Progress has been very slow through December, due to both project related issues, and real-life getting in the way (which is actually not a bad thing).
Project issues include a cmos 555 timer with a strange fault, a damaged RaspberryPi USB socket, and discovering my choice of relay was a bad one.
Although the relay had a 1A contact rating and looked suitable for a Raspberry Pi drawing less than 500mA, this rating is for a purely resistive load. (see earlier post for relay circuit details.)