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View Full Version : persistence of vision led text display (and my failure to find it)



Fred Hawkins
05-12-2008, 01:41 AM
Msng n Lbry, among my stacks and stacks of Circuit Cellars and Dr Dobbs (before it was Borged), is an article about using a vertical string of leds to spell out messages or simple logos.

Importantly, the gimmick was that the string didn't move; if you looked at it directly it would mean nothing.· But as you looked around, your peripheral vision and rapid eye movement would spread out the flashes, leaving a message. Bi-symetrical logos was optimum because you never·'saw' something backwardly.

I like the idea and see it as a·natural for the prop to handle. But I would sure like to find that original article. As I recall the article, the writer's main tech-problem-to-be-solved was getting a big enough eeprom to store his messages.

Alas, I've googled until I'm black and blue, and never quite seem to find it. I've even spent whole afternoons looking through Circuit Cellar's search results. All to no avail.

So I ask this·high-skill resource, aka the prop forum: do you know where this article or a close facsimile is?
·

Peter Jakacki
05-12-2008, 09:55 AM
It's no different from a normal or even a rotating led display except of course it's your eye that moves. Basically if you multiplex the message onto the column of leds as if it were the next column in a display at a slow enough rate then your eye will see the individual "strobed" columns. If you happen to have an old led calculator and you wave it around in front of you it looks like the characters dance because your persistence of vision only works when it's stationary.

A simple routine would read a character from the display buffer, look it up in the font table and then output a column at a time with a delay between each column before moving on to the next character.

*Peter*

Forrest
05-12-2008, 10:08 AM
Lots of POV projects listed here www.hackaday.com/search/?q=pov (http://www.hackaday.com/search/?q=pov)

Beau Schwabe (Parallax)
05-12-2008, 02:04 PM
Fred,

Look about midway down on this link...
http://forums.parallax.com/showthread.php?p=574524

...there, posted by Dave Scanlan (http://forums.parallax.com/member.php?u=45436)·is a POV demo that I wrote early on when the Propeller was first released.

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Agent420
05-12-2008, 05:34 PM
Fred Hawkins said...


So I ask this·high-skill resource, aka the prop forum: do you know where this article or a close facsimile is?

I am an avid Steve Ciarcia / CC·fan and have most·issues going back to the very first.· I recall for fact the article you refer to, and I know I have it.· My CC collection is not indexed, so it may take me a day or two to browse through them all to find it, but I have a general idea what group it's in. I'm pretty sure it's early 90's.

It is interesting you bring this up, because I have recently been thinking about doing the same thing :)

Many years ago I saw a 'professional' installation of this type of display at the Boston Museum of Science.· It was a vertical box about 3 ft high mounted across the room and appeared as a simple line of light when viewed head on, but an image was able to be seen in the corner of your eye as you turned your head.· Very cool.

It seems to me that this type of application must be very similar, if not identical to other pov applications - even if they are moving.·But perhaps there is some specific timing that makes all the difference for a stationary display to be effective.


Post Edited (Agent420) : 5/12/2008 12:20:07 PM GMT

Fred Hawkins
05-14-2008, 08:12 AM
Nods to fellow CCfan, I gave up on Byte when they decided to go 'commercial' and drop Steve in the bit bucket. I did buy a couple years of back issues from them but those were from when they still published code fragments that were worth reading. (83? I'd have to knock the dust off)

So early 90's, eh? That means downstairs and diving into the boxes. Seems just like yesterday to me.

Fred Hawkins
05-16-2008, 06:17 AM
Well, still no luck, but I've looked in some very interesting places full of stuff that was important once. Found issues 1-46 fully intact and still interesting.

And an apparent double mailing of an issue during a resubscription, remarkable because it has an article by one Chuck McManis. Something about a little car. I'd be more specific but now I can't find that article either.

Newbie alert: if you want some interesting challenges, get a cd of back issues from circuitcellar.com. The cd's are all worthwhile, and implementing many of the projects ought be fun to with a propeller, and probably simpler than the original solution.

Agent420
05-16-2008, 05:14 PM
Sorry I haven't replied earlier... it's been a an unusally busy week and I haven't had time to go through the racks...· I'll get to it this weekend for sure.


And yes, CC is probably the best periodical dedicated towards embedded circuit design and related electronics.· An excellent investment imo.

·

Fred Hawkins
05-16-2008, 09:49 PM
Plus since Jan '03, Parallax has had the back cover ad. This one caught my eye during my search:



February 2003 back cover said...


"Parallax developed the Stratix SmartPack for use in
our own ASIC development efforts, but we made it
universal enough that you can use it for your
projects too.

