From: Kevin McMurtrie
Subject: Re: LED flickering problem
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Mon, 23 Dec 2002 07:08:35 GMT
NNTP-Posting-Date: Sun, 22 Dec 2002 23:08:35 PST
In article ,
>I am working of LED display with microprocessor
>Given 5 I/O pins, transistors and D0-7, I controlled(refresh in a cycled
>order) 5 groups of LED(4 7-seg and 1 group of 8 ultra bright), but even the
>refresh rate is as high as 400 Hz(each refresh 80 times a second already),
>there is still flickering.
>I looked at the datasheet, though higher refresh rate, shorter duration of
>pulse needed, but seems that duty cycle must be larger than 20%(my
>understanding is no matter how high the refresh rate, it's still only 20%
>duty cycle, rite??) so the problem cannot be resolved unless some other
>brand LEDs are being used??
>I am sure this is not insufficient current problem coz if it is static, the
>LEDs are fine. And I tried to drive it with signal generator, seems
>~50-60Hz, already no flickering percepted.
>Thanks for your professional and valuable comment/suggestions.
Are you sure you're really getting 80 refreshes a second? Check on a
scope. The display chip may not produce an output on each clock cycle.
There's also a chance that the duty cycle, intensity, and frequency
together are a problem with eyes. Eyes only see changes of light both
in time and space. They must constantly move very slightly to maintain
a full image. Your bright and short bursts of light could be
projecting multiple flickering images onto a moving retina. If this is
the case, a frosted shield over the display or a higher frequency should
 - If you really, really concentrate you can stop your eyes from
moving. Any objects that aren't moving will vanish from both sight and
perception. Outlines of objects will appear here and there from slight
movements in your head, sometimes vividly. This happens naturally too
when you daydream. I used to be able to do it for about 20 seconds in
High School. High School was soooo boring...