LCD contrast problem

[Futterama]

Hello forum,

I'm currently working on a project including a 16 char 2 line LCD display.

My problem is about the contrast. The datasheet circuit is attached below.

I use a 1k pot to regulate the contrast (shown as R1 & R2 in the diagram).
When I regulate the contrast, from dark to dim, the LCD turns completely off before I reach the desired contrast. Also, the contrast needs to be set very dark (almost unreadable) on power on, otherwise the LCD will stay blank until I adjust the contrast to very dark, and then I can adjust it a bit back.

I have tried with different capacitors for the C1+ to C1- and for the V5 capacitor. I have tried with 0.1uF and 1.0uF poly, 1uF and 10uF electrolytic. I have also tried with a 10k pot and a 200ohm pot, but nothing helps.

I use 4.6V supply, the datasheet says limit is 7V for both logic and LCD supply, so around 5V should be ok.

Any suggestions?

Edit: I tried with 3.0V supply, and at this voltage, the contrast is almost too dim, no matter how I turn the pot, but the contrast can be set to maximum readability. At 3.5V I can adjust it to both dim and dark. At 4.0V, I have the same problem as with 4.6V. At 3.0-3.5V I have no power on problems.

What should I do to use 5V supply and be able to adjust the constrast right?

 

[Nigel Goodwin]

Is this an Hitachi compatible LCD module?, if so check my tutorial which gives the hardware connections and suitable PIC software.

 

[Futterama]

Is this an Hitachi compatible LCD module?, if so check my tutorial which gives the hardware connections and suitable PIC software.

Nigel, thanks, but I already have the connections and the software right, I can write to my display, and I can adjust the contrast, but not to optimum at 5V. I have connected my 1k pot the same way you do in your schematics.

 

[Nigel Goodwin]

Nigel, thanks, but I already have the connections and the software right, I can write to my display, and I can adjust the contrast, but not to optimum at 5V. I have connected my 1k pot the same way you do in your schematics.

So is it NOT an Hitachi compatible display?, the extra capacitors seen rather strange?.

I've used many different LCD displays, they all plug directly into exactly the same circuit - the only difference between them is a slight adjustment of the contrast control.

 

[Futterama]

So is it NOT an Hitachi compatible display?

I don't know how a Hitachi compatible display works, but this is a display with 4-bit interface, and in the datasheets I've got, there is no reference to Hitachi compatibility.

Maybe I just have to run it with 3.0-3.5V, the other components in the system works fine down below 3.0V (PIC16F627A and ADT7301), exept the MAX232A I use for serial communication, it needs 5V.
But what is the proper way to connect a 5V input and output to a PIC running at 3V?

I took some pictures of the display itself:

**broken link removed** **broken link removed**

 

[Nigel Goodwin]

Maybe I just have to run it with 3.0-3.5V, the other components in the system works fine down below 3.0V (PIC16F627A and ADT7301), exept the MAX232A I use for serial communication, it needs 5V.
But what is the proper way to connect a 5V input and output to a PIC running at 3V?

That picture doesn't look like it's Hitachi compatible?.

For the 3V/5V it's pretty simple, PIC to MAX232 can be coinnected directly, MAX232 to PIC just requires a series resistor to limt the current, the PROTECTION diodes in the PIc will clip it to 3V for you (actually just over, but it's designed to be used like that).

 

[Futterama]

Thanks Nigel,

Otherwise, MAXIM has some 3.3V RS-232 drivers I could use, and then it wouldn't require 2 different supplies.

I'll go with the 3.3V supply for this project.