The SmartPack is based upon the new Stratix chip
from Altera. The Stratix is, in our opinion, the best
FPGA architecture in the world today - it has
scalable RAMs, DSP blocks, and PPLs, all meshed in
an enhanced logic fabric. And it's FAST - built in a
0.13µm all-copper process, the RAMs are rated for...."



Prices were $495 and $995

In March 2003 the ad went back to a compass, a breadboard, a prototype board, an LED terminal (4 digit alphanumeric), Stamp Modem, a audio amp and a sound module w/ a speaker. Prices range from $29. to $89.



Post Edited (Fred Hawkins) : 5/16/2008 3:00:46 PM GMT

Steel
05-16-2008, 10:12 PM
Here is the first project that I came across using this. It was originally used in Burningman 2001 and shown at raves subsequently after...
http://sub-zero.mit.edu/fbyte/ledart/obelisk2/

Agent420
05-16-2008, 10:24 PM
Things like that are why I love this hobby :)

I hope to make it to a Burning Man one day...

Fred Hawkins
05-17-2008, 09:41 PM
Creeping Doubt Arises: it finally occured to me that I should pose this question in the CC forum (poor slobs only 1500 registered users after 20 years). Which I did. The sole reply thus far suggests I misremembered a Nuts & Volts article. The only good thing about that is that I only subscribed to N&V for two years and they're lots easier to spot cause they're a bigger magazine. (back to house cleaning aka magazine hunt).

Searider
05-19-2008, 08:41 AM
Other than the missing N&V article, can anyone point me to some technical behind POV displays? I am curious about how fast should each line of the image be clocked out? What governs the optimal speed? Are there ON/OFF duration limits?

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Searider
--------------------

kelvin james
05-19-2008, 10:16 AM
This might save you some exploring. Also, it looks like Scott Edwards did a chapter in his stamp programming book #2.

Graham Stabler
05-19-2008, 03:35 PM
Searider, the speed the columns are clocked out depends on how fast they are going to be moved. If you will move it by hand then you need to consider the average arm wave speed, if it spins on a motor you can synchronize if it is on a bike wheel you can again synchronize with the rotation speed. Going too fast or slow will just crush or stretch the text respectively.

Graham

Leon
05-19-2008, 03:54 PM
The optimal speed depends on the properties of the human visual system. I forget the details, but you should be able to find them quite easily. There are quite large individual differences, so you need to select parameters that suit, say, 90% of the population.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Agent420
05-19-2008, 05:30 PM
Sorry for the delay...· My stacks of 14 year old mags are not too organized ;)· Plus, it's easy to get nostalgic and start re-reading issues of CC, BYTE and Compute·http://forums.parallax.com/images/smilies/cool.gif·· How times have changed.

Fred Hawkins said...
Creeping Doubt Arises: it finally occured to me that I should pose this question in the CC forum (poor slobs only 1500 registered users after 20 years). Which I did. The sole reply thus far suggests I misremembered a Nuts & Volts article. The only good thing about that is that I only subscribed to N&V for two years and they're lots easier to spot cause they're a bigger magazine. (back to house cleaning aka magazine hunt).
It's shameful for a CC forum member to confuse a CC article with a N&V article·http://forums.parallax.com/images/smilies/blush.gif·· I wonder if the hard core CC members are still logging in via BBS (those were the days http://forums.parallax.com/images/smilies/smile.gif ).

Here's the CC article 'Lightstick II' from Feb '94, attached below.·The strobe timing seems to default to 466us to begin with, with the option to increase or decrease it on the fly.· The article doesn't discuss much of the technical aspects of pov parameters, but seems based on a previous project named 'Lightstick" by artist Bill Bell...· I believe Mr. Bell's display was the one I saw at the Boston Museum of Science.· A Google returns several results on Bell's Lightsick.· I'm sure more detailed information may be obtained on one of those sites - he seems to be a pov guru :)

http://www.google.com/search?hl=en&q=bill+bell+lightstick

Some other good technical information seems to be in Bell's original patent #4470044.· I've not had time to fully digest all of this, but these stationary displays are somewhat unique in that they are based on involuntary 'saccadic' eye movements we make all the time.· No doubt it is the synchronizing to this timing that is important.

http://www.freepatentsonline.com/4470044.html



In human vision the eyes fixate for a period of time on one point in the visual scene then rapidly move to fixate on another point for a period of time and so on. The rapid jerky movements between points of fixation is so instinctive and natural that most people are unaware of it. Psychologists refer to these eye movements, which may also involve some head movement, as "saccadic" movements. Researchers in psychology of vision have made measurements of the angular rate of saccadic eye movement and found the angular rate to be quite uniform from person to person, and largely not consciously controllable. Angular rates reach a peak at the middle of the saccadic movement, and may be about 220-250 degrees per second for movements of 5° in angle and about 450-500 degrees per second for movements of 20° in angle.

