I2C LCD Display Problems

I hope this post can be a rescue ring for anyone struggling with this problem. I purchased several lcd displays on eBay from a seller named 'Frentaly.' Now the seller was fantastic, located in the US so shipping was fast and the build quality was very good, however, I couldn't find a library that would work with it. I contacted Frentaly and they didn't know what the problem was, but offered me a refund, which I declined because I was certain I was doing something wrong (how could three lcds be defective?!).

I became an expert on these lcd screens. I used every library and sketch I from every how-to video and blog out there, but nothing seemed to get the display to show text. I could flash the back light, read the device's address, but I couldn't see the text I was sending it. I gave up for a week in frustration, and finally reproached it yesterday at work when I found a page on i2c lcd displays (not the solution, but still a great resource). I thought maybe I had found the right library, because that page says some lcds have different wiring. I got home, ran it, and it wasn't fixed. Undeterred I scoured the net looking for customer reviews, hopeful someone else would share this problem. I finally found a useful review on amazon (no link because I can't find it again, but thank you amazon reviewer!).

The lcd I bought came with a pre-soldered 'backpack' on it, which would make it possible to use only I2C wiring, instead of several i/o pins. On this backpack is a small blue box with an adjustment knob on it, can you guess what that knob does?

Contrast, Bingo! Turn it until you can see the text on your screen, and voila! Miraculously, my screen works now. If your display doesn't have one of these backpacks you can pick one up on ebay for 99 cents, and it'll make your life a whole lot easier!

Thanks for reading, and as always, check your wiring!