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# PPOV

Proportional Persistence Of Vision

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Near scientific attempt to design POV (persistence of vision) LED rotary display where each scanline (scanring?) produces exactly the same brightness per area without software tricks.

DISCLAIMER: I've never built POV circuits before and this is my 1st experiment...

Lets assume that the LED running longer trace in the same amount of time will produce weaker light than LED running shorter trace. So single LED on radius R will run 2*Pi*R trace and 2*R will do 4*Pi*R (2 times longer). It means that in order to produce the same amount of light we need to put 2 LEDs on radius 2*R. Similarly we put 3 LEDs on radius 3*R and so on.

Another benefit of such approach would be the fact that having more LEDs on the same line will give us ability to reduce speed of rotation and it should allow to use simpler and slower circuit to control it...

• ### LongExpo and 2nd test

Then I tried long exposition effect by LongExpo app (on my iPhone5s):

Here center is visually more bright! Next experiment - adding 2nd LED to outer ring:

And LongExpo is definitely showing outer ring a little brighter! But if our hypothesis is correct, middle ring with 1 LED should have the same brightness as outer ring with 2 LEDs (because it's twice as far from the center), but it looks a little brighter. See different LongExpo shot with background light:

This is picture of this 2nd experiment with normal exposition (to see 2nd LED):

So my initial assumption was that brightness is linear, but @Yann Guidon / YGDES suggested that "the eye has logarithmic sensitivity and adapts very well", so may be my approach has to be re-calculated from linear to logarithmic?...

• ### First test

I have these gear motors kit from RadioShack:

It has 3 kinds of gear motors:

with white tip - 40 RPM

with blue tip - 120 RPM

with black tip - 165 RPM

40 RPM is too slow for POV so let's try 120 RPM (blue tip):

Initially I put on the disc 4.5V battery pack and just 3 constantly emitting LEDs - one closer to center, one somewhere in the middle and one at the edge:

It looks like my initial assumption was WRONG - I don't see any brightness reduction for far LEDs...

• ### Preparations

In order to create the disc I took prototyping PCB from RadioShack and drew circles with step 0.2''

Then I cut unneeded parts of the board:

And drew the pattern 1-2-3-4-5-6-7-8-9:

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