Minimizing device power use is a constant battle for many Internet of Things (IoT) product designers, particularly when the only power source is a small battery and the equipment needs to operate for years at a time without intervention.
Designers must therefore optimize their code and, critically, choose their components carefully. Low-power sensor? Check. Energy-efficient microcontroller and wireless communication system(s)? Check. So far, so good. And how about a display? Cue head-scratching.
The IoT display dilemma
Displays can add significant value to IoT applications and improve the user experience by enabling people to read off information without the need for a second device and internet access. Use cases include smart device tags, energy meters or remote sensors.
But this is where the big challenge comes in, because displays are typically extremely power-hungry. Imagine you need your IoT equipment to run for at least five years, using just a CR2032 battery. Such coin-cell batteries typically have a specified capacity of 220 mAh, which translates into an operating capacity of 193.6 mAh, or 696,960 mAs (88 percent of specified capacity). For this example, say you’ll need to update the display in your IoT design six times per day, perhaps with a sensor reading.
To maintain a constant image on a traditional TFT LCD, the display needs to refresh every 20 ms. To be bright enough to be legible, a two-inch TFT LCD would consume around 30 mA, reducing to 3 mA in standby. In a day, such a display will require 259,203.54 mAs, meaning that over the five-year lifespan, it’ll need 473,046,460.5 mAs. Put another way, the device would require 679 CR2032 batteries in this time, meaning you’d be changing the battery every 2.69 days. Clearly, this is not a viable approach.
E-paper and IoT: The perfect match
Compare this to what you can achieve with e-paper, and it quickly becomes clear why this latter display technology is ideal for IoT applications like the one described above. A two-inch e-paper component, combining a display and external timing controller, uses 2.33 mA for 2.32 seconds to refresh the contents of its display. The rest of the time, when what’s on the display doesn’t change, it uses no power. Six refreshes a day therefore have a daily power consumption of 32.43 mAs (2.33 mA x 2.32 seconds x 6 refreshes), which, over five years, gives a total of 59,191.32mAs. This is just 8.5 percent of the 696,960 mAs operating capacity of the CR2032 battery.
[This graph illustrates the energy consumption of an e-paper display during a screen update. (Source: Pervasive Displays)]
Why e-paper displays require so little power
So how does e-paper achieve such transformational levels of power efficiency, compared to same-size TFT LCDs? The answer is two-fold.
Firstly, e-paper displays don’t require a backlight to be legible: they’re reflective technology, meaning external light reflects off the image on the screen to make it readable.
Secondly, as indicated above, e-paper only consumes power when you change what’s on the display. Because it’s bistable technology, e-paper requires no further power to keep an image visible, unlike a TFT LCD, which must refresh numerous times per second.
An e-paper display is made up of a matrix of capsules, each containing black and white particles. The white ones are positively charged and the black ones negatively charged. By applying positive charge to the top of a capsule and negative charge to the bottom, the black particles rise to become visible to the eye, creating a dark area on the screen. Invert the charges, and the white ones rise to the top to produce a clear area. You therefore create an image by applying the right charges across the whole display matrix.
Crucially, the charge is only required to get the particles in position: once they’re there, they’re stable and remain where they are until you apply the opposite charge. For an IoT display that doesn’t require regular updates, e-paper enables designers to add functionality or improve the user experience without burning through their limited power budgets in just a few days – as they would with a TFT LCD equivalent.