LCD contrast issue

[jeg223]

I'm using a HD44780 controlled LCD, but the display is almost invisible. I'm using a 10k pot from +5 to ground with the wiper to VL to control contrast as the data sheet suggested. If I go from +5 to -5 the contrast is great. Has anyone had similar problems or any suggestions to avoid the dual supply?

 

[gramo]

Wierd, What is VL? I'm not familiar with it from my LCD's... What Pin# is it?

The first three Pins are usually Vss, Vdd, Vee, and I wire it up like this;

**broken link removed**

 

[ericgibbs]

I'm using a HD44780 controlled LCD, but the display is almost invisible. I'm using a 10k pot from +5 to ground with the wiper to VL to control contrast as the data sheet suggested. If I go from +5 to -5 the contrast is great. Has anyone had similar problems or any suggestions to avoid the dual supply?

If its for sure a HD44780, then IMO there is someother problem with the LCD than the contrast voltage. Vo +5V thru 0v, using a 10K0 should be OK.

Is the Common 0V line connected from the LCD controller pcb to the PIC 0V line?.

If you must use a -Vo, do you have a MAX232 RS232 ic on the PIC pcb?, if so, you can tap into the -9V on the MAX.

 

[ericgibbs]

G'd Day Gramo,

Please check the naming of the LCD power connections in your pix, above.

The Hitachi manual for the HD44780 says:
pin#1 Vss, signal ground.[0V]
pin#2 Vdd, power supply for logic.[+5v]
pin#3 Vo, power supply for LCD driving voltage.[Vo=> +1.5V]

The recommended variable resistor is 10K0 thru 20K0

Regards

 

[Gayan Soyza]

I'm using another type of LCD panels.For the contrast I'm using 5K V/R and sometimes in ohms scale fixed resistor value.

My LCD manufacture recomends that you must connect the V/R in between the V0 - contrast reference supply & Vss - Ground.

If the back light is too bright also its hard to adjust the contrast well.also some back lights like blue colour draws much more current than others.

 

[gramo]

G'd Day Gramo,

Please check the naming of the LCD power connections in your pix, above.

The Hitachi manual for the HD44780 says:
pin#1 Vss, signal ground.[0V]
pin#2 Vdd, power supply for logic.[+5v]
pin#3 Vo, power supply for LCD driving voltage.[Vo=> +1.5V]

The recommended variable resistor is 10K0 thru 20K0

Regards

Well there you go

My little datasheets said 4.7K to 10K

I'll look around and confirm it

 

[justDIY]

you must have an 'extended temperature' lcd, which requires negative voltage for contrast.

stealing a little -V from a max232 is an awesome and easy idea, you can also generate some using a charge-pump consisting of two capacitors and two diodes driven by a sq. wave from your microcontroller.

another option is a dedicated charge pump controller - Microchip has an easy to use SO8 verison that works with inexpensive ceramic caps.

 

[ericgibbs]

you must have an 'extended temperature' lcd, which requires negative voltage for contrast.

stealing a little -V from a max232 is an awesome and easy idea, you can also generate some using a charge-pump consisting of two capacitors and two diodes driven by a sq. wave from your microcontroller.

another option is a dedicated charge pump controller - Microchip has an easy to use SO8 verison that works with inexpensive ceramic caps.

Hi,
I use the -V from the max for graphic displays.

You can steal upto 10mA from the +/- lines of the MAX232, I usually make the MAX caps 4.7uF or 10uF, when I do that.
Use the +/- 9v thru 78/79L05 for opa supplies.