CMOS-NAND gates control sump pump

-May 14, 2009

With just a few NAND gates, you can control sump pumps and other pumps that keep your basement from flooding and maintain water levels in tanks. The circuit in Figure 1 receives 12V signals from L1, the lower water level, and L2, the upper level, of an underground tank. You adjust the gap between these two levels to avoid short cycling of the pump. When the water level touches the maximum level of L2, the pump switches on to fill up the overhead tank. When the water level falls below the low level of L1, the pump switches off.

When the tank is empty, sensors L1 and L2 and Gate D are at low levels because the outputs of gates B and A are high. When the water level rises and shorts 12V through L1, the gate outputs remain the same. When the water level further rises and shorts 12V with L2, then the output of Gate A becomes low, which forces Gate D to a high level. That action, in turn, latches Gate B’s output low. A low output on Gate B pulls down the SSR (solid-state relay), which turns on the sump pump (Reference 1). Simultaneously, the high output of Gate D turns on the gated oscillator and sounds the piezoelectric buzzer.

When the water level lowers below level L2, the pump remains on because of the latched B and D gates. If the water level falls below sensor level L1, the output of Gate B becomes high, which turns off the pump. This action makes the output of Gate D go to a low level, which stops the oscillator and thus the piezoelectric buzzer.

The circuit uses HCF4093B Schmitt-trigger-input NAND gates to square the slow signals. The input resistor, R1, has a value of 560 kΩ. Checking the circuit with a glass of filtered water shows an improved conductivity for ground water. Raising the value of the input resistor to a higher value is also not objectionable after you account for pickup and the voltage drop across the resistor due to the input leakage current.

The solid-state relay may have back-to-back connected SCRs (silicon-controlled rectifiers), random turn-on, and snubber circuitry to handle the motor load (Reference 2). Choose an SSR with a voltage rating that is double the working voltage and five to 10 times the current rating of the motor for withstanding dV/dt and the surge current. You should also use fast-blow fuses or semiconductor fuses with less than the I²t rating of the SSR, where I is the current and t is the duration of current flow in seconds. Choose appropriate SSRs for different ratings of pump motors.

This circuit uses sheathed, single-strand, thick-gauge, edge-stripped copper wires as sensors. You can connect the sensor wires in two-way porcelain connecters, which you house in a box and place at the top of the tank. The parallel sensor wires avoid the chance of a moisture interface between the wires when the water level falls below the sensors. You can also use any other high-conductivity and noncorrosive wire material in some configurations. The power supply is floating.

With few modifications, the circuit in Figure 2 can perform a slightly different function. Assume that you have a tank in which you want to maintain a level of water or any conductive liquid. Mount sensors L1 and L2 in the tank the same as those in Figure 1. Switching on the power supply causes the pump to begin to fill up the liquid in the tank. When the level reaches L2, the pump turns off. The pump remains off until the level falls to L1. When the level falls below L1, the pump again starts filling the tank until it reaches L2. The piezoelectric buzzer announces that the pump is running.

You can also control pumps with three-phase motors using a three-phase SSR or adding one appropriately rated single-phase SSR to this circuit. In this case, you can connect the inputs of the two SSRs in series. One SSR on each phase controls two of the phases, and you directly connect the third phase.


  1. Solid-state relay applications,” Western Reserve Controls.

  2. Single-phase SSRs,” ERI Solid State Relays, 2007.

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