Level Sensing Circuits

Reed Switches and Reed Sensors are used in circuits with resistors or special reed sensors with integrated resistor to give out a Potentiometric feed back across two outputs as a foamed magnetic float moves along the PCB’s length. The PCB tracks can be designed in such a way that a full tank gives high resistance and an empty tank gives low resistance, or vice versa. Resistor values should be selected according to the shape of the tank.

All resistors on PCB circuits can be of one value if the tank being used is cubical or rectangular or a vertically mounted cylindrical tank. In the case of cylindrical tanks mounted horizontally, resistor values should be highest in the center of the PCB and should decrease towards the bottom and top of the PCB to equate to the circular cross-section of the tank.

Increasing resistance as tank empties

The following circuits can be used to monitor a liquid level where a full tank gives least resistance and an empty tank gives highest resistance. The second and third circuits show additional level alarm points which are used to trigger external indicator lights. The alarm point in use can be a reed switch mounted on the PCB at say 1/5th tank level.

increasing resistance as tank empties with alarm point Example 1

Decreasing resistance as tank empties

The following circuits can be used to monitor a liquid level where a full tank gives highest resistance and an empty tank gives lowest resistance. The second and third circuits show level alarm points which are used to trigger an external indicator lights.

Decreasing resistance as tank empties with Alarm point Example 1

Increasing Or decreasing resistance as tank empties

The following circuits are a combination of the above circuits and can either be used to monitor a liquid level where a full tank gives least resistance and an empty tank gives highest resistance or vice versa. The second and third circuits show level alarm points which are used to trigger external indicator lights.