ATTINY Led Keychain is a LED display badge using a mix of SMD and DIP parts powered by a coin cell. 12 LEDs charlieplexed off an ATtiny85, driven by 4 GPIO pins instead of 12.

Hardware:

ATtiny85-20PU, SOIC-8

12x LED0805 yellow-green LEDs, charlieplexed in a 4x3 grid

4x 68 ohm series resistors on the charlieplex lines

10k pull-up on RESET

100nF decoupling cap

CR coin cell holder for power

How it works:

Charlieplexing drives 12 LEDs off 4 GPIO pins instead of wiring one pin per LED. Each pin can be set to three states, high, low, or floating, and each LED is wired between two of the pins in a specific direction, since an LED only lights up when current flows the right way through it. To light one LED, its anode pin goes high, its cathode pin goes low, and every other pin is set to floating so it doesn't interfere. Wiring LEDs in both directions between the same pair of pins lets one pin pair address two LEDs instead of one, which is how 4 pins end up controlling 12 LEDs instead of just 4.

Only one LED can actually be lit at a time this way, so the chip cycles through all 12 fast enough that it makes it look like several are lit at once. The firmware controls which LED is on and for how long by switching the pin states rapidly in a loop.

You can find a demo video at https://youtu.be/Yd-Kx_JBXS8