14M2 2-4 line LCD PCB

lbenson

Senior Member
#1
5 years ago I made a PCB using a 14M2 to provide a serial interface (from another picaxe) to a 2x16 LCD with the Hitachi interface using Hippy's 4-bit code.

14LCD_Ser1.JPG

It worked (and works) fine. I had a use for it with a 4x20 LCD, so I plugged in the board. I thought it wasn't working, but happened to glance at it from an angle and could see the expected text very very faintly. I'm suspecting that it's because I tied pin 3 directly to ground (which had worked on my breadboard and worked on the LCD I first used). I see that some designs show this pin connected to a 10K pot. Can someone say what exactly this pin is supposed to do?

I can cut some traces and solder jumpers and a (say) 4K7 pullup to this pin if that is likely to help (if I can find a blank PCP after all these years--I'd rather not modify the one which works on the 2x16 LCD).
 

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AllyCat

Senior Member
#2
Hi,

Yes, it's normally called the "Contrast" control, i.e. the ratio of Foreground / Background intensity, and generally is required for LCDs. However, it's not needed for the OLED displays so may be omitted now (although controlling the OLED brightness can be another issue, if required). Have you tried adjusting the "5 volt" rail down or up (a little) as it can affect the LCD contrast quite a lot? Alternatively, you might be able to use the "DAC" output (B.0), but its source impedance is very high so you would actually be controlling the source impedance (in ~5k steps to Vss or Vdd) rather than the potential divider voltage level as such.

Which reminds me that one of my (many) unfinished projects is a similar 08M2 version to use with one of the "I2C Backpack" PCBs (which nearly always do include the contrast pot) often attached to these LCDs. My (interrupt-driven) RS232 reception, I2C interface and even optional PWM brightness control of the backlight, (via I2C) all worked well, but I was planning an "improved" RS232 command set (to replace/enhance the normal 253 /254 / 255 commands).

Cheers, Alan.
 

hippy

Technical Support
Staff member
#3
As said, that is the pin which a contrast pot connects to. Contrast is higher when towards 0V, and connecting directly to 0V usually works, if anything perhaps giving a slightly too high contract.

There are however some LCD displays which require a negative voltage on the pin to create a proper contrast. It could be the case that you have one of those. Do you have manufacturer or product details, or a link to a datasheet of what you have ?
 

lbenson

Senior Member
#4
Thanks for the responses. No great likelihood of product details or a datasheet. I ordered a 4x20 "IIC I2C" LCD (with the typical 4-bit backpack), and got this LCD with no backpack. The vendor gave me a refund, but does not sell a like item, so I don't know where to look for a datasheet.
 

Janne

Senior Member
#5
Too dim on contrast usually means too low contrast voltage, so It's likely the LCD you're having would require a negative contrast voltage to work in 5V system. If you had a spare PWM pin (looks like you don't on that design) you could use it to generate a negative voltage with a charge pump arrangement.

Only time I've stumbled upon such a display was when I got some mid 90's 2x40 Char LCD's. Upon examining the pcb's they came out I figured they had a 7660 generating a negative voltage just for the contrast pin.
 

hippy

Technical Support
Staff member
#6
It might be worth removing the PICAXE. When powered up without any PICAXE it should show a single line of solid blocks.

If they are very faint you could try isolating the contrast pin, jumpering any required 0V around it, then connecting a 1.5V battery with +Ve to 0V, -Ve to the LCD pin, perhaps with a current limiting resistor, to see if that improves the contrast.

The worst you are likely to do is destroy the LCD and it doesn't seem it's going to be much use as it is anyway.
 
#8
A already mentioned pin 3 is usually used to set the contrast. However, some LCD might differ. I use a NHD-0420D3Z-NSW-BBW and is able to communicate over I2C, SPI and serial. The contrast is regulated with a command to the processor of the LCD. What is the partnumber of your LCD so we can search for the necessary datasheet.
 

lbenson

Senior Member
#9
Thanks for the replies. I had hoped for a very simple fix, but I'll probably just drop it and use a 20x4 serial LCD (non-I2C) that I know works (if I can just find it).
 
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