Unusual reed switch latter for water depth sensing

All credit to Thomas Langewouters for an unusual reed switch ladder circuit for liquid level sensing. You can find it on his website here.

He went on to use a more traditional two-terminal design, but this four-terminal circuit, as he points out, can be used to get a little more information.

Fluid level sensing with reed switches involves a fixed column of reed switches as high as the potential fluid level change, actuated by a floating magnet.

These might not be as simple as they first appear – the magnet has to operate at least one reed at any time, and therefore will often be operating two simultaneously as the magnet passes one for another, and each switch has magnetic hysteresis.

Having access to every second switch on two separate outputs with the Langewouters circuit means, as far as I can puzzle, that a little extra resolution can be squeezed out of the system as it can be used to tell the difference between one switch operated, and two switches operated simultaneously – with any hysteresis blurring precision to some amount or other depending on the detailed magnetic design.

All that is needed is a simple ADC-equipped MCU at the top to decode the two ladder outputs and produce a sensible signal for onward transmission.

Reed switch ladders do need a lot of switches if a reasonable depth needs to be sensed – for example in a garden water butt, so a waterproof ultrasonic distance sensor working from the top might be a better idea.

Waterproof ultrasonic sensors that sense between 3 and 450cm are inexpensive (type A02YYUW works with 5Vand 3V3 MCUs) – although mounting one 25cm above the highest expected level could be annoying – maybe mount it close to the upper liquid level and have a 25cm reed-switch ladder measure the last bit?

Hats off to Thomas Langewouters for a though-provoking design.

The image was copied from Thomas Langewouters’ website. If it has been used inappropriately, please get in touch and it will be removed asap.