Driving 7 segment LED

[rnielsen]

Greetings fellow blood-shot-eyed readers....

I am making a 7-segment LED display, driving multiple displays. I will be multiplexing them and was wondering if anybody has experimented with the durration of driving each segment before moving on to drive the next segment. I don't want the displays to appear to be flashing so I will ask what frequency, or length of 'ON' time, I should drive each segment.

Thanks,
Ronald
_________________________________________________
At my age... sometimes I stop to think and forget to start again.

[PCM programmer]

Search the forum for the keyword: flicker

[newguy]

I've read in a few different places that the human eye has a LP response out to about 10 Hz. That said, I once hooked up a LED to a signal generator just to see where the flashing seemed to stop for me. The flashing seemed to disappear around 20 Hz. If you shoot for about 25 Hz, you should be okay. 30 Hz is the frame rate for TV, and movies are 24 frames/sec.

At 25 Hz, that's 40 ms between refreshes. The hard part is driving the LEDs hard enough so that they seem bright.

[Humberto]

Hi Ronald,

Regularly the most common LED displays do not specify its Luminous efficiency
(unless you are using a special type of some know brand) so to get the right brightness
without flickering is a matter of error and trial test.
Unfortunately I do not know an easy trick to give you the exact info you are waiting
for, but I can give you some tips to consider:

1) Driving a LED at frequencies below 30 Hz, it will appear to flicker, so this is one
of your low limit.

2) The human eye bright perception is more efective when a LED is driven with a
high current peak at low duty factor than at low current (10 time less)
with a 10X times duty factor.

3) The bad news is that driving a LED with such a high current peak, will get it significantly
more hot, and LEDs have more light output at lower temperatures.

4) The human visual system is non-linear . With the right choice of duty factor,
driving current and pulse rate, perception will correspond more to the peak
brightness than to the average brightness. This is your high limit.

regards,

Humberto

[bscogg]

I know cost is always a factor, but we just used the MAX6956 chip in a similar application. It drives 28 segments from an I2C bus at 16 different intensity levels for about 2 bucks. I think there is a basic driver in the code section.

Brad