Choosing the Right Display for IoT Devices

Forecasters assure us that the Internet of Things (IoT) is going to be BIG, with billions or even tens of billions of devices becoming connected over the next decade to the networks that make up the Internet. These ‘Things’ will be diverse in nature: IoT is being used as a catch-all phrase for everything from wearable fitness bands reporting your health stats daily via a mobile, to embedded sensors scavenging energy from their environments and occasionally passing small amounts of data to a server over low-energy mesh networks.

Choosing the ‘right’ display for such a diverse set of applications means going back to first principles; understanding each application’s requirements and then finding the display that best serves those needs. That’s not to say, however, that there aren’t opportunities for the process to happen the other way around – with a particular display technology’s characteristics creating opportunities to add functionality to existing IoT devices.

Consider IoT devices such as the aforementioned environmental sensors. For these devices, it is vital to minimize their power use to extend the operating time between charging cycles. Such devices don’t have the energy budget to run backlit, high-resolution color displays. On the other hand, in many situations it would be useful to have a display on each device to show simple information, such as battery charge level or system diagnostics data, without having to get that data by using another device to log in to a cloud-based server.

Bistable displays such as electronic paper, which has been used with great success in e-readers such as the Amazon Kindle, fit the bill for this kind of application, and there are two key reasons for this. The first is that the display only consumes energy when it changes (Figure 1). This saves the energy used by other display technologies to refresh a static display. The second is that e-paper displays are reflective, and so don’t need an energy-hungry backlight for their contents to be visible – at least in reasonable ambient light.

Figure 1: Electronic paper is either black or white depending on the charges of the electrodes.

This low-energy technology creates opportunities to add displays to even the simplest IoT devices, so that they can present a direct, human-readable interface. If your IoT device can receive data from its cloud as well as sending it, that creates opportunities to make, for example, a payment card that shows your current credit balance, or membership cards that hold a scannable entry barcode or notifications of special offers.

Of course, an e-paper display is much less useful in situations where an IoT device needs to show rapidly changing data, or work in a dark place. For that kind of application, TFT LCDs and OLEDs can provide useful alternatives.

The quality of LCDs has improved enormously over the past couple of decades, driven by their widespread adoption in flat screen TVs, mobile phones and computer monitors. The resolution of some screens is now so high that it is virtually impossible to see individual pixels with the naked eye, while gamers and action movie enthusiasts have driven up display refresh rates to the point where even the fastest action can be clearly displayed.

This means that LCDs are best suited to use in IoT applications that demand a rich multimedia experience, or which are designed to be used in dimly lit places where the backlight can provide the necessary screen contrast to ensure legibility. For the same reason, LCDs are less readable in bright light, because of the decreased contrast this creates.

However, LCDs are an inherently more energy-hungry display technology than e-paper, so may best be reserved for IoT devices such as thermostats that are permanently powered, or for devices such as mobile phones, whose users are happy to recharge them regularly. The fact that LCDs need a backlight also means that an LCD module will be thicker than the e-paper equivalent (Figure 2), while the complex driver electronics of high-information-content displays will also present cost and energy-consumption drawbacks.

Figure 2: Cross-section showing the construction of a typical TFT LCD.

A third option for IoT device displays is OLED technology, which has started to appear in high-end TVs, mobiles and smart watches. OLEDs have theoretically better power efficiencies than TFT LCDs because they use electroluminescent materials to emit light when a current is applied, rather than using liquid-crystal materials to act as a shutter over a backlight, as in LCDs (Figure 3).

Figure 3: Typical cross-sectional view of an OLED display.

OLEDs can control the brightness of individual pixels, enabling the display of pure blacks and very high contrast ratios. They also offer better viewing angles than many LCDs, and can be made on a plastic substrate to create unique shapes, as in the Apple Watch. As with most technologies, though, OLEDs have disadvantages, such as ageing issues with some of the electroluminescent materials, sensitivity to water, which tends to necessitate costly encapsulation, and the same sort of daylight readability issues as LCDs. In addition, they’re still relatively expensive, compared to LCDs.

The term IoT is so broad as to have become almost meaningless. Choosing the right display for your IoT design simply means understanding the application’s requirements and then choosing a technology that matches. Clever designers will also ensure that they look at how the available technologies such as e-paper can bring additional, perhaps unexpected, functions to their Things and so help set them apart from the billions of other devices that will make up the IoT.

Pervasive Displays parts for your design are available from DigiKey. These parts include a variety of display modules, evaluation boards and kits, evaluation expansion kits, and an RF evaluation development kit.