This circuit will power up a water pump when the water reaches a predetermined level. Then it turns itself off when the
water recedes to another predetermined point. Gates U1A through U1C each have their two inputs tied together, and serve as probes. The probes are then placed at various levels to trigger a particular function at a predetermined time. The ground side of the circuit is placed below the minimum water level. The inputs to each gate are tied high through a 100-KOhmhm resistor connected to the + 12.5-V bus. As the water level slowly rises to probe 1, the input to Ul A is pulled low by the conduction of current through the water to the ground probe. That turns Ql off and Q2 on. With Q2 turned on, the circuit is placed in the standby mode, ready to activate the pump when conditions are right. Probe 2 is placed at the maximum water level. If the water level reaches probe 2, the input of U1B is brought low, turning Q3 on, which, in turn, causes current to be applied to the gate of SCR1, turning it on. The circuit through Kl, Q2, and SCR1 is now complete to ground, and the water pump is now turned on, which causes the water level to recede. When the water level falls below probe 2, U1B goes back to logic high. However, because of the latching nature of SCR1, the pump continues to run until the water level falls below probe 1. At that point, the ground circuit opens and de-energizes Kl, which turns the pump off. The pump will not turn on again until the water level again rises above probe 2. Probe 3 was added as a warning. If the water level reaches probe 3, LED2 indicates that the pump is not working. Switch S2 is a manual override and SI powers the sensing circuit. LED3 indicates that power has been applied to the pump. LED1 indicates that power has been applied to the sensor.