I also need a circuit like this. I would like for a microcontroller, (parallax propellor) to be able to turn itself off to save battery life. I would like to turn the circuit on using a momentary contact switch, then to turn off the circuit with a signal from the controller. the circuit is powered by 9v battery to 5v regulator to 3.3v regulator. controller output pins are 3.3 volts. I believe this "auto-off" ability should be a common problem, but i haven't been able to find any information on the web! any help is greatly appreciated!