The end-result works pretty well, and thanks to an ATtiny13 microcontroller on the back side it can do a couple of nice visual effects. It accepts either single-color or dual-color 0805 LEDs.
The current blinking pattern is stored in EEPROM. To cycle through the patterns, short the two centermost pins on the ISP pads (marked by silkscreen) with a paper-clip and power up the device.
The minimum supply voltage varies depending on which LED colour is used. It starts up at 1.8V (RED, AMBER, ORANGE) and will therefore run fine with a 3V CR2032 lithium cell. GREEN, BLUE and WHITE work as well, but will cease working much earlier when the battery wears down.
Step 1: Schematic...
For the latest design files (KiCad gerbers...) visit github.