a water - flow detector with an electronic time delay . there is a light - emitting diode and photo - transistor detector pair , a timer comprising an oscillator and a counter , and an alarm relay . the output of the photo - transistor is applied to the reset input of the counter , and the counter output is applied to the relay . the electronic assembly is positioned over the actuator arm of a paddle - type flow sensor assembly in such a way that movement of the arm by water - flow blocks the light path between the emitter and detector , which causes the photo - transistor output to go low , which releases the reset on the counter . the counter counts a predetermined time delay sufficient to avoid nuisance alarms due to temporary surges in the water conduit , and then activates the relay .