My main concern however is that the cable is said to be compatible with the 350D, 300D Canon's and I'm just wondering whether that automatically means it will be compatible with the 400D.
Any Canon experts out there willing to soothe my fears regarding blowing up my camera? Any help will be much appreciated.
I'm no canon expert, however there are/is a design available for a long exposure setup that uses an opto-coupler rather than the transistor circuit you are looking at. The opto-coupler will isolate your camera from the pc, so from that point it will not let the smoke out of your camera. It stands to reason that as long as your camera uses a 2.5mm stereo socket that the circuit configuration will work.....search for the link to the optocoupler design, if nobody else can point you in the right direction, I'll see if I can find it, but you should be able to google around and find it.
cheers,
Doug
Yep I've built the exact same one almost 2 years ago and works on all my cameras, 300D, 20Da etc.
It's a great little gadget and the bi-LED is terrific, you can see instantly what is going on.