Do you think I'm a walking charity?

That's what an employment contract is for.

If you want me to work on weekends, put that in the contract.

I can then negotiate an appropriate wage.

If it's not in the contract, get lost.

Alternatively propose a new contract so we can re-negotiate my compensation.

If we negotiated for 160 hours of my time / month, then that's what you're getting.

And if you can't wrap your head around that, you are not qualified to run a company and probably shouldn't be trusted with any kind of business transaction at all.