How much equity should I give an electrical engineer working 10-15 hours on simple hardware product and if they are not an employee, do vesting?

Generally you don't vest if they're not employed. It doesn't make sense if you think about it -- vesting means "you earn more and more of this amount unless you leave." But they're not employed, so how would you know whether they've left?

From what you say, I wouldn't offer equity at all. Equity is long term compensation to promote loyalty and engagement.