LCD common cathode display fading

Thread Starter

graml

Joined Mar 25, 2019
1
Hello all,

After running the circuit from 2 to 5 minutes the individual segments in a 6 digit 7 segment common cathode (not anode as in the title) LCD display begin to fade. Pulling power on the breadboard for several seconds and repowering will make the display act normally but it they will start to fade again. Using a CD4511 driver for each segment. The LCDs are rated at 5V and the circuit power is 5V as I am also using 74LS90 decade counters.
My students have all built identical circuits and all have the same problem.
Thanks for any ideas.

Mod edit: changed title to common cathode. JohnInTX
 
Last edited by a moderator:

danadak

Joined Mar 10, 2018
4,057
Sure sounds like the LCD bias is floating or getting loaded thru leakage or a
poor R divider ladder connection/soldering.

Regards, Dana.
 

ian field

Joined Oct 27, 2012
6,539
Hello all,

After running the circuit from 2 to 5 minutes the individual segments in a 6 digit 7 segment common cathode (not anode as in the title) LCD display begin to fade. Pulling power on the breadboard for several seconds and repowering will make the display act normally but it they will start to fade again. Using a CD4511 driver for each segment. The LCDs are rated at 5V and the circuit power is 5V as I am also using 74LS90 decade counters.
My students have all built identical circuits and all have the same problem.
Thanks for any ideas.
Are you driving it with DC? - they tend to polarize and lose their ability to twist (or whatever) the liquid crystal molecules.

The theory I've read so far - said something about also driving the backplane electrode with the output from a logic gate so the total waveform applied vaguely resembles AC. If its a module like the 44780 or similar - all this should be on board - unless its conked out...……..
 
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