Water level switch - what is this component and how on earth does it work?

Thread Starter

Taymar

Joined Jan 13, 2017
31
Hi all, hoping I can get some help in solving this mystery please.

I have a small circuit from a commercially available miniature humidifier. It runs a small piezo disc at it's resonant frequency of 2.4ghz, which atomizes the water into fine droplets to create a 'mist'.

I need to repurpose the circuit into a different tank, and am stumped about this one component. It looks like a small metal disc that's clipped to a spring. Only the other end of the spring is soldered to the PCB. When installed, the spring is compressed slightly and the disc end presses up into a 'bulge' in the bottom of the plastic water tank.

I'm pretty sure it's acting as a water level switch, but I have absolutely no idea how it does that job. It clearly works - if I remove the pcb from the tank but leave all the wires connected, the circuit won't run.

Please could anybody shed some light on this? In the new configuration I don't have room to mount it below the tank, so I'd need to either:
1. bypass it entirely and install a separate float level switch in the main power line
2. replace this switch with a float level switch.

I've tried searching for an answer but haven't been able to find anything useful, it looks like no other water level switch I've found pictures of. I can't figure out how it does anything at all with just a single electrical connection.

thank you very much for any insight, would love to solve this mystery.




diagram.jpg
sensor2.jpg
sensor4.jpg
sensor3.jpg
sensor-bubble.jpg
 

Thread Starter

Taymar

Joined Jan 13, 2017
31
No, I hadn't - thanks for that! Definitely seems like the same question, though without a definitive answer.

Assuming it IS a capacitive sensor, could it work by 'completing' the circuit through water alone, even though this isn't touching the water? And it completes the circuit through the water to the piezo disc which is also touching the water would be ground?

That sort of makes logical sense to me in layman's terms, but I don't really know how any of this works.

thank you very much for the link.
 

Analog Ground

Joined Apr 24, 2019
456
Assuming it IS a capacitive sensor, could it work by 'completing' the circuit through water alone, even though this isn't touching the water? And it completes the circuit through the water to the piezo disc which is also touching the water would be ground?
A capacitive sensor used for measurements such as this one is detecting either air or water. The idea is the capacitance of a "parallel plate capacitor" varies with the material between the plates. Different materials between the plates have different "dielectric constants" which change the capacitance. So, air gives a different capacitance than water. Sense the capacitance and you sense the presence of either air or water. Like any capacitor, the material between the plates is not conducting. It is an insulator.

Now, in this case, the plates are not in parallel this: | |. They are in the same plane like this" ___ ___". Even though the plates are in the same plane, they still have capacitance between them and the material between them is either air or water. A close look at the sensor on the spring may reveal two halves of a circle forming two plates of a capacitor.

What I do not see on your sensor is two wires inside the spring going to the two possible plates on a capacitor. So, I would not go too far with the capacitance sensor theory.
 

Thread Starter

Taymar

Joined Jan 13, 2017
31
What I do not see on your sensor is two wires inside the spring going to the two possible plates on a capacitor. So, I would not go too far with the capacitance sensor theory.
Thank you for this - is it plausible that one of the 'plates' is the spring and disc (it does only have one wire), and the second 'plate' is the piezo disc which is also in the water? That has two wires - could they share a common ground?
 

Sensacell

Joined Jun 19, 2012
2,919
So, microwave frequencies to boil off water, makes sense.

I seriously doubt it's 2.4 Ghz. These things vibrate mechanically to break up the water into droplets, it's not thermally boiling the water. (in that case a simple heater would be cheaper)
More like 24 Khz, which is reasonable for a piezo ceramic element, while 2.4Ghz is totally too high for this concept.

It is most likely a capacitive water level sensor- the spring keeps the electrode in contact with the tank- as close as possible.
The bulge helps the plastic to "de-wet" when the level drops, so the film of water on the plastic does not register as "full" - which could occur with highly mineralized water. (conducts well)
 

ericgibbs

Joined Jan 29, 2010
14,190
hi Tay,
I would say the spring part is just to keep that metal disk in firm mechanical contact with the water reservoir base.
The stepped base level ensures during normal operation the metal disk and spring are the method of acoustic feedback to the HF oscillator. The vibrations from the Piezo are conduct via the water to the metal/disk and spring and then into the main HF osc circuit.

When the water level falls below the stepped base the acoustic coupling is lost and the main HF osc stops running.

E
 

ericgibbs

Joined Jan 29, 2010
14,190
hi 747,
Where is the Spring assembly [ as per the TS.s query] shown in either of those two YouTube video's, that I referred to in my post #11, regarding the auto water out cut off.??:rolleyes:

E
 

Thread Starter

Taymar

Joined Jan 13, 2017
31
hi 747,
Where is the Spring assembly [ as per the TS.s query] shown in either of those two YouTube video's, that I referred to in my post #11, regarding the auto water out cut off.??:rolleyes:

E
Neither of the YouTube videos I posted use that method of water level sensing. Sorry for the confusion. The circuit I have now with the spring is from a separate device - a USB humidifier.
 
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