This circuit uses opamps and mosfets to automatically switch between different power inputs. This can be done very easily with diodes. However mosfets have a lower voltage drop than diodes which conserves available power and reduces the heat generated.
The upper input starts with an over voltage protection stage set around 5.3V. The second stage provides backflow protection with the lower input. As many of either input as are needed can be used allowing multiple 5V power sources to be safely combined in a project.
The upper circuit is good for a primary DC input such as a barrel jack or screw teminals that a user could accidentally plug in to anything. The lower circuit is then good for "known" voltage sources such as a USB input, a battery pack, or an onboard voltage regulator.
To test the circuit begin by turning on the highlighted switch in the middle to provide power to the lower input. See that the power flows through the lower circuit to provide 5V to the load resistor on the right. Then turn on the switch in the upper left corner to enable the upper input. See how the current now flows through the upper circuit because of its higher voltage. You can then play with the three switches in the bottom left to test the other states of the circuit.
