Then you go from being a prototype designer and software/hardware developer into a businessman.
If you are building something that can acquire a patent, you do that first. Make sure your patent applies to the big markets (US, Europe, China).
You'll now need to hustle to discover manufacturers in the worlds factory (aka China). You may also be able to find an OEM in Taiwan or another high-tech country that can help you refine your designs and turn the prototype into a working product.
You'll also need to develop sales/marketing channels, work out logistics, costing, life of product, etc.
Basically, if you are more interested in the technical side of things, you'll quickly grow frustrated with everything else.
Before thinking of building a great hardware product, first see if your profile fits that of an entrepreneur.