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Trying to figure out how ultrasonic humidifier detects low water level Answered

Hello all,

I'm currently working on a home automation system and have decided I want to throw my fragrance diffuser into the mix. What I'm trying to accomplish is I'd like the diffuser to come on whenever the unit has power so I can simply control it with a Sonoff SV. I was originally planning on just replacing the guts with an Arduino Nano or something until I tore into it and realized I have absolutely no clue how this thing detects when the water level is low! There's no sort of float switch or resistive fluid detection. In fact, the only part that has contact with the fluid is the ultrasonic transducer. I'm curious if anyone can look at the PCB and give me some more insight as to what's going on. On the far left the five pins coming into the board is the cable connector for the buttons and LEDs, the two large pins beneath that are the buzzer for making beeping noises when buttons are pressed, The SMD IC towards the middle is from what I can tell the brains of the operation. The three large pins (two with solder) near the top center are where 24vdc comes in. The two large pins on the upper right are for the fan and the two large pins on the bottom right are for the transducer. If anyone has any insight as to how this thing reads a low water level it'd be very much appreciated. Thank you!


Jack A Lopez
Jack A Lopez

1 year ago

I see the picture of your circuit board now. This one:


I am not sure why I did not see it yesterday. My browser might be doing something goofy, and only loading pictures when it feels like it. Sorry about that.

I also think I now understand what you are saying about the big mystery surrounding how this thing senses low water level, since you have disassembled the thing so completely that you should have discovered wires going to, coming from, such a sensor, if it there was a sensor there to be found.

My best guess for how this thing detects low water, is it does this by measuring the impedance of the transducer, and it does that by sensing current flowing through it, by way of measuring the voltage across that big wide resistor labeled R9. I have to wonder about that resistor: Does "R330" mean 33 ohm, or is it smaller than that? I would kind of expect a smaller value, like 3.3 ohm, or 0.33 ohm, for a current sensing resistor.

By the way, the signal driving the gate of that big MOSFET that drives the transducer, comes from a little voltage divider, made of R7 and R8, and the signal driving that appears to be coming from that little IC named U3, and I think that is the IC you were refering to as the, "brains of the operation".

Also the voltage across R9, which I am guessing is a current sensing resistor, is measured through resistor R10 ("102" = 1000 ohm), and the wire on the other side of R10 trails back underneath U3, somewhere, and it runs right next to the wire I mentioned previously that carries the signal driving the MOSFET.

Jack A Lopez
Jack A Lopez

1 year ago

Are there supposed to be pictures attached to this question?

I mean, I guess what I am saying is that it would not hurt to attach some pictures, of the circuit boards you describe, like, you know, so the rest of the class can see them.

Also it would not hurt to mention the model and manufacturer of your humidifier. That way, in the absence of pictures, the interested reader could use an image search, to see if pictures of this humidifier, and maybe its internal parts, already exist on teh interwebs.

By the way, I have only ever seen two kinds of sensor, for sensing water level in an ultrasonic humidifier.

One kind looks like a bare wire, that just touches the water. I am guessing this wire stub is part of some circuit that sends a small current through the water.

The other kind I have seen looks like a glossy black sphere, kind of resembling an eyeball on a stalk, like the eyeballs that snails have. I am guessing the mechanism for this water level sensor is some kind of internal optics, that can sense the difference between water and air by the difference in index of refraction. Like maybe there is a beam of light that gets reflected back, but only when the sensor is covered by water. Actually that's just a wild guess. I do not actually know this kind of sensor works exactly, but I know that it does work, because I have seen it work.

Upon request I can send you pictures of either of these sensors: the stubby wire, or the black eyeball, because I am pretty sure I have examples of both kinds, in my collection of humidifiers.