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Level Sensing Circuits using Reed SwitchesReed 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. Click to view or download this page as a PDF. Increasing resistance as tank emptiesThe 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.
Decreasing resistance as tank emptiesThe 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.
Increasing and decreasing resistance as tank emptiesThe 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.
Complete PCB layouts which use the above circuits and the R3-S and MK-xxxx reed sensors are available. Do contact us for more details.
Click here to download this page as a PDF.
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Reed Relays and Electronics India Limited, 2011
