Sensors and other devices destined for the Internet of Things (IoT) are no different. While the primary requirement for most connected equipment is to send and receive data to and from the cloud, there are lots of examples where having a display on the device itself would make it even more compelling to potential buyers.
For example, imagine a network of connected, battery-powered temperature or humidity sensors in a manufacturing environment. For someone on the ground tasked with monitoring the factory, being able to take an instant sensor reading without using a separate, cloud-connected device is quicker, easier, and, in some cases, safer. As a result, a sensor with a built-in display could facilitate faster and better-informed operational decision-making.
The Power Budget Conundrum
However, getting a display into a remote, battery-powered IoT kit is typically thwarted by the limited power budget. Because many sensors have to run for months or even years on a single battery, such as a CR2032 coin-cell, traditional TFT displays are out of the question. An always on, two-inch TFT LCD on its own would eat through over 1,000 such batteries in a year – and that’s before you even consider powering the sensor itself, its communications chip or other components required to make it work.
There are two main reasons why TFT LCDs require so much power. Firstly, to display even a still image, such as sensor reading, TFT displays need to be refreshed around 50 times a second. Each refresh uses power, which quickly adds up. Secondly, TFT LCDs then require a power-hungry backlight to make the image visible.
So the key to getting a display into a device running on a small battery is to eliminate these two big users of power, and this is exactly what e-paper does.
E-Paper: The Low-Power Game-Changer
E-paper displays are commonly found in e-readers, including the Amazon Kindle. They’re different from TFT LCDs in that they don’t require constant refreshing to keep the same image visible, and only use power when you change what’s on the display. For IoT applications, where sensor readings are sometimes only updated a handful of times a day, e-paper is a real game-changer when it comes to power consumption. For example, an always-on, two-inch-diagonal e-paper display, updated six times a day, would require just over 0.001% of the daily power the same-size TFT LCD would.
Amazon's Kindle with e-paper display. Image courtesy Xxolgaxx
This is made possible by the way e-paper displays work. Simply put, each ‘pixel’ is a tiny capsule containing ‘ink’ particles, which are typically black and white (though red is also available). The white particles are positively charged and the black ones negatively. When you apply a positive charge to the top of a capsule, you draw the black particles into view, creating a dark area on the display. Applying a negative charge brings the white into view, resulting in a clear area. You do this across the whole screen to build up an image. The beauty of it is that because the technology is ‘bistable’, once the particles are in position, they’ll remain there, using no power, until you apply an opposing charge.
This approach of creating an image using physical ink particles also means e-paper displays don’t require a backlight. Instead, ambient light reflects off the display in the same way it does off printed ink and paper, creating a visible image that human eyes (and barcode scanners) can see.
Getting Started with E-Paper Displays
Another thing to consider when it comes to e-paper displays is how easy and low-cost they are to try out, whether you’re a professional product designer or a tech enthusiast.
For examples, you can use an Arduino UNO or MEGA 2560 with a dedicated e-paper display extension board. This particular board enables you to try out displays up to 4.2 inches in size. Once you’ve got your kit, it’s a question of downloading some code files from GitHub, creating the image you want to display, updating the code to point it at your new file, and uploading the image to your Arduino.
An Arduino with an e-paper display extension board. Image courtesy of Pervasive Displays
A Popular Choice for the IoT
E-paper is a game-changer when it comes to adding displays to equipment with very limited power budgets: it opens up a whole new range of possibilities for designers to dramatically enhance the user experiences their products offer. Moreover, the ease with which you can experiment with e-paper in your designs–not to mention the relatively low investment required–explains why so many in the IoT world are now including e-paper displays in their equipment.
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