i recently made something similar, except mine was dmx controlled.
instead of a separate power supply, i just used a really small servo. since all i had to do was move a small bit of plastic.
how exactly did you make the circuit to disable the power to the relay?
i suppose i could just do it with a power transistor. this is one thing i'd like to add to my design.
unfortunatly i only had 3 pin xlr connectors in the workshop so i committed the shamefull sin of not using 5 pin xlr for my dmx.
the problem i had was that many theaters actually don't like to give you dmx on stage(we used the projector in a black box on the front of the stage) so i had to run annother dmx cable from the desk to the stage. and to further complicate matters my lightfactory licence only has one universe and i didn't have a dmx splitter. so i had to set up annother (little, chinese)lighting desk just for the douser.
i think that there is a better way of solving the douser problem though:
wouldn't it be much better not to have to lay another cable?
maybe it'd be better to build a circuit that detected black video and automatically activated the douser?
i wonder if there are any preexisting vga-in circuits for arduino?
or for manual control it would be possible to use the ddc pins on the vga cable, (ddc is just an i2c buss that is used to transmit information about the display.)http://en.wikipedia.org/wiki/Display_Data_Channel
i may look into this.