You'll probably be aware of this already, but your existing contract with your employer will most certainly state that you agree to work for them exclusively and that any ideas/products that you develop whilst employed by them are 'their' intellectual property.
I would be careful about keeping 'quiet' if you are to work in parallel (in your spare time) and specifically if your new interest will see you 'competing' with them in any way.
Also ensure that your protect yourself from any legal action arising, by being aware specifically of your existing 'contractual' obligations.
I speak from experience
EDIT - similar post to Dave_3 - who got in before me ;-)