There's a 3xAAA battery pack under the Geiger counter that powers the Moteino Mega. The 3.3v output from pin 0 is soldered to the positive terminal of the Geiger counter's battery case to power the Geiger counter, as it only draws 10 mA. The Geiger counter power switch is set to the on position, so the Moteino Mega controls when it's powered by changing the pin 0 state.