Pages: [1]   Go Down
Author Topic: Persistence of vision question  (Read 1413 times)
0 Members and 1 Guest are viewing this topic.
Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

I am designing a PCB to incorporate POV with LEDs. My question is: do the LEDs have to be aligned with the radial line or can they be offsetted and still work? My suspection is not aligned version gives italic letters.

A) not aligned
B) Aligned

Logged


North Yorkshire, UK
Offline Offline
Faraday Member
**
Karma: 104
Posts: 5531
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Well think of it as how they will appear when it is rotated.

On the radial line, they will be as they are and all the LEDs will pass the radial line at the same time. Therefore when all the LEDs are lit up, the line will be straight out from the center.

As, you offset the LEDs, either the first or last LED will reach the radial line first as it spins and if you all the LEDs were then lit up, you would get a diagonal line.

You could account for this in code though. If the offset is small then I doubt you would notice much. Most of the time POVs do not look perfect anyway due to differing LED brightness, slow code (which may give italics as it is) etc.

If you have to offset the LEDs then as you will be measuring the speed anyway (presumably) so you can calculate in code the delay you will need in between lighting each LED.

Well that really is long winded - I must be tired.

So... basically yes using code designed for centralised LEDs but no if you account for it in code.

Obviously the further out you put the LEDs, the larger the gap in the middle of the POV.

Mowcius
Logged

Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Thanks mowcius, I discarded that idea and went with coaxial arrangement. Here's a perf board version:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1292541248

It works ok. I need a faster rotating platform! Guess what platform I used.  ;D
Logged


SE USA
Offline Offline
Faraday Member
**
Karma: 40
Posts: 3783
@ssh0le
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

your arm
Logged


Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Trust me Osgeld, I tried. I was spinning in place trying to show my wife what the message is. I became dizzy and gave up. You would have to shoot the picture from directly above my head if I used my arm. The hint in on the post "El cheapo POV" on this board  ;D
« Last Edit: December 16, 2010, 08:26:53 pm by liuzengqiang » Logged


Left Coast, CA (USA)
Online Online
Brattain Member
*****
Karma: 331
Posts: 16540
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I was spinning in place trying to show my wife what the message is. I became dizzy and gave up. You would have to shoot the picture from directly above my head if I used my arm. The hint in on the post "El cheapo POV" on this board  

That reminds me of the very first el cheapo POV display I saw in the late 70s. They used a very flexible 12" ruler, mounted the leds along one edge (seven as I recall) along the top inch, and had a mercury switch that would toggle at the end of each arc to give a timing reference. They just held the ruler in their hand and gave it a continuous gentle whipping motion and you could see the message pretty clear. I was impressed.  8-)

Lefty
« Last Edit: December 17, 2010, 01:05:23 am by retrolefty » Logged

Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Mystery revealed! I taped my cheap POV on my chair on caster wheels. I had to take off the back support in order to rotate the chair fast enough. I'll upload some pictures. Now my setup is on a $600 lab rotation stage.
Logged


NYC
Offline Offline
Full Member
***
Karma: 0
Posts: 125
The singularity is near!
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

An arduino with 8 LEDs is easy. The hard part of doing a project like SpokePOV or Hokey Spokes is supporting more LEDs and synchronizing multiple spokes.

Synchronizing multiple spokes is needed to obtain persistance without spinning so fast. Supporting 25-30 LEDs is difficult because of the wiring. How do you wire an atMega, 4 8-bit latches and ~30 leds without a PCB? The wiring gets a bit messy.

And you can't ignore cost. With three spokes per wheel, you are looking at 6 spokes per bike. I've got 4 bikes.

Some kind of open source pcb, group buy is needed...
Logged

Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Andy,

Actually there is a place where you can buy spokePOV, adafruit.com
They sell spokePOVs, which is the source of my inspiration. They have more LEDs and you can mount as many of them as there's space on your spokes. Sync is not a problem at all. There is only one trigger, a magnet on the inside of the fork. It triggers all the POV spokes. Unless you will be displaying things that will change with time, like displaying clock, you don't need to sync. All POV spokes basically do the same static image over and over.

From my testing, having 2.5 rev/s is the miniumu for one spoke to see letters. Images may require higher speed.
Logged


NYC
Offline Offline
Full Member
***
Karma: 0
Posts: 125
The singularity is near!
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Right, but it's $99 a wheel. That's $400 just for my kids bikes.
Logged

Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Only put one on each wheel and have them ride really really fast at night! Well, that's not safe. I'm making something that is cheaper (one control and multiple display) but not as many leds. I'll post my results once I get a PCB and assemble it.

Can these POVs be seen under day light? I only used the cheapest LEDs (low brightness) so I don't think mine can.
Logged


NYC
Offline Offline
Full Member
***
Karma: 0
Posts: 125
The singularity is near!
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

The horrible one I made was visible during daylight, but much more dramatic at night. The pics of SpokesPOV look light dusk and dark.

I'm sure AdaFruit spent some time seeing which LEDs mounted at which angles worked best. I think she put 2 rows of leds, one facing right and one facing left: http://www.ladyada.net/make/spokepov/make.html

This has less info, but sounds like the same general idea:
http://www.hokeyspokes.com/faquestions.htm

Are you making a pcb? Do you make them yourself or send them out?
Logged

Central MN, USA
Offline Offline
Tesla Member
***
Karma: 65
Posts: 6914
Phi_prompt, phi_interfaces, phi-2 shields, phi-panels
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

I don't see two rows but only one row on adafruit's POV.
Logged


NYC
Offline Offline
Full Member
***
Karma: 0
Posts: 125
The singularity is near!
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Look at the pic: http://www.ladyada.net/images/spokepov/make/cutled.jpg

The row of LEDs facing down has been inserted. You can't see the LEDs, but you can see the leads which are ready to be soldered and clipped. The row of LEDs facing up go in the top row of circles.

And the parts lists says 60 high intensity LEDs. Each row has 30.

This also answers your question about row positioning. It apparently does not matter if you shift the row parallel to the spoke.


Logged

Pages: [1]   Go Up
Jump to: