View Full Version : Parallax Serial LCD Contrast Control

03-10-2010, 11:48 AM
I am working on a project using the serial lcd and it would really enhance the end product if I could control the contrast externally using a parallel connected pot.ˇˇHasˇanyone dug into, disassembed or whatever one of the units and determined the installed pot's value and which of the 16 pins the pot leads are connected to?

03-12-2010, 10:33 PM

Generally the POT is connected to pin #3 on an LCD. This can be different for LCDs with total display characters greater than 80 (e.g. 4x40 LCDs), and even some of them can vary. If you check out the Parallax PBASIC LCDCMD documentation you will see how a parallel LCD is attached to a BASIC Stamp and how they've configured the POT on pin #3. A serial LCD is a parallel LCD with a serial to parallel board attached to the back. The POTs on these are generally 10K in various documentation I've encountered for attaching LCDs to micro-controllers, but that can be a matter of preference and even with the LCD manufacturer. Once you "dial-in" the contrast you prefer, you can measure the resistance across the POT and determine the value and use a resistor of that value instead of a POT. Attaching the connection to GND with no POT gives maximum contrast. It sounds like you need more flexibility but did not specify exactly what in your post. There can be varying situations during use of the LCD (e.g. ambient light conditions), that may require that the contrast be variable while in use. In my case I tried to find a happy medium under all conditions day and night and with the backlight on. For my 4x40 LCD, that meant a combination of a 1K resistor and a 1K POT to fine tune it near 1.7K because a 1K POT was all I had laying around at the time and I didn't feel like building up a series of resistors to get that value. My hope is that if needs change I can open my project and tweak the POT instead of tearing up the board. That's really the reason for the POT to begin with.

Hope this helps.


03-13-2010, 01:01 AM
Looks like you've "been there" and "done that" at least once. Thanks for the input. WRT my app, the display may be in bright sunlight or indoors; therefore, I'd like as much flexability as possible. I am going to be doing some experiments, non-leathal I hope, and equiv R calcs to get it to where I want to be.