A couple of months ago I started designing earrings with LED's, controlled by a micro-controller. I knew I was going to use the ATtiny, but I did not know which device to use. After some research I found an ATtiny with a very small foot print, the ATtiny84a with a 20-VQFN (3x3 mm) foot print.
The ATtiny84a has 12 general I/O pins, 8 can be used for an analog in. It supports an external interrupt to wake up the device from sleep mode. Using it in a project is very simple, it just needs a capacitor next to the Vcc pin, and the Arduino UNO can be used to program the chip as an SPI programmer.
There is just one little issue, this controller is really tiny, and hard to solder by hand. I have been using a stencil and reflow oven, and use a camera to check the connections.
For upcoming new projects I like to create some concepts on the bread board. Unfortunately there is no development board for the ATtiny84a out there, so I decided to make one myself. It will have just the essentials to upload firmware over USB with Micronucleus, a push button to wake it up from sleep mode, and a capacitor on the input pins.
I really like the sleep mode on this micro-controller for very low power consumption applications, and for that same reason there will be no LED's on this board (finding place for these was difficult anyway).