Admit it, you still love LED throwies, everyone does! So does David. After viewing an article on very low power LED fireflies by Karl Lunt, David decided to build the project (and does a good job of documenting the process to reproduce it). The LED fireflies use the LED as a photosensor to detect when there is light so it knows when to go to sleep, thus by saving power by not doing anything with the lights on.
The build works off of a CR2032 coin cell battery, and everything is dead-bug soldered together. The microcontroller is an ATTiny13a which is already a low power microcontroller, but by using the LED as a light sensor periodically, the microcontroller knows when to sleep and doesn’t wake up until the light level changes below a threshold. This allows the firefly to last a really long time on that single coin cell.
David added a little spin on the original by wrapping it in a waterproof enclosure (à la sauce container) and a magnet (so it can be a throwie). This looks like a fun build.