POV vs PWM

[bouvett]

hi all, i am trying to build a display which uses both POV (persistence of vision) and PWM, in order to produce some display effects.

now pwm is used to eg fade leds, by varying the duty cycle... POV involves lightning say one led at a time and it would still seem like all the leds are lit.

now my problem is that when applying POV i would also sort of doing PWM due to the switching on and off the led and thus i think that the leds will fade...

can anyone help please.. thanks

 

[geko]

now my problem is that when applying POV i would also sort of doing PWM due to the switching on and off the led and thus i think that the leds will fade...

With POV LED display you're not doing PWM you're multiplexing so they're as bright as they are.

 

[geko]

On re-reading your post are you wanting to use PWM to control the brightness of the POV display?

If so then you need to use a PWM frequency very much higher than the rate at which you change the POV data.

 

[bouvett]

i want a display to do both but not all the time...
if i first light two leds with pov and then light then leds, the duty cycle for each led varies and thus they will dimm... that's the prob

 

[kpatz]

Keep the duty cycle the same for each LED regardless of how many are lit. Then the brightness of each LED will stay the same.

Let's say you have 10 LEDs and they're multiplexed so you light one at a time in rapid succession.

To light all 10, you'd light the first LED for 1/10th of the time, the 2nd LED for 1/10th of the time,... until you reach the 10th LED, then repeat.

To leave an LED off, you'd just skip that LED (have all LEDs off) for the amount of time that the LED would have been on. So, to light the 1st and 4th LEDs, the sequence would be: LED 1 on for 1/10th time, all LEDs off for 1/5th of the time (since LED 2 and 3 don't need to be lit), LED 4 on for 1/10th time, and so forth, all LEDs off for remainder of time, and so forth.

 

[geko]

To light all 10, you'd light the first LED for 1/10th of the time, the 2nd LED for 1/10th of the time,... until you reach the 10th LED, then repeat.

This won't work for a POV display because the LEDs are moving. The rate the LEDs are changing has to be quite slow so that as they are physically swept across your field of view you see the text/image.

If you 'PWM' the LEDs in the way you've described, they won't lower in brightness, you'll just see the image flicker.

 

[bouvett]

i want a display to do both but not all the time...
if i first light two leds with pov and then light then leds, the duty cycle for each led varies and thus they will dimm... that's the prob

 

[geko]

i want a display to do both but not all the time...
if i first light two leds with pov and then light then leds, the duty cycle for each led varies and thus they will dimm... that's the prob

Which as I've already said means you will have to do the PWM at a very high frequency in fact it may be inpractical to do so. Perhaps it would be better to avoid PWM and look at controlling the LED current as a mean of varying the brightness.

 

[kpatz]

Maybe use PWM but put capacitors across the LEDs (or transistors driving the LEDs) to eliminate the flicker.

 

[bouvett]

which as i've already said means you will have to do the pwm at a very high frequency in fact it may be inpractical to do so. Perhaps it would be better to avoid pwm and look at controlling the led current as a mean of varying the brightness.

how would you control the current? With pwm right? With some sort of voltage controlled current source..
You will still have to use pwm

 

[geko]

how would you control the current? With pwm right? With some sort of voltage controlled current source..
You will still have to use pwm

Not neccessarily. Are you trying to contol the brightness of the display as a whole, or individual 'pixels'?

You need to specify clearly what functionality you want from it

 

[bouvett]

I am just willing to understand the rather complex issues of when you have a display with multiple leds and you want to light one at a time with the frequency fast enough that you don't notice the motion....

At the same time having complete control of the luminosity of the leds

sry for the upper case ... I was assembling

 

[geko]

I am just willing to understand the rather complex issues of when you have a display with multiple leds and you want to light one at a time with the frequency fast enough that you don't notice the motion....

You've lost me

 

[kchriste]

Probably the most effective way would be to adjust the LEDs power source with a buck converter controlled by the hardware PWM in the PIC. Then you would simply do the POV LED pulsing like you normally would. It would certainly make the code easier to write.