Those parts should be fine, but since they are SOT-23, you'll need adapter boards to use them on a breadboard.
With Ebay there is always a chance the parts are clones of the genuine parts, particularly at those prices. It probably doesn't matter in this case, but you want to check the RDSon performance when the Arduino is powered up. You would need to measure the voltage drop across the mosfet. There should be very little drop, less than 0.1V.
Take care that V-in is not higher than Vgs max of the mosfet used.
12volt in case of the AO3401, but many small smd fets are only ±8volt.
A solution is to split the 100k pull up resistor into two 47k resistors, and connect the gate to the tap of that voltage divider.
Leo..
ShermanP:
Those parts should be fine, but since they are SOT-23, you'll need adapter boards to use them on a breadboard.With Ebay there is always a chance the parts are clones of the genuine parts, particularly at those prices. It probably doesn't matter in this case, but you want to check the RDSon performance when the Arduino is powered up. You would need to measure the voltage drop across the mosfet. There should be very little drop, less than 0.1V.
Thank you very much!
I'm used to work with SMD components and have equipment for soldering (hot air) SMD components on PCBs so making a test-circuit shouldn't be a problem.
Wawa:
Take care that V-in is not higher than Vgs max of the mosfet used.
12volt in case of the AO3401, but many small smd fets are only ±8volt.A solution is to split the 100k pull up resistor into two 47k resistors, and connect the gate to the tap of that voltage divider.
Leo..
Thanks!
The voltage in my case will be 4.2v max (fully charged battery)
I fail to see why you would need an atmega328 for this, buy a attiny13a, or any other attiny ic. That would be much effective and efficient
kaseftamjid:
I fail to see why you would need an atmega328 for this, buy a attiny13a, or any other attiny ic. That would be much effective and efficient
Correct! The Atmaga328 is a bit overkill for my project, but It makes it all much easier for me. I need severeal inputs and ouputs and the Attiny dont have that many.
ShermanP:
If you want to do the mosfet switch, the attached schematic is the simplest I know of which lets the push-button turn on the power and also be an input while the power is on.
TomGeorge:
Hi;
@FSharmanP circuit;
Tom....
Can I use Pin 13 (D13) for the push button in this case?
(I have a standalone Atmeaga328p without the resistor and LED on Pin 13)
Oh well, you seem to insist to insist on doing it the hard way! 
Such is life!
I think any I/O pin would work for input or output.
