I am finalizing details for a battery-powered water tank level sensor (based around a 5v differential pressure sensor; MPX5050).
I intend powering the unit using a 9v battery; but only "switch on" the pressure sensor during a reading every 5-15 minutes or so (9v regulated down to 5v).
I have successfully prototyped this using a digital pin (plus a transistor/N_MOSFET - 2N7000) to control a P-MOSFET - BSH203. My research indicated that a transistor or N_MOSFET was desirable between the digital pin and P-MOSFET to protect the pin from the switched voltage which is substantially above Vcc (in this case 9v).
Here is an example article on the approach I am using -
http://parttimeelectronics.blogspot.com.au/2016/06/basics-fet-as-switch-part-2.html.
My query actually relates to the design of the WeatherShield, which uses a digital pin to briefly switch on a P-MOSFET to take battery readings. In WeatherShield's case, there is no intermediate transistor or N-MOSFET, even though Vin could be substantially higher than Vcc (such as 9v). I don't want to include a transisitor/N-MOSFET if I don't really need it.
Could Felix (or other knowledgeable contributors) provide their perspective on WeatherShield's particular design considerations around that P-MOSFET component, and driving it directly from a pin in the case that Vin is 9v (I haven't been brave enough to hook up 9v to my moteino and drive the P-MOSFET directly off a pin yet - testing "off moteino" indicates that Vin will be present at the pin?!).
Mark