I²C LCD alternative

Free GPIO pins by moving from parallel LiquidCrystal to an I²C backpack module when the Ethernet shield consumes the pin budget.

Top view of a blue Arduino Uno microcontroller board on a white background
Arduino-compatible boards run the sketch that polls probes and publishes JSON. Photo: Wikimedia Commons contributor. CC0

Parallel mode uses six GPIO lines plus backlight drive: tight on an Uno stacked with W5100. I²C backpacks (typically PCF8574) need SDA/SCL only, freeing pins for extra probes or backlight PWM. The tradeoff is another library, address jumpers, and shared bus timing with any future I²C sensors.

Parallel LCD versus I²C backpack pin count comparison on Arduino Uno
I²C backpacks reduce GPIO use to two data lines plus power: helpful when SPI pins are reserved.

Pin budget math

Review reserved SPI pins and parallel map: two DHT22 lines plus six LCD lines leaves little headroom. I²C reclaims four to six pins depending on wiring.

Migration notes

  1. Install LiquidCrystal I²C library and set backpack address (often 0x27).
  2. Rewire SDA/SCL. Uno A4/A5 on classic boards.
  3. Update display strings in LCD format: character width unchanged.

When to stay parallel

If your shield stack already works and pins suffice, parallel mode remains simpler: DHT22 and LCD overview. Consider I²C when adding a third probe or PWM backlight on a crowded pin map.