Thin Film Transistor LED Displays - Visual I/O

To add a resistive touchscreen on top of a TFT display, Jeff explores the technology offerings and libraries needed to accomplish his task. Calibrating the touchscreen's computed coordinates with the actual pixel coordinates of the display proved difficult with a modular framework but separating functions allows easy expansion.

- Jeff Bachiochi

Last month I chose to use a TFT display, based on the simple fact that it was compatible with Arduino UNO shield (Circuit Cellar #409, August, 2024 [1]). That means the display mounts atop an UNO and requires no wiring whatsoever to operate. My micro of choice nowadays is an ESP32. Officially there is no UNO-format ESP32 from Arduino. They do make a NANO format ESP32, but that lacks the Arduino expansion format. Third-party companies such as Wemos offer one, but it's hardly a standard. Using an UNO or MEGA format board with a shield-compatible display board made a lot of sense.

Despite its advantages, a TFT screen cannot emit light on its own, and needs a backlight to generate an image. In this month's column, I add a resistive touchscreen on top of the TFT display

HOW TFTS AND TOUCHSCREENS WORK

TFT display: A TFT or "thin-film-transistor" technology display has a sandwich-like structure with liquid crystal material between two glass plates. Referring to Figure 1, you can see the two polarizing and RGB (Red/Green/Blue) color filters, which, combined with two alignment layers, determine the amount of light allowed to pass. An uncharged pixel allows the vertically polarized light to pass unaffected, then it gets blocked by the second horizontal polarizer. When charged, the liquid crystals bend the vertically polarized light 90°, allowing it to pass through the second polarizer. Each pixel in the active RGB matrix is paired with transistors that includes a capacitor to give each sub-pixel the ability to retain its charge. Thus, the TFT layer controls the amount of light allowed to flow through its color filter. The TFT screen, itself, however, can't emit light like an OLED display; it must have a backlight to generate the picture.