Matching Display Type to Your IoT Device Needs
Most IoT devices would benefit from the addition of a display—if it’s chosen to match the functional needs and energy constraints of the device.
The hype surrounding the Internet of Things suggests that the world will soon be host to billions of connected things, linked to each other and to centralized data-collection and analysis services over the rapidly evolving internet. Some of these devices will be connected versions of products we already use—such as home thermostats and industrial machine monitors. Others will be entirely new, using the availability of low-cost connectivity to enable new services—such as fitness monitors that allow you to compete with complete strangers over lap times for your local park run.
Many of these devices could be enhanced by adding a display, even if it is not strictly necessary. Go back to the fitness band example—since they are all about data capture and sharing, they could be implemented as a plain band and rely entirely on a Bluetooth-linked mobile phone, or internet connected laptop, to display the resultant fitness data. In practice, almost all fitness bands have a basic display to give their users feedback. Similarly, environmental monitors such as industrial thermometers, strain gauges for civil infrastructure, and pollution monitors don’t need a display but would be a lot more convenient to work with if they had one.
If you accept the premise, the next step is to ensure that the display you choose fits the functional needs and design constraints of your IoT application. A high-resolution, full color-gamut display may look great—but it’s not much use if it drains the battery of a remote monitoring device in a day, or can’t be viewed in the bright sunshine of its intended location. So what are the options, and where is it most appropriate to use each?
E-paper — a bistable, reflective display technology
Bistable displays such as e-paper only use power to change what they are displaying, holding an image without consuming further power. The displays are reflective, so they don’t need a backlight, and offer high contrast that continues to work well in the bright lighting that would wash out an alternative technology such as liquid crystal displays (LCD).
How is this possible? Rather than acting as a two dimensional array of tiny ‘shutters’ that allow or stop the passage of light from a backlight to the display’s viewer, an e-paper display uses millions of tiny capsules, each containing positively charged white particles and negatively charged black ones, suspended in a clear fluid. When you apply a charge across each capsule, the ink particles align with the applied field, creating an image. Ambient light is reflected off the display, so it doesn’t need a backlight and sustains a high contrast ratio, even in bright lighting.
Fig 1: An e-paper displays uses a matrix of tiny capsules, each containing charged black and white ink particles, to make an image. Source: Pervasive Displays
The charged pigment particles in e-paper have to physically move within their capsules to change the displayed color, so refresh rates are limited. On the other hand, e-paper is very energy efficient. Its reflective nature means that it doesn’t need a backlight (although it will need illumination if used in dark environments). It also saves energy by not needing a power-hungry refresh cycle to keep a static image on display.
Over a year, the e-paper display consumes 3.29mAh, or less than 2% of the capacity of a 220mAh CR2032 coin-cell battery: the TFT consumes 262,800mAh—or nearly 1,200 CR2032 batteries. The energy consumption of e-paper is so low that e-paper displays can even be driven with energy harvested from the environment, from solar cells, vibration-driven micro-generators, or scavenged RF energy. For example, it may be possible to make credit cards with embedded displays that show your credit balance, updated using energy scavenged from the NFC connection the card makes when presented for payment.
The liquid crystal display
E-paper has its advantages, but the IoT is such a broad church that other display technologies will also have a role to play.
Thin film transistor (TFT) liquid crystal display (LCD) technology has benefitted from a program of constant improvement over the past three decades, driven by its use in computer monitors, flatscreen TVs and more recently, smartphones. It’s not so long ago that a 40in high-definition LCD TV was a status symbol. Today we think nothing of having the same 1920 x 1280 pixel resolution on a display in our pockets.
Today’s LCD can offer a combination of wide color gamut, rapid refresh rates, very high resolutions and very small pixels.
Fig 2: TFT LCDs produce an image by shining light through a layer of liquid-crystal material that acts as an individual ‘shutter’ for each pixel. Source: Pervasive Displays
The advantages of LCDs come at a cost, which in the context of the IoT is mainly their power consumption. This is due to the backlight, which provides the light that the liquid crystal shutters modulate on its way to the viewer, and the energy cost of regularly refreshing the LCD panel.
TFT LCDs are appropriate for use in a different class of IoT applications than e-paper, particularly those that have access to mains power, or whose users are conditioned to charging devices daily, such as wearables—although there’s a fine balance to be struck here between the extra utility of a richer display and the inconvenience of more frequent charging.
The vast volumes of LCDs that have been produced over the past thirty years drove economies of scale that have made TFT LCDs attractive for use even in cost-sensitive devices. It’s worth noting though, that it takes quite a lot of peripheral circuitry to drive such panels, including video memory, relatively powerful MCUs, an LCD controller, and inverter to drive the backlight—all of which adds to the costs when compared with e-paper.
TFT LCDs are also relatively thick, because of the backlight, and have poor outdoor visibility in bright ambient light.
OLED – an emissive display technology
If neither e-paper nor LCDs fit the bill for your IoT application, a third option has emerged recently in the form of organic light emitting diodes (OLED) displays—an emissive technology that is already in use in high-end TVs, music players, and smart watches. OLEDs have better contrast ratios and (theoretically) better power efficiencies than LCDs, so are they a good fit for IoT applications?
OLED displays rely on the electroluminescent properties of certain organic semiconductors. An OLED display sandwiches this material between an array of electrodes, such that when a voltage is applied electrons are deposited in the emissive layer and removed from the conductive layer. As electrons are removed from the conductive layer, holes are left that are then filled by electrons from the emissive layer, giving off light in the process.
Fig 3: OLED displays rely on light-emissive materials and so do not need backlights or polarizing filters. Source: Pervasive Displays
The advantage of this approach is its simplicity. It doesn’t need a backlight, or the polarizing filters used in LCDs, enabling thinner, lighter displays with better optical characteristics. OLEDs can also control the brightness of each pixel, which makes it easier to achieve pure blacks and high contrast ratios. OLEDs don’t leak light, as an LCD will from its backlight even when it is supposed to showing black. And the lack of polarizing filters in OLED displays also means greater viewing angles than LCDs.
OLED materials can be deposited on plastic substrates to create flexible displays for devices with unique form factors, which could be useful in some novelty-driven consumer markets.
Finally, OLED technology is not a panacea. OLED displays currently cost 20 to 30% more than similar TFT LCDs, their power consumption is only marginally better, outdoor readability is about the same or worse, and the technology has lifespan and durability issues. Other drawbacks include lower brightness than backlit LCD, and the sensitivity of the organic materials used in OLEDs to water, which leads to the use of costly encapsulation technologies. OLED materials also have limited lifespans compared to that of TFT LCDs.
The IoT offers myriad opportunities to create new services by connecting existing products to the internet, and to create new products thanks to the broad availability of network connectivity. Whichever approach you’re taking, most, if not all, IoT devices could be enhanced by the inclusion of some sort of display. The key to doing this successfully is to match the type of display to the use of the device, and the constraints under which it will operate.