During saccadic movement, stationary objects in the field of view are not clearly perceived, but an image of the previous scene fixed upon persists for up to about 1/4second, with diminishing intensity. The previous image is immediately supplanted by a new image as the eye rests on the next point of fixation, typically within 1/15 to 1/25 second. This action, which may be easily demonstrated using a device as described in the present invention, takes place so rapidly that one is aware only of a smooth continuum of vision, free from blue caused by movement.



Means (6) for energising and modulating the LED's (2) is provided. Each LED (2) can be switched on and off independently. In operation, the modulator (6) switches each LED on/off at a rapid rate in accordance with a pattern that is determined by the specifics of the desired image. This action will be made explicit in following paragraphs, but for now let it be noted that the modulation is such that the entirety of the desired image is generated within about 0.005 to 0.025 seconds and repeated at a rate of about 20 to 40 times per second.

Post Edited (Agent420) : 5/19/2008 1:06:37 PM GMT

Fred Hawkins
05-19-2008, 10:48 PM
Agent420 said...
Sorry for the delay...· My stacks of 14 year old mags are not too organized ;)· Plus, it's easy to get nostalgic and start re-reading issues of CC, BYTE and Compute·http://forums.parallax.com/images/smilies/cool.gif·· How times have changed.

Fred Hawkins said...
Creeping Doubt Arises: it finally occured to me that I should pose this question in the CC forum (poor slobs only 1500 registered users after 20 years). Which I did. The sole reply thus far suggests I misremembered a Nuts & Volts article. The only good thing about that is that I only subscribed to N&V for two years and they're lots easier to spot cause they're a bigger magazine. (back to house cleaning aka magazine hunt).
It's shameful for a CC forum member to confuse a CC article with a N&V article·http://forums.parallax.com/images/smilies/blush.gif·· I wonder if the hard core CC members are still logging in via BBS (those were the days http://forums.parallax.com/images/smilies/smile.gif ).

Here's the CC article 'Lightstick II' from Feb '94, attached below.·The strobe timing seems to default to 466us to begin with, with the option to increase or decrease it on the fly.· The article doesn't discuss much of the technical aspects of pov parameters, but seems based on a previous project named 'Lightstick" by artist Bill Bell...· I believe Mr. Bell's display was the one I saw at the Boston Museum of Science.· A Google returns several results on Bell's Lightsick.· I'm sure more detailed information may be obtained on one of those sites - he seems to be a pov guru :)

http://www.google.com/search?hl=en&q=bill+bell+lightstick

Some other good technical information seems to be in Bell's original patent #4470044.· I've not had time to fully digest all of this, but these stationary displays are somewhat unique in that they are based on involuntary 'saccadic' eye movements we make all the time.· No doubt it is the synchronizing to this timing that is important.

http://www.freepatentsonline.com/4470044.html



Thanks!http://forums.parallax.com/images/smilies/yeah.gif My better half·thanks you as well.

I was surprised when you said it was from the 90's, and really stunned when it turns out to be the early 90's (and back when CC was still saddle stapled).· It was really just a little while ago -- one or two years, maybe four years ago... Does this mean I am old now?

So, found the issue and reading it now.


·

Agent420
05-19-2008, 11:37 PM
Most amazing of all is that I can remember an obscure 14 year old electronic article, yet have trouble remembering my weekly chore list :)· Short term memory loss is aptly named.

edit -

I've attached the patent (in pdf format) below for those that don't wish to go through the Freepatent's registration....· It's easier to read and includes diagrams.

Post Edited (Agent420) : 5/19/2008 5:12:38 PM GMT

Fred Hawkins
05-21-2008, 09:03 AM
Has this one's term been extended? It's more then twenty years old, so it should have expired.

rjo_
05-21-2008, 09:33 AM
Fred...

Sorry for the late response... I am an expert on this... No-one understands rapid eye movements. All of the texts are wrong. To get a reliable POV for everyone you need to implement it in the standard way. This kind of device would work for most of the people most of the time... but not for all of the people all of the time.

This kind of device would be very interesting for visual scientists, who would like to quantify and study various pathologic eye movement disorders (genetics, progression etc) It would also make a dandy screening device.

Build it and they will come.

Rich

Fred Hawkins
05-23-2008, 01:48 AM
rjo_ said...
·This kind of device would work for most of the people most of the time... but not for all of the people all of the time.


Rich,
I think that's this item's charm -- it remains enigmatic until you 'get' it.·My current·plan·is to use·a·two extra offset LEDs·that momentarily·flash to attract attention, and feed the data pdq to the column.·Here's a thought:·have the attractors blink in a simple pattern that·'trains' the eye to make the movement, then spring the·trap.

