OK, if there's pressure detected then a timer circuit is triggered to run the pump for 2 hours (if I'm reading 1/2 hours to mean one to two hours). When the tank is full the pump can be shut off, even if there's supply available. With a sensor and timer, when pressure is detected and the pump comes on the pressure may fall below the level of the sensor. That's why you run a pump for a set period of time - or until the tank is full.The problem I have is that water can come in tap at any time during the day for 1/2 hours then I have to detect the flow of water through the tap. when water is detected in that tap the pump turns on and fills the house tank, when the house tank is full a turns off the pump.