The arrival of some goodies from RS Components this morning prompted me to get on with the power supply side of the project. The issue here is that on power down it is necessary to cleanly close the Flex radio interface and save configuration/operational data for next time. It's not acceptable to just dump the power!
I fabricated the circuit on a prototyping board. A 7.5VDC "wall wart" provides the raw power and this is regulated down to 5V, as required by the system, by a 7805 voltage regulator.
When the power switch is set to ON the solid state relay conducts and sends power to the equipment. The control processor then sets pin 48 to high which lights the LED and also makes the transistor conduct, holding the relay closed.
When the power switch is set to OFF, this sends a shut down request via switch 31 to the control processor, which then closes everything down cleanly before dropping Pin 48 to low, thus removing power from the equipment.
A simple circuit that does exactly what is needed.