Cool Off With A Piezo And A Glass Of Water

Some cool-mist humidifiers work by flinging water at a vaporizer, but our favorite kind uses a piezoelectric transducer. These work by using high-frequency sound waves to pound the surface of the water with mechanical energy. That energy introduces standing waves that force the water to break apart into a fine mist on the surface of the piezo disk.

The driving circuit for this DIY mist maker uses a 555 to generate 113 KHz, a trimmer potentiometer to fine-tune it, and a MOSFET to amplify the signal. You don’t need much more than that and a handful of passives to recreate this cool junk box experiment, but the spec of the piezo disk is quite important. The circuit is designed for atomizing transducers, which have a resonant frequency of 113 KHz — much higher than your average junk box piezo. Check out the demo and build video after the break.

Atomizing transducers can do way more than than moisten the air for our comfort. They’re not picky about where the water comes from, so if you have enough of them, you can dry a load of laundry in a few minutes.

21 thoughts on “Cool Off With A Piezo And A Glass Of Water

    1. It was sticking my finger in the water that led me to this post! Thinking that there must be something in there worth harvesting for other projects since it caused me pain haha. At first it felt like a shock or a sting, and then my finger felt really funny like pins and needles for hours.

  1. very clever. I was going to pump water through a tiny hole. this looks to be more interesting. If only there were a way to get it to shoot water downwards, now that would be very cool.

    1. Before it cooks them, it hurts lots. Found out the hard way when I came across one in a shop many years ago. Concluded that if it can smash water about, it can smash bits of finger about.

  2. What is the purpose of the 220uH toroidal inductor? Maybe the inductor is there to increase the peak voltage across the piezo transducer? I don’t think so, the transducer is rated for 5-12V and this circuit is using 15V already. The Piezo’s capacitance is 1700pF which with the 220uH inductor and 100nF capacitor resonates at 262,449 KHz, not 113 KHz. But that’s using the specified static capacitance of the piezo transducer, the dynamic capacitance (and other parasitics) are not specified at self resonance, and there’s no datasheet on offer. You don’t need the inductor to make the 555 a 113 KHz astable multivibrator, a simple RC network will work, and that’s exactly how this circuit does it. Why not just dump the inductor and the 100nF series cap and drive the piezo directly with the MOSFET?

    I’ve seen almost the same or similar circuit many times on the web (see example link below), but I have never seen a full circuit description.

    If I were designing this circuit I would use a voltage-controlled oscillator (VCO) whose frequency is automatically adjusted in a feedback loop by the detected transducer’s output amplitude. The ubiquitous and inexpensive 74HC4046 phase-locked loop / voltage-controlled oscillator comes to mind. The 74HC4046’s high impedance VCO control voltage input makes it particularly attractive so as to not load the transducer.

    Here’s another almost identical ultrasonic mister circuit with an inductor, and no circuit description:

    1. I think your first idea is correct – the inductor is there to increase the peak voltage across the piezo. At the link you provide, there is a scope capture of the voltage across the piezo – it looks like about 60 volts, from a power supply of only 20 or so volts.

    2. From looking at the circuit, the inductor L1, does three things:
      Impedance matching for the piezo( forms a higher voltage sine wave), forms a tuned circuit with the capacitance of C3 and the piezo and helps to filter out noise that would get on the power and ground lines.
      With the tuned circuit and the driver transistor, I don’t think the 555 timer is even necessary.
      I could be made to oscillate on it’s own without the timer.

  3. This video/project looks misleading. In the clips and examples of a piezo disc he shows ones used in ultrasonic submerged transducers like used in ponds and fountains (and some comments here mentioning putting your finger in the water which will burn you).
    BUT the piezo element he’s using in the video and actual demonstration are commercial diffusers that are a piezo ring around a metal plate that has microscopic holes in it, working in a different principal. These will not burn you because they don’t vaporize the liquid and don’t work when submerged (you see him flicking off a droplet of water because it stalled the element). The one he is using vibrate the mesh disk which causes micro droplets of water to spray up, where a piezo disc focuses energy at a certain point a few cm away flash boiling it

    1. hi Erik, could you please explain the difference between the two types a bit more in detail? i have found a lot of explanation of the submerged type, but i am using the vibrating metal mesh version in a school project and I cannot really find sufficient explanation of its working principles. Thank you!

      1. Basically the submerged ones expose the piezo element to the water directly and creates a focused beam of 1.6MHz ultrasound a few cm above it (which is why those foggers have a specific working depth, and why they splash so much). If you put your finger in the water here it will burn you from the focussed energy. They also rely on submersion to stay cool or the piezo element will heat and crack – which is why they all have a little sensor bar near the element to disable it if it runs dry

        The ring/mesh ones use a piezo ring around a metal mesh, with water sitting just behind the mesh and air above the mesh. The mesh is vibrated really fast by the ring, still ultrasound but barely at a few tens of KHz (can be audible if no water is present). They work by basically slapping the water surface using inertia to force water through the holes acting like a tiny squirt gun, a pumping action not vaporizing action, and must not be submerged for this reason

  4. A friend bought a commercially produced mist pump which causes the water to rise up in a narrow jet then fly apart into a cool vapor at the top of the jet. The directions said to not stick a finger into the jet. He did, very briefly. Got a tiny little painful bruise.

  5. A little whiff of ammonia vapor will quickly drive a groundhog out of his lair. I used household ammonia with a battery powered piezo mister to get the critters out from under my shed. Only the mister went under the shed. The controller sits outside and activates the mister for about 15 seconds every few minutes. Low cost, fast acting and lots of fun – not for the groundhog.

  6. @odonatax said: “I think your first idea is correct – the inductor is there to increase the peak voltage across the piezo. At the link you provide, there is a scope capture of the voltage across the piezo – it looks like about 60 volts, from a power supply of only 20 or so volts.”

    Ah, I missed that. Thanks…

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.