The Rainfall Sentry model RS3 is a microprocessor controlled device designed to monitor rainfall and operate external pumps and/or solenoid valves to control the discharge of storm water into a sewage system. The unit operates from a 6V DC battery and can switch a user’s slave relay/ contactor to control a mains power operated pump. It detects pulses from a Tipping Bucket Rain Gauge, and when a set number of tips have been detected within a selected time period, a relay is operated to control a pump. Another settable timer determines how long the relay is held on for. If pulses from the TBRG are detected while the relay is activated, then the relay hold timer is restarted.