This scheme does not work. The reason is that the switch press to turn off the device has the FET once again shorted via the other pole right at the time the micro is trying to turn the FET off. You end up with a race condition.

Very many of the power on/off control type circuits you find will have various short comings. Some of these are:

- Race conditions
- Operation at release of switch
- Stuck On if switch if pressed continuously by accident
- Requirement to hold switch at On or Off time for certain length of time
- etc etc

Here is the logic I have come up with to manage On/Off for a battery powered portable device:



In this design a cross coupled pair of FETs forms a flipflop that prevents the problems listed above. The P-FET is selected to pass the required current for the application at hand. A logic level type FET is suggested. For an 9V battery application an IRLMS5703 type may be suitable.

Michael Karas