I do like this detail from Bell's patent: "Researchers ...·have·made measurements of the angular rate of saccadic eye movement and found the angular rate to be uniform from person to person, and largely not consciously controllable. Angular rates reach a peak at the middle of the saccadic movement, and may be about 220-250 degrees per second for movements of 5° in angle and about 450-500 degrees per second for movements of 20° in angle."

So one expects a working solution depends on the·environment. Something installed·on a street corner would be different·than a breadboard·on ·a workbench.

Fred.

rjo_
05-25-2008, 05:15 AM
Fred...

My dentist has auto-stereo prints hanging on his wall... the idea is to cross your eyes and the 3D comes out after a while. I think there is a huge commercial potential for anything that looks interesting and distracts a person's attention for a few minutes.

That's what I love about this forum... just about everything gets discussed.

nice thread.

Rich

Fred Hawkins
05-28-2008, 05:34 PM
Still spinning my wheels -- lately I am wondering is there a newer, bigger, faster chip than the 74hc595 used in this implementation?

Rookie Alert: Plus I don't get how the data is put into the 595's. Unless 'cascading' means a bucket brigade brute force·bit by bit shoving down the line.... All in all, it doesn't look like the i2c bus that the propeller environment has taught me to know and love.

So I poked around TI and turned up this nerdy 'fun lights' pdf: http://focus.ti.com/lit/an/scpa043/scpa043.pdf·which uses their TCA6507 7 led controller. http://focus.ti.com/docs/prod/folders/print/tca6507.html


TI also has a PCF8875C which seems to be saying that it can handle 16 leds. http://focus.ti.com/docs/prod/folders/print/pcf8575c.html

It doesn't seem to have the same bells and whistles as the 6507 but maybe the 8875's·presentation is not dumbed down as far as the 6507's.

Agent420
05-28-2008, 07:03 PM
rjo_ said...
·I think there is a huge commercial potential for anything that looks interesting and distracts a person's attention for a few minutes.


I am a complete sucker for distracting flashy beepy things.· A good percentage of my hobby projects serve no purpose other than to entertain.



Fred Hawkins said...

Still spinning my wheels -- lately I am wondering is there a newer, bigger, faster chip than the 74hc595 used in this implementation?

Rookie Alert: Plus I don't get how the data is put into the 595's. Unless 'cascading' means a bucket brigade brute force·bit by bit shoving down the line.... All in all, it doesn't look like the i2c bus that the propeller environment has taught me to know and love.


·Yes, the 595's are fed one long continuous bit stream. The serial data flows through each 595 via the Ser(14) and OH(9) pins.·No changes take place at the 595 outputs until the Strobe signal is sent.· So you could have virtually any number of cascaded 595's, you just have to spit out 8 bits for each one, and then end it with the Strobe to trigger all the outputs simultaneously.· The 1st 8 bits out would be for the last 595 in the cascade, and the last 8bits out would be for the 1st chip in the cascade.

This serial method really bears no comparison to I2C.· It's not any std 'protocol', it's just a bit banged interface.· You have to create the logic and io programmatically.· Really, a simple repeat loop would do it.

This idelology is not far removed from my original Hello World led matrix project... take a peek at that code for an idea.· Specifically, the repeat idx loop takes 16bit word data and outputs it 1 bit at a time while also cycling the clock pin.· If you expand that idea to accommodate even longer bit lengths (or, alternatively viewed, groups of bytes) you have the foundation for dealing with cascaded 595's.

http://forums.parallax.com/showthread.php?p=725126

I'm sure there are a couple of newer chips like those you found, but really the 595 is cheap, easily located, available in friendly pdip form·and handles some good current (important for strobed led applications), so I wouldn't dismiss it so quickly.·· Also, in this application I think it is important to be able to strobe all the led's at the same time, something that might be difficult to do if you use·other controller chips on an i2c or spi bus.

Post Edited (Agent420) : 5/28/2008 1:17:36 PM GMT

Fred Hawkins
05-28-2008, 08:19 PM
Agent420 said...
I am a complete sucker for distracting flashy beepy things.· A good percentage of my hobby projects serve no purpose other than to entertain.
....
·the 595 is cheap, easily located, available in friendly pdip form·and handles some good current (important for strobed led applications), so I wouldn't dismiss it so quickly.·

And I have discovered, there·are different·flavors of·595 datasheets. I was reading Fairchild's and getting nowhere fast.·Happier now that I have a full set plus a nifty application note from Basic Micro, Inc.

flashy beepy: YouTube is jam packed with LED POV stuff. This one, a·globe made from 256 smd white and 36 red LEDs,·is very cool: http://www.youtube.com/watch?v=oLygWkHo9nw

·