The Adafruit Push-button Power Switch is a tidy little design that lets you control a DC power source using an everyday tactile button. The breakout uses a latching analog circuit that is triggered by a push of the button. Press once to turn on, then press again to turn off. The circuit uses a 3A P-FET to connect and disconnect the IN pin to the OUT pin. Works great from 3V to 14VDC and up to 3A (although the FET gets a little toasty at continuous 3A draw) yet has only 0.5uA quiescent current draw.