Since my last post on this topic, there has been quite a lot of interest; this post is to cover some slight tweaks to the box itself and to include some extra details. I’ve added a second parallel socket to the side of the box and connected each pin to the corresponding pin on the other socket. This allows one socket to be connected to the computer and the other connected to further parallel devices. I plan to build several more similar boxes, which will be able to be controlled simultaneously from a single parallel port.
There are a couple of caveats. The second parallel cable must never be connected to a second computer; this can short the data lines and cause damage to one or both of the machines. Additionally a parallel port can only be trusted to source 12mA, with boxes like this requiring about 2mA each it would be unwise to connect more than 5 on the same data line; you can of course use up to 5 on each of the 8 different lines.
You’ll notice that there is only a single diode on the diagram whereas my circuit has 4; the other 3 are not really required for a circuit this simple. The choice of components for the circuit isn’t particularly important; any transformer and relay that work at the same voltage will be fine, most npn bipolar transistors will do the trick for switching the relay and any old diode and electrolytic capacitor can be used for the power supply.