LEDs, current and PWM

[Acetronics2]

Hi, Robert

just consider you need a certain amount of energy to make your lights glow ...

WHATEVER the way you give them that energy.

NO miracle, as I told you ...

so, the only way to get them all together lit is to reduce current to the minimum " readable " ... ( here PWM can be useful ... )

or use very high efficiency leds ... as there's a lot here ... current can be reduced to ~ 5 mA per Led to get a nice visible glow.

Alain

[Ioannis]

You can increase the current to the maximum the LED's can accept in pulse operation and then light them in turn with a low duty cycle.

Ioannis

[sayzer]

May be I should have been more specific.

Check MBI5026.
This is a shift register. Use as many as you need.
And the current can be adjusted.

[Demon]

AN234, figure 11
http://ww1.microchip.com/downloads/e...tes/00234a.pdf



This is the technique I meant by blinking the LEDs in sequence (multiplexing right?).

- Let's say a complete 7-segment LED consumes 20mA.
- If I turn Q1 on/off, then Q2 on/off, and so on, really fast.

Wouldn't it only consume 20mA since only one 7-segment is lit at one time?

The human eye would see them as all on, no? Kind of like screen refresh rates.

Robert

[Ioannis]

If you light a LED for 10% of the time, fast enough, then your eye will of course see it ON but not bright. Maybe not 10% of the LED normal brightness but sure dim. So you have to increase the drive current.

That is what Alain meant by "No miracle when energy speaking ... never ! "

Ioannis

[Demon]

Ok, so multiplexing LEDs really is the same as PWM but in a group.

Time-sharing lightbulbs.

I guess I won't have a choice but to make a compromise on what Alain suggested; PICs go through a pre-programmed sequence lighting a group of LEDs across all panels.

Robert

[Demon]

...

or use very high efficiency leds ... as there's a lot here ... current can be reduced to ~ 5 mA per Led to get a nice visible glow.

Alain

I put 16 high efficiency LEDs on a MCP23016 just to see what it looks like. They were just a bit too bright full on, so they come out just about right when pulsed in sequence, and current is kept at a minimum.

I could even use a bit lower than the regular 330ohms too.

Robert