I am trying to figure out what type of sensor I have that is used to detect water flow. This sensor is placed near a cylinder of water with some type of float. When the water pressure causes the float to pass the head of the sensor some kind of output is registered with the micro-controller indicating the flow of water. However, I have now idea how this device is used in a circuit and connected to a micro-controller.

Because this sensor has only two wires I can't figure out how it works. Is it possible that it just produces a current when a magnetic field is present? And if so, would it likely be necessary to amplify the signal so that a micro-controller analog to digital input can detect the change in voltage?

It came out of a "Pool Warden" which is used to automate pool water chemistry by measuring pH, ORP and temperature. This device is used to let the device know when water is flowing through a water block.

Anybody know how this sensor works, what it might be and how I would use it?

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  • \$\begingroup\$ Do you have access to the rest of the equipment it was designed for? Checking what voltage it was driven at might help you. I have absolutely no idea, so I can only broadly guess that it might be a switch (closing/opening), and analog (resistive) encoder, or some kind of digital two-wire encoder. It is presumably powered from a moderately high-impedance voltage source, and knowing details would allow you to safely try out what happens then, maybe looking at the current (voltage across current-limiting resistor in the supply) with an oscilloscope. \$\endgroup\$
    – user30985
    Nov 20, 2013 at 6:16

1 Answer 1


A simple sensor used in such an application is a magnetic reed switch.

Reed switch (source)

The simplest normally-open reed switch has two contacts, which are separated when there is no strong magnetic field incident on the switch. When the switch is exposed to a strong enough magnetic field, the contacts close and a circuit can be established through it. In your case, the float would have a small magnet on it, which passes close to the reed switch when the float moves.

The advantage of this approach is that no current flows, and hence no power is consumed, while the switch is open. Power requirement of the sensor is marginal even when the switch is off - just sufficient to overcome a pull-up resistor at the microcontroller sense pin, for instance.


simulate this circuit – Schematic created using CircuitLab


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