Practically the basic idea centered around the construction of a suitable switch mode regulator, the modification of a cheap USB hub and the construction or purchase of a RS232 Interface circuit.
After some initial research a simple switch mode regulator design was found in the LM2576, the Data Sheet gives basic design ideas and the circuit below was one of them.
The circuit was then transferred to a stripboard design.
The components were then obtained online and the circuits was load tested.
Secondly the a cheap USB hub was obtained online and then this was then stripped and modified in order to allow case mounting.
The USB hub was equipped with a 5V power input, with the removal of the black power socket (seen above on the PCB) this then provided a method of powering the HUB as well as the Raspberry Pi via the hubs USB plug.
Connection of physical components
The ribbon used by the Raspberry Pi GPIO is a re purposed parallel ribbon from a PC header, these are described online as a "Serial Parallel PCI Slot Header 9 Pin Male DB9 DB25 Pin Female Cable Bracket" and was purchased for 99p. For this project only the 25 way connector was used. For the design it was decided that a female 9 way "D" socket would be used for the serial port. This would then allow a standard USB to serial converter to be directly connected to the Raspberry Pi and would not leave any exposed pins on the module.
In order to make this ready for installation some connections needed to be tapped for power, rerouted for external UART access and the last of the GPIO pins needed to be re-directed in order to make all of the useful GPIO pins accessible on the 25 way "D" type connector.
Without any modification the 26th GPIO pin is unavailable as below:
If this is a problem then by opening up the ribbon clamp at the raspberry pi end and moving the last cable over and re-sealing, all the useful GPIO pins become accessible.
It is a bit tricky to split the connector but moving the last ribbon core the "D" type will be connected as below:
The level shifting for the UART signals has been handled by this RS232 module based on the MAX3232 device, these are available online and referred to as "Mini RS232 to TTL Converter Module Board Adapter MAX3232CSE 120kbps 3-5V" again this one was obtained for 99p.
The routing of signals can be depicted graphically in the circuit below. On the ribbon all signals are straight though from the 26 way connector of the Raspberry Pi to the 25 way "D" type connector. This is with the exception of the input signal to pin 10 on the GPIO bus, this is directed with a single pole two position switch so that the input can either come from pin 18 of the 25 way "D" type connector or from the RS232 module. The connections shown on the 25 way "D" type are those available if the 25th ribbon core is relocated to the 26th position.
The case to house the power supply and hub was then measured up, cut, filed and painted.
Finally the units internal structure was assembled.
Basic tests where then perfomed to ensure the stability of the power supply. So far the unit has been tested with a USB WiFi dongle, keybord and mouse connections as well as basic GPIO tests on the 25 way connector.