This circuit is designed to accomplish over voltage protection, under voltage protection, and over current protection using simple components.  The circuit is designed to protect an atmega328P or other similar 5V microcontroller.  There is a enhancement mosfet (Q1) which is normally open and closes when the gate reaches at least 2V.  There is a voltage divider (R4 and R2) which makes it so this mosfet will only close when the input voltage increases over 4V.  There is a depletion mosfet (Q2) which is normally closed and opens when the gate reaches at least 1.8V.  The voltage divider (R3 and R9) make it so this mosfet opens when the voltage increases over 5.8V, thus protecting the circuit.  I chose a resettable fuse with a 150 mA trip current to protect the circuit from a short.