Document version: 2015-08-31

It's possible to control high power loads via the output pin from any
micro-controller MCU. The MCU itself can source loads with a couple 
of mA. The solution is to use MCU to control a switching circuit. FET 
transistors is a normal choice for such circuits.

The FET transistor must be able to switch with TTL logic levels used
the by the MCU. So special care must be taken for choosing FET.
IRL 3803 is used in this in our case. The circuit is simple and is
called an open-drain circuit.

A FET is controlled by Gate-Source voltage. The MCU TTL pin is connected 
to the Gate and Ground (GND) to the Source.  The load is connected between 
and (external) power and drain.

A simple design can design on veroboard:
http://www.radio-sensors.com/pictures/TTL-FET-PWR-PCB-edited.jpg

A an example a sensors node is controlling an SSR (Solid State 
Relay) is depicted below:
http://www.radio-sensors.com/pictures/TTL-FET-PWR-overview.jpg

The setup is powered by the USB cable (5V). This powers the sensor node 
and the 5V is also used as open-drain voltage for the IRF 3803 N-ch FET.

Pin PB2 is from ISP connector is used for logic control. This pin can be 
set on/off by the firmware.