworks they have here at "Stockholm Water Festival" every other night.
>>One of us had a walkman. It was dark, and the walkman had this little red
>>LED to indicate power is on. When he moved the walkman, this little LED
>>seemed to kind of float around, like it was not attached at all to the
>>walkman!!!!!
>>It was nothing wrong with our eyes, we all saw it, and one of us did NOT
>>wear glasses. Really freaky!!!
>> 
>>/Kjell Rilbe
>Strobed (rapidly blinking) LED's can play such tricks on your eyes.  Most
>LED calculators and digital watches didn't actually display all of the digits
>at the same time - the display was multiplexed.  This saved some circuitry
>(you only needed one BCD to 7-segment decoder) and also saved power (By
>running a higher peak current to your displays, you lost less power in
>the current-limiting resistors, while keeping the average brightness
>constant.)  It's possible that the walkman's LED was strobed in this way:
>the 5 or 10 mA (constant) it takes to light it up is not a ridiculously small
>amount of the total current draw (especially if radio instead of tape is
>being used.)  I've seen the insides of lots of walkmans (walkmen?) but have
>never seen this done, actually.
>
>In any event, these strobed displays can appear to jump up and down if
>you watch them.  I think that what happens is that the feedback system
>(i.e. your brain) that keeps your eye pointed in one direction constantly
>makes adjustment to where your eye is pointing.  This is particularly true
>when your eye is tracking an moving object.  Now if your eye
>is following some sort of smooth average motion, but the object you are
>watching actually moving irregularly and jumpily, the blinking of the LED will
>occasionally happen during a jump your eye doesn't follow.  The image you
>see is the integrated image over the last twentieth of a second or so looks
>smooth, except for the LED which blinked out of place!
>

I know what you're talking about, but I don't think this LED was strobed.
I have this shaving machine (Philips Philishave something), which has 
strobed LED when you're loading (Is that what it's called in English?)
the batteries. And then, when you wave it back and forth you can see the
LED blinking on the "trace" you moved the machine.

But this thing with the walkman was *VERY* different. It had to be dark,
because when we held it up where there was more light, the phenomenon
was not at all as big.

I think it rather has to do with the different receptors in the eye responding
differently fast. You know we have different receptors for daylight and 
twilight/darkness. I would think that the ones for daylight respond faster
than the others. These are also the ones with color, the ones for twilight
only see shades of gray. 

One of the reasons for this theory is that the LED seemed to move ahead of
the machine, not as you might expect, and they way it looks at this
shaving machine I mentioned, the LED coming after.

Try it yourself! Should work with any (red?) LED you might have, and enough
darkness! I think you might be surprized at the effect! Really cool!!!!

Regards
Kjell Rilbe