The design for both PCBs is flexible in terms of power management strategy. The primary configuration the PCBs are designed to handle entails that an ATX power supply is connected and the relay controls the switch circuit. Obviously any power supply that provides a switch circuit would work. The relay can also be used to control A/C power to the D/C power supply or the relay circuit could be used between the D/C power supply and the RepRap Controller (using two relays of appropriate capacity).
Below is a picture of the larger PCB (with relay on the board) mounted on top of my Prusa I3's electronics stack. In this implementation the gas and temperature sensors are surface mounted on the back of the PCB. This places the temperature sensor at the very top of the electronics stack and the gas sensor a little higher. The temperature sensor is focused on the controller but the gas sensor will react to issues from either the electronics stack or the print area. The buzzer is mounted on the printer frame.
Here is a smaller PCB mounted on my Prusa I3 just above the electronics stack with the relay just below the monitor PCB. The two sensors are on a stalk that is attached to the top of the printer controller. The gas sensor is at the top of the stalk while the temperature sensor is inside the lower part of the stalk closer to the electronics.
More in Part III