.8mm PCB used to minimize thickness, and two can be nested together for easy carrying in wallet etc. 1.6mm total thickness with LED, 3.2mm thick with CR2016 battery installed. 

While a very simple PCB, it is correspondingly easy to make, inexpensive, and easy to transport/store. In other words, you can give them out without too much hesitation much like a normal business card!

Thanks to PCBWay for making these PCBs and sponsoring this project!