I2C Adapters and Displays Explained
I2C lets you use only 2 wires to connect an LCD display
The most commonly used displays are 1602 or 2004 (20 characters on 4 lines) and to use them with I2C they need an adaptor module - usually called an I2C Backpack as it solders to the back of the display.
To add the adaptor module to your display, solder the 16 pins of the adaptor to the pads on the display, so that the body of the adaptor is oriented over the back of the display with the 4 pins pointing to the edge.
The jumper lets you turn the backlight on or off - if you remove it you can connect a switch, trimmer or circuit (eg light sensor) to control it using the two pins. The blue trimmer on the backpack needs to be adjusted for contrast so that you can see the characters clearly.
You then have 4 pins to connect - the power line of 5v and Ground, and then 2 Data lines labelled SDA and SCL.
With an Arduino UNO, just connect SDA to pin A4 and SCL to pin A5 and you are ready to setup your Arduino sketch. Click here for example code and explanation: