Displays can be used to communicate with a user/operator. They work much similar to a computer screen but typically are much smaller. Displays can be as simple as a matrix-style LCD monochrome text display and go all the way up to sophisticated true color graphics screens, some providing touch input support.
Technologies
These are the commonly used display technologies:
- Signal LEDs Basic Signal LEDs can signal state, In DIY projects, simple Signal LEDs are common place (requiring one GPIO per LED. That’s why today, Signal LEDs are often replaced by programmable WS2812 RGB LEDs that can be daisy-chained. A single GPIO is required to control any number of such LEDs individually.
- LED Matrix Displays: When multiple LEDs are combined, LED matrix displays evolve and can display numbers and text. More sophisticated LED matrix displays are comprised of freely programmable dot matrices where each dot is represented by a single LED. These displays can visualize simple graphics, however the individual dots are large, and the resolution is very low. Generally, LED-based displays offer high brightness and energy efficiency but are limited in color depth and resolution. LED Matrix displays are typically multiplexed using specialized driver ICs such as the MAX7219 which can drive a 8x8 matrix display with three GPIOs, and these matrices can then be daisy-chained.
- LCD (Liquid Crystal Display): LCD technology emerged to provide higher resolution and more color accuracy than LED displays, at much lower energy consumption. LCDs use liquid crystals sandwiched between glass layers and requires a backlight for illumination. LCDs became widely popular in the 1980s for watches, TVs, computer monitors, and mobile devices. Today, LCD displays are commonly used for very low-cost devices. In DIY projects, LCD displays are still very useful for very bright environments (like devices that are operated in direct sunshine). Such displays are limited to text characters and typically accessed via the simple I2C interface (requiring two GPIOs).
- TFT (Thin Film Transistor): A refinement of LCD technology, TFT displays use active-matrix technology, where each pixel is controlled by its own transistor. This results in faster response times, better color reproduction, and higher resolutions compared to earlier passive-matrix LCDs. However, TFT still require backlight, and due to their low contrast, they can be hard to read in direct sunshine. Today, TFT is used for most computer screens because of its cost-effectiveness and the circumstance that computer displays are typically not operated in super bright environments. In DIY projects, TFT is still the only affordable and robust technology for large color displays. Since such displays have high resolution, a large color depth, and potentially high frame rates, large amounts of data need to be transferred which is why the SPI interface is commonly used here. It requires four GPIOs.
- OLED (Organic Light Emitting Diode): The latest advancement, OLED displays feature organic compounds that emit light when an electric current is applied: each pixel becomes its own small light source, and no backlight is required anymore. OLED provides superior contrast, deep blacks, and flexibility in design, allowing for thinner and more energy-efficient screens. They work equally well in darkness and bright environments and are today used in high end smartphones and TV screens. However, OLEDs are much more costly than TFT, have a much shorter lifespan, and are prone to ghosting (when a static image “burns in” over time). In DIY projects, typically very small and monochrome OLEDs are used. Since they do not display moving video or high resolutions, the slower but simpler I2C interface is commonly used. It requires only two GPIOs.
- E-Paper/Electrophoretic Display (EPD) Technology: Inside an e-paper display are millions of tiny microcapsules suspended in a fluid. Each microcapsule contains positively charged white particles and negatively charged black particles (or other color combinations). When an electric field is applied, the charged particles move within the capsules. E-paper is bistable: once an image is displayed, it remains on the screen without requiring additional power. The display only consumes power when updating (i.e., when the particles move), making it highly energy-efficient (for static images), and an energy hog when updating the display rapidly. Since it reflects ambient light, E-Paper is highly readable in bright sunlight and less straining on the eyes. It almost looks like real paper. Its use in DIY project is limited because of its high cost and its very slow screen update process during which the screen can visibly flicker multiple times. E-Paper is a promising technology, however there hasn’t been much technological progress in this area for many years now.
| Technology | Cost | Energy | Sunshine-Environment | Requires Microcontroller | GPIOs |
|---|---|---|---|---|---|
| Signal LED | low | medium | +++ | - | 1 |
| Programmable Signal LED (like WS2812) | medium | medium | +++ | yes | 1 (for any number) |
| LED Matrix | medium | medium | ++ | - | 3 |
| LCD | low | low | + | yes | 2 (I2C) |
| TFT | low | medium | – | yes | 4 (SPI) |
| OLED | high | low | + | yes | 2 (I2C)/4 (SPI) |
| E-Paper | high | low | +++ | yes | 4 (SPI) |
Operating Voltages
All the different display types uses internal voltages that can be as high as 15-30V (as in the case of the gate driver voltage in TFT displays) and as low as 1.8V (as in LEDs or OLEDs).
Display Drivers (or simple current-limiting resistors with Signal LEDs) take care of safely converting the operating voltage to the voltages needed by the displays. That’s why you must always make sure the driver logic of your TFT, OLED, LED Matrix, or whatever else matches the operating voltage your microcontroller or project uses.
There are displays that can handle a wide input voltage range and work equally well for 3.3V and 5V circuits, and there are other displays that have very specific requirements.
LED-based displays (like LED Matrices) often use simple resistors to limit the current. Such displays are then typically rated for 5V and the classic Arduino world. To use them with 3.3V (with modern microcontrollers such as ESP), you’d have to remove (or change) the current-limiting resistor.
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(content created May 05, 2024 - last updated Oct 16, 2024)