By Steve Bush 15th May 2024
All credit to Thomas Langewouters for an unusual reed switch ladder circuit for liquid level sensing. You can find it on his website here. Time Delay Relay 120v
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.
Tagged with: EinW Engineer in Wonderland magnetic relay sensing
That does look interesting… and I should find time to read it in detail (sorry!). It reminds me a bit of some commercial sensors that are built of stainless and not inexpensive. A bit of poking around the web pulls up this… https://www.deeterelectronics.com/us/product/float-switches-level-sensors/vertical-mounting/lvcs-series-continuous-vertical-liquid-level-sensor/ It does mention using hall devices, and I have to wonder if they aren’t hall switches in a similar configuration to the circuit you reference. I suppose you could arrange some linear hall sensors to sense the approach of the magnet in the float, and then a processor to merge the outputs of a number of sensors to produce a useful output signal. That would be truly a continuous output signal, as the name implies. I notice that the web page says that they make a 2 meter version of this, so it would be a candidate for Thomas’s application, but the lure of just throwing something inexpensive together is hard to deny. 🙂
Your email address will not be published. Required fields are marked *
Get our news, blogs and comments straight to your inbox! Sign up for the Electronics Weekly newsletters: Mannerisms, Gadget Master and the Daily and Weekly roundups.
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Keep up with developments relating to space technology - satellite technology, PNT, thermal imaging, SatIoT, spaceports and more
View our busy aerospace section »
Read our special supplement celebrating 60 years of Electronics Weekly and looking ahead to the future of the industry.
Read the Electronics Weekly @ 60 supplement »
Read the first ever Electronics Weekly online: 7th September 1960. We've scanned the very first edition so you can enjoy it.
Read the very first edition »
Keep up with developments relating to the Internet of Things (IoT) - Industrial IoT, sensors, Edge AI, battery technology, SatIoT and more
View our popular Internet of Things section »
Keep up with developments relating to Power electronics - MOSFETs, batteries, PSUs and more, for example
Solid State Relais View our busy Power section »