The AAduino is an Arduino compatible microcontroller board, created by Swedish embedded systems engineer Johan Kanflo, that’s the size of a AA battery. The AAduino is designed to fit comfortably inside of one of the bays of a standard 3xAA battery holder, resulting in a reasonably powerful, wireless node will endless application possibilities.
The AAduino is software-compatible with Arduino and powered by an ATMega328p microcontroller and fitted with an RFM69C radio transceiver, two DS18B20 temperature sensors, and an indicator LED. Johan used Keystone battery spring clips attached to the PCB to allow it to fit inside of a bay of the holder and to make electrical contact with the battery power source. The RFM69C board was somewhat wider than a AA battery, so he had to shave down the edges on the PCB to get it to fit.
You can get all of the files, Bill of Materials (BOM), and code on the AAdunio’s GitHub page. Johan also has a post on his blog which fills in more of the details.
Source: Meet the Arduino Clone That’s the Size of a AA Battery