How to raise money for a hardware startup that needs money upfront to even produce a prototype?

I want to start a hardware startup where I have a solution for home water monitoring. The problem is: some investors are already requiring a prototype and most of them are requiring an existing customer base, and a validated market. But how can I validate my product and make a prototype if I can't even get started? I tried raising cash from the family, but no one has an amount available for me. My other options were selling my car (it is worth only US$ 2k) or saving myself. But I would need to save for at least 2 years and I don't want to wait all that long or I'll lose momentum. Notice that I am from Brazil, some points to consider: 1) Some states are going thru water shortage crisis. So my solution would come in good time. 2) There are just a few angels here (around 100) and they aren't that open for projects without a customer base. 3) Banks require credit proof that are 150% of the amount that I want to lend. So if I want 50k they require a guarantee of 150k.