To the OP: I'm guessing you're not experienced in product design, so here's a bit of a clue as to what we're all thinking and why.
Ideas alone are worthless. Seriously. This is one of the hardest lessons to learn, it comes as a major surprise to a lot of people, and usually provokes a response along the lines of "surely not!" - but it's absolutely true.
Offering just 10% equity in your business says several things:
- that you can't afford to simply hire an engineer at a fair market rate, otherwise why wouldn't you?
- that the total, accumulated value of all the time, effort, expertise that the engineer is expected to invest, will be no more than 10% of the total value that you and other participants will invest. Even though the engineer will probably be bringing the majority of the technical expertise, and developing most of the value in any eventual IP that comes out of the project, nine times as much value is still going to be generated by other people. You'll need to be able to explain where all this value comes from, in a way which is exceptionally clear, detailed, and justified.
- the eventual value of your business will be such that 10% of it is likely to end up being worth enough to cover the engineer's time, at a fair market rate, times some reasonable multiplier to account for the risk involved. Otherwise the opportunity just isn't worth it.
Let's make up some figures. Suppose the design required is fairly simple, that your business wants to own the resulting IP, and that it amounts to no more than a few weeks' work for an engineer. You might, if you're very lucky and don't require anything in terms of ongoing support, need R&D work amounting to about £10k worth.
That immediately values your business at £100k.
Then suppose that this £100k valuation is justified by the company's ability to earn that amount in profits within 3 years, and that it does so by selling its product at a 20% margin. If you can make £33k/yr on 20%, then you need to turn over about £165k p.a. averaged over that period.
Feel free to substitute your own projections, of course, but they need to be realistic.