you can copyright designs and prototypes - but not ideas - as ideas cannot be owned or proved.
if you make a prototype or sketches of designs you can copyright them, but it costs and you must search to make sure something similar has not already been thought of and copyrighted - turns out most have. if they have all you will be able to do is produce you product and sell the finished pieces.
i would suggest approaching companies that the idea would be relevant to and explain your idea and that you own the copyright - they may buy it off you if they are interested - they could of course just nick the idea and make their own version so try ot keep certain key things secret until they sign you up - as long as they can see the thing works that is all they should need to know up front.
one thing though, many many ideas are thought up everyday and copyrighted and you be surprised ho many fall down at the first hurdle or turn out to be useless - just watch the dragons den to see how many just collapse and fail, so don't get your hopes up or quit your job.
ideas on their own are no good - you need the working model