I would like to buy a pressure activated water valve (for irrigation), so that when the water pressure is above P1, the valve will turn on, and when water pressure drops below P2, the valve will shut off. Does anybody know any product that would serve this purpose? Thank you.
What you are looking for is generally called a
Spring loaded Check Valve. A spring presses against a ball which seals the line. Flow only occurs when the pressure can overcome the force of the spring. But that only gives you a single pressure to work with.
See Check Valve for an overview