Not getting good results out of LCD display

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
I have two displays doing the same thing. I it appears to be a contrast issue. Below is the best I can get the contrast adjusted. Notice how part of the P and part of the M appear to be washed out and there are artifacts all over the screen.

I have never had this problem before and think I have used 10k trimmers in the past.

upload_2017-2-25_18-41-57.png

I am using a 10K single turn trimmer. Considering swapping it out for something larger but before I do I wanted to run it by others first.

J2 is the LCD display.

upload_2017-2-25_18-48-40.png
 

AlbertHall

Joined Jun 4, 2014
12,345
If the display looks like that with the program stopped then I would think that it is a faulty display. The contrast appears to be correctly set so the 10k pot isn't the problem.

I see you have the same problem with two displays so it would be a good idea to check the actual supply voltage and that it is clean.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
I never said anything about the program being stopped. I tried a second supply (my bench supply). Same issue.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
Looks like a bad couple of displays. I have another display I use for prototyping. It has a SIP connector so I can easily hookup jumper wires. That one works perfectly. The only thing used on the prototype LCD is the current limiter for the backlight and doubtful that is the issue.

Bet I ordered both of these at the same time and got a bad batch.
 

Picbuster

Joined Dec 2, 2013
1,047
Disconnect all data lines an see result you must be able to go from block to blank.
If this fails look at decoupling when pwr is insufficient this could occur.
Measure for oscillation on the pin contrast and power pin. (decouple >10Uf and 100nF parallel)
Picbuster.
 

Kjeldgaard

Joined Apr 7, 2016
476
A few more questions:

Where is trimmer set, when the contrast is the best?

Is it one of those rare HD44780 displays, that actually need a small negative contrast voltage?

You mention two displays with the same error, but do you have more display of exact same manufacturer and type?
 

AlbertHall

Joined Jun 4, 2014
12,345
A few more questions:
Where is trimmer set, when the contrast is the best?
Is it one of those rare HD44780 displays, that actually need a small negative contrast voltage?
You mention two displays with the same error, but do you have more display of exact same manufacturer and type?
Please read post #5
 

GopherT

Joined Nov 23, 2012
8,009
The price keeps falling in eBay. 5-years ago, I was happy to find one under $4, now prices are about $1.75 including shipping. I'm guessing I'll have to order 3 to get one good one.
 
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