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

 

Please post schematics and if you're using a microprocessor the codes... it would help us to help you

 

Sure sounds like the LCD bias is floating or getting loaded thru leakage or a
poor R divider ladder connection/soldering.

Regards, Dana.

 

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...……..

 

Are you sure you don’t mean LED? The CD4511 will apply a DC bias to a Liquid Crystal Display that will eventually ruin it. As noted above the voltage between the backplane and the segments must be a net zero i.e. AC drive to avoid damage.

https://www.ti.com/lit/ds/symlink/cd4511b.pdf

See Page 12 of this apNote:
http://ww1.microchip.com/downloads/...Driver-Module-of-8-Bit-PIC-MCUs-00000658C.pdf

 

If you're using 7-segment LCD displays, I would recommend using cd4543 drivers instead. They are designed to drive LCDs, but will drive nearly any type of 7-segment display.
Datasheet here: http://www.ti.com/lit/ds/symlink/cd4543b.pdf

 

Either that or use 7 segment LED modules but they will take more power.