In the lead up to this year's Embedded Systems Conference, Boston, there is a lot of buzz about the limited-edition, mesh-networked attendee badge that will be available to a lucky hundred or so attendees. This clever item comes from the same minds that created the networked CapNet propeller beanies a few years back, but with a twist. The badge was made to be hacked and its full design is available as open-source so those unable to get their hands on an original can replicate one. At EDN we've dug into the design to give you insights into the design thinking behind the ESC 2016 Collectable Badge as well as a head start on building or modifying your own.
The high-level concept for the badge called for it to address several requirements. It needed to support business networking among badge wearers by using wireless networking to draw together those with similar interests. It also needed to provide an educational opportunity for wearers to learn more about mesh networking and embedded development. Finally, because all work and no play makes everything dull, it needed to provide opportunities for just plain fun. And, as always, cost was a consideration – especially as these were to be given away.
Initial design thinking
A preliminary design phase, including several e-mail design reviews, established much of the badge's functionality. The developers started out by exploring the badge format's potential for the project, analyzing the designs of other electronic badges that have appeared at various trade shows. This investigation established the size parameters that would be tolerable for a badge-like device and revealed that weight was not a significant consideration. The research also suggested that the badge advertise a user's interests to other attendees to help support business networking.
The team wanted to perform this advertising in two ways. One was to use a visual display that would communicate wearer interests to everyone around. The other was the use of wireless mesh networking to communicate interests specifically to other badge wearers. The same networking technology as used in the earlier propeller beanie design – SNAP from Synapse Wireless – would be used in the badge design. This approach would provide both the wireless connectivity and a user-programmable application processor in a single package.
The nature of the visual display was a significant consideration, however. The display needed to be large enough to be read at a distance and dramatic enough so that badge wearers could spot one another in a crowd. It also had to be versatile enough to provide more than text messaging, but it could not be too expensive. This combination of needs quickly eliminated such options as graphical displays, e-ink, and LCDs.
The developers settled for a matrix of individual LEDs, chosen for their low cost, high visibility, and simple interface. This matrix would support both text as well as simple graphics, with text scrolling across the display for extended messages The final decision was to use simple red LEDs. Tri-color LEDs received consideration, but the LED price difference as well as the more complex drive requirements outweighed the benefits of tri-color for this application.
Power was another key consideration in the initial design thinking, with 10 hours of continuous operation a design goal. Battery power was obviously essential, but the lack of a weight restriction on the badge opened many options, including use of standard cylindrical cells instead of more expensive and lower energy coin cells. The team considered rechargeable cells, but concluded that the cost and added complexity of the charging circuit (especially if designed to avoid potential fire hazards due to overcharging) outweighed any benefits. The final choice was to use alkaline AAA cells.
While AAA cells would provide a nominal 1.5V each, however, they do not have a flat discharge curve, as seen below. This discharge curve complicates the power situation, especially when trying to maximize the badge's operating lifetime on a single battery. Using two AAA cells would provide a supply voltage that started at 3.0V but would decline to just under 2V over time. Three AAA cells would provide 4.5V declining to just under 3V. While there is a large body of electronic devices that work with a nominal 3.3V power, their full operating ranges vary. Some will not tolerate voltages above 3.6V while others will not operate below 2.4V. So, either the devices used would have to be restricted to those that work over the full discharge curve, or some sort of power regulation would be necessary.
Figure 1 – Typical alkaline battery discharge curve
The use of LEDs for the display also encouraged the use of power regulation. With an unregulated battery supply the display's brightness would vary over time, gradually dimming as the batteries discharge. While such dimming is not a failure, exactly, it works against the objective of having a highly visible display.