2 x 16 LCD contrast issue...

Howdy :)

I have an L1642 2x16 LCD. I have it hooked up in 4 bit mode and it works, I can see the text on the screen, but it is extremely light. I initially tried a 10k potentiometer but found that I in order to see it best I had to trim all the way down which meant that the 10k pot wasn't really being used. I took that out of the circuit and just tied the VLC (contrast) pin to ground and that is the best I can get (contrast wise) but I seriously cannot really see the text unless I am specifically looking for it.

I have grasped in a few different places that this means that I might need to feed the contrast a negative voltage, but I have no idea how to do that. I have read a few things where people say something about two diodes, but I am pretty confused. I have searched the forum for LCD and contrast but didn't find the info I needed. Would someone be able to point me in the right direction?

I am a newb when it comes to circuits, my forte is programming. Does anyone know of a "circuit cookbook" that might help in situations like this?

Thanks, kdittyr

You had the pot wired between 5v and Gnd with the wiper to pin3 on the LCD ? . A 10k pot usually works nicely, higher value ones may "pull" the voltage divider it creates when wired up.

All the ones I've used work this way. Once I've established what it works with I sometimes substitute a pair of fixed resistors in place of the pot.

kdittyr:

I have a few LCD modules that require a negative voltage on pin 3. Those devices are completely blank with pin 3 grounded or with any positive voltage. Since you can see your characters with pin 3 grounded it is not likely that your device requires a negative voltage.

The 10K pot should work. You will probably find that the best display comes with the pot almost at the end of it's range, with the contrast voltage just barely above 0v.

Don

Does your display have a backlight ?

Did you enable it ?

Interesting. Thanks for the input.

My LCD does have a backlight and it is on. I guess I could try another one of these lcds as I bought ten for $0.75 each. Maybe the first one is slightly bad, or something.

If the next has the same results I will come back with more info. Maybe have to try a negative charge pump and possibly sacrifice one. I wish I had a video camera so I could show you guys what I mean, though. Maybe later I can take and post pictures.

Thanks again for the replies, though. It is always great to have other input on problems.

Well, I hooked another LCD up (same type) and the contrast was the same... Barely visible. I tried several different resistors between ground and pin 3 and the higher the value, the less I could see the text. I am pretty certain that it needs negative voltage.

Anyway, I soldered some headers onto another LCD (different type than the other ones) that doesn't have a backlight and the contrast is wonderfull. I picked up a few max232's to grab a negative 5 volts from it but I didn't pick up the right capacitors. At some point I will get that circuit working and try it on the other lcds.

I tried several different resistors between ground and pin 3

are you applying voltage to the resistor?

here is the datasheet for your display, flip to page 27, ignore most of it, but on the right hand side it shows you how to wire up a potentiometer

5 volts on one outside pin, ground on the other outside pin, pin 3 of the lcd on the middle pin :)

Oooo, I will take a look at that when I get home. The datasheet I was going off of didn't have any wiring diagram. Thanks for the info, I'll let you know how it goes.

[EDIT] Yeah, that is what I was doing... still the same. :'( Thank you for trying, though. [/EDIT]

I picked up a few max232's to grab a negative 5 volts from it but I didn't pick up the right capacitors. At some point I will get that circuit working and try it on the other lcds.

For testing purposes you can just use a 9v battery to get the negative voltage.

Don

For testing purposes you can just use a 9v battery to get the negative voltage.

You know I have a AA battery pack that I could tap into.

I'll try that to see if it works and let you know. Great idea :)

[EDIT] I just tried a single AA battery wired in reverse... it worked like a charm and I could see the text without having to specifically look for it! Thank you for the idea! [/EDIT]

Not readable with a + voltage but you could see the letters very light. Thanks for the info, I am feeding mine a - .5V and the contrast is Very good. No idea why this is not documented but does explain the low cost. :)