On/off Circuit with an AVR and a push button
You can replace the 2N2222 with a P-mosfet and drive the gate with a small transistor that will operate as a level translator so that you can control the mosfet with voltages at a level of 3v or lower.



The load represents the devices you are going to power from the mosfet output.

You should select a logic level P-mosfet that can be turned  on fully with a Vgs of -5v and have a current capability high enough for your application.

The resistor values can change if you have different needs but for static operation and low consumption the provided values should be good enough (about 0.2mA base current with 3v base voltage and about 0.4mA collector current)