Is my LCD fried?

Hi Guys,

Finished building my motherboard today. Power circuit is fine, Micro-controller seems to be working as buttons and LEDs responding as expected… Although I can’t get my LCD to work/display. It just lights up purely green with no obvious characters. The contrast pot does nothing and changing the backlight resistor just affects the brightness of the green glow. I thought the display I ordered was supposed to be black background with green characters.

At first power up I used a 33 Ohm resistor as I wasnt sure if this model was one that didnt require the resistor or not. I changed to a jumper to see if it improved, but no.

Is anyone aware of what my issue might be? Is my LCD faulty? Are there any tests I can do to determine issue?

I am using the following LCD:
ERM4002DNG-1
http://www.buy-display.com/default/lcd-display-40x2.html

I have attached a picture trying to convey what the LCD is doing, one with flash, one without.

> The contrast pot does nothing

It might be a bad solder joint on the contrast trimmer.

I have reflowed the trimmer pot pins as well as the header pins of the LCD. I can also see the resistance changing on the corresponding pin of the LCD Header when I wind it.

I think you should read a voltage changing on the LCD header.

I was checking resistance with the power off. But I will check again for changing voltage while powered to double check.

So just to confirm these model LCDs are the ones that don’t require limiting resistor for backlight?

The brightness of the backlight seems OK. If your LCD had needed a resistor and you didn’t put one, it would have been excessively bright for a short while and then completely dark. If it didn’t need a resistor and you did put one, it would have been a bit dim. Not getting anything on the display is typically caused by the trimmer - either because it’s defective, because it’s not soldered correctly, or, most often, because it has to be turned what seem to be an endless number of turns to do anything. Please try to adjust it again, first something like thirty full turns in one direction and then in the other. There’s no stop at the end positions, just a very faint click.

Hmmm this is for Ambika, so the trimmer has a very short course (330°, not 7920°).

Maybe he has destroyed the Trimmer by turning it 7920°… :wink:

He measured the resistance changing so trimmer should be ok. Maybe it’s not getting voltage on the trimmer.

Trimmer is single turn, and it is OK.

I am measuring ~0-5 volts between pin VO and GND, on both the Ambika PCB side and the LCD PCB side.

How do these LCDs achieve black background and green characters out of curiosity? I mean why is my screen lighting up bright green, when it is supposed to be black background?

> I mean why is my screen lighting up bright green, when it is supposed to be black background?

Even with a “green on black” display some of the backlight “bleeds through”, but I’m not sure what the photo is really showing (is the LCD the only light source here?).

Maybe you have a faulty LCD?

The photo demonstrates pretty closely what I am seeing, ie. bright green, LCD is only light source… not exactly bleeding through!

Oh well, I think may be faulty… :frowning:
I have emailed buy-display, not sure if they have some sort of customer service…

thanks for your suggestions anyway :slight_smile:

I might temporarily tack solder some breadboard jumpers from the pins on Ambika to to the corresponding pins on my MidiALF LCD to confirm LCD is faulty. Obviously with the MidiALF not on.

If it works on MidiALF screen, I will buy a replacement screen for Ambika.

Would this be a bad idea?

LCD displays work by polarising the light. The crystals are one layer and on top is another thin polarising film layer. By reversing the top film you get either black on green or green on black. It’s a wild guess but maybe the top layer is missing on your LCD. You could test this by looking at it with Polaroid sunglasses.

Another thing to measure is if the display gets 5v power for its logic to function.

Got 5v on VDD.

:frowning:

I think my lcd for the Ambika kit must have been faulty. It didn’t require a limiting resistor, but nevertheless some white smoke appeared first time I powered on.
Had a replacement lcd luckily which didn’t require a limiting resistor either and that worked, so I knew the mobo was ok.

From the color it looks more like a ERM4002SYG-1. What color is it when turned off? The ERM4002DNG-1 should be black, the ERM4002SYG-1 remains approximately the same color. The website which you mentioned has changed a bit and now lists the different versions side-by-side. That still does not explain why it’s not working though. It starts to seem pretty likely that it’s defective.