Reed Relays and
Electronics India Limited
Manufacturer of Reed Switches, Reed Sensors and Reed-based products
Reed Relays and Electronics India Limited Incorporated in 1971
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Level Sensing Circuits

Reed Switches and Reed Sensors are used along with resistors or special reed sensors with integrated resistor to give out a potentio-metric 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 a PCB 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 circuit has an additional alarm point which can be used to trigger an external indicator light. The alarm point in use can be a reed switch mounted on the PCB at say 1/5th tank level.
Increasing Resistance circuit
Increasing Resistance circuit with alarm
Increasing Resistance circuit with alarm

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 circuit is an alarm point which can be used to trigger an external indicator light.
Decreasing Resistance circuit
Decreasing Resistance circuit with alarm
Decreasing Resistance circuit with alarm

Increasing and 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.
Increasing and Decreasing Resistance circuit
Increasing and Decreasing Resistance circuit with alarm
Increasing and Decreasing Resistance circuit with alarm
Complete PCB layouts which use the above circuits and the R3-S and IR-xxxx reed sensors are available. Do contact us for more details.