Abstract: A PC board containing electronic components of a normally closed switch is enclosed in a case and connects to a circuit using two wires. Electrically conductive sensor-probe pins extend from the PC board and are positioned for detecting liquids at various levels within drains, drain pans, tanks, reservoirs and pipes of various dimensions. An optional remote probe, having a pair of electrically conductive sensor probe pins, connects to the PC board and is adapted for positioning at a location that is separated from the enclosed case, such as in an auxiliary drain pan of an HVAC system. The normally closed switch is actuated to an open state upon a rising liquid level contacting the tips of the sensor probe pins. Actuation of the switch to the open state changes the status of the circuit from a normally closed circuit condition to an open circuit condition, thereby interrupting operation of a system (e.g. HVAC) on the circuit.