[Q] how to apply negative voltage to lcd

[John E. Finster]

hi, how do i apply negative voltage to a character lcd?

 

i have 2 clcd with blue backlight. i got them to work, the characters are showing, but there is no backlight.

also the Luminance of the lcd could not be changed with the corresponding 10k pot on the core. i got that to work by soldering two cables from pin 15 and 16 to two contacts on the back of the lcd labeled A and K, but somewhere in the wiki i read that some lcds need negative voltage for the backlight, and now i am a bit lost. can this be don the same way as the glcds ( with a max 759cpd chip ) ?

 

i attached the datasheet i got from the distributer. my display is a mc4002d-sbl, stn.

 

thanks and greets

20825.pdf

Edited February 3, 2013 by JEFinster

[kpete]

Pins 15 & 16 are completely isolated from the rest of the 14 pins. If you are sure they are reversed, all you have to do is put a twist into the last 2 wires going to pins 15&16. Remove the connector going to the LCD and separate the last 2 wires from the ribbon cable. Put a half twist on these last 2 wires and re-attach a new 16 pin ribbon connector.

[John E. Finster]

hi, thanks for the answer.

you mean, just change the polarity? i did not try that yet, but i will now  :happy: .

thanks.

[Shuriken]

Looking at the datasheet i did not see any mentioning of a negative voltage.

[freddy]

Looking at the datasheet i did not see any mentioning of a negative voltage.

 

Well, pins 15 and 16 are not connected based on the attached datasheet (though A+ and K- is depicted in the block diagram), maybe the pins are located elsewhere (I don't see them on the diagrams, though they could be located on the other side of the LCD for other MC4002D-series display I found datasheets for). This could be the case if swapping the two pins doesn't make the LCD light assuming the trimpot was turned there and back to make sure.

 

Hth,

freddy

[kpete]

Rereading your original post and reading the post from Freddy, maybe I gave you some bad information about twisting the cable. The spec. on page 2 indicates they added pins 15&16 to the connector. They indicated they are not connected to anything on page 8. It would only make sense that they be connected to the back light but when your dealing with China you never know. Maybe you just have to add these wires you talked about to the back of the LCD to get the back light to work.  Try looking at the traces on pin 15&16 on the LCD board to see how they are connected to the backlight.  May they aren't connected at all.

Edited February 4, 2013 by kpete

[John E. Finster]

i tried twisting the cables, still no backlight. thanks anyway for your effort, kpete.

 

there are several connector pairs (pads) on the back of the lcd labeled "K" and "A" (4 or 5). i figured, they must be the backlight connectors. so with the 16-pole lcd cable from the core connected to the lcd i wired two cables from the top of the lcd from the 15th and 16th pin to the connectors on the back (i get home in 2 days, i can make a picture then). i tried soldering the wires to the A/K pairs one after another, but still no backlight.

if i try to reverse the polarity of the wires connected to the back, the core module won´t boot. after i tried this 2 or 3 times, i figured, maybe i let this idea go and look for something else  :sweat: .

 

normally i like to puzzle with the electronics (i studied something completely different), but this one really drives me nuts  :rofl: .

 

thank you all for the effort. i will get back to it on the weekend and puzzle some more  :happy: .

[/tilted/]

Heya.

I had an issue with contrast on a pair of 40x2 LCDs.

It was the old issue of negative contrast voltage rearing its head again.

I solved it using this schem:

http://www.stephenhobley.com/blog/2011/03/28/creating-a-negative-voltage-from-a-positive-one/

I tried it using 3.3v from the J5 port, no dice. Likely because I'm using an NE555 timer, would probably work with the CMOS version.

Anyway, it solved the issue I was having.

Good luck!

/t/