Everything for Electronics

From the Q&A

With TJ Byers

Whatcha Gonna Do When The Well Runs Dry?


My sprinkler timer engages a well pump via a relay. But when the well goes dry, the pump keeps running. I need a circuit to shut the pump off when the well is empty and prevent the pump from starting again until the well is filled. The season dictates how long it takes for the well to fill, so another timer isn’t the answer.

Ken Schultis
Salmon Arm, B.C. Canada


Because your timer operates a relay, I decided to build the circuit using just relays. They can withstand harsh environments, are immune to power surges, and are generally more trouble-free than their semiconductor counterparts for this application.

RLY1 is the top sensor and RLY2 is the bottom sensor. RLY3 is the controller relay. In order for RLY3 to lull in, both RLY1 and RLY2 must be engaged. This is done when the water level is high enough to contact the Well-Full and Empty sensors. when these sensors come in contact with water, current flows through the relay coil and closes its contacts. Once RLY3 is engaged, it remains that way even if RLY1 turns off because of the unique latching circuit seen on the left contacts. In fact, it remains engaged until RLY2 releases, which happens when the water level drops below the Empty sensor and current ceases to flow through the coil. This breaks the current flow to RLY3 and disengages the latch circuit. However, it isn’t until RLY1 is again activated that RLY3 can pull in, preventing the pump from starting until the well is full.

The two sensor relays — RLY1 and RLY2 — are RadioShack 275-233 reed relays that will pull in with just 8 mA of current, which all but the purist of water can conduct easily. If you have problems triggering the sensor relays, the insert shows a modification using a switching transistor that requires just 1 mA of sensor current. The sensor contacts can be made using two pan-head screws screwed into a length of 1/2-inch PVC plastic pipe, which you can buy at any hardware store. The sensors are then positioned in the well at your desired trigger levels.