FWIW, the Ruida controller has several digital inputs including one for "water protect". (Others are for other safety inputs). If the incoming signal is not active (I forget if it's looking for Vcc or GND) then the controller software will not activate the laser. IIRC, the HV power supplies have an "Enable" and a "Power" input that come from the controller.
My point is that perhaps your DIY laser can do something like this and avoid having to relay the HV power supply.
I presume you are building your own flow sensor because you're interested in the challenge, and not because you're unaware that they exist and may be purchased.
