Come back whenever you find the power/current specifications for the panels. The Max chip is an oscillator; like the 555 is. But it is about driving power, not frequency. your 3 panels in parallel for 6V may do it for the item on post #14 if they are 0.7W each.
I do not think you will get a better deal without knowing what panels you have.

These little panels I believe are rated at only .3W each. The more research I'm doing the more I realize to drive a piezo with purely solar would be a complex task seeing the power requirements. I am learning a lot in the process though. So thanks!

 

Hi Trimick, you really need to drive the piezos with a high voltage sine wave, this nearly always done using a self resonant circuit so it always finds the resonant most efficient frequency of the piezo. Using something like a 555 is liable to over heat something. There are lots of examples on the net for resonant driving of a piezo, but unless you want to do something more complex with it your best just buying a driver off the net, and then incorporating it into your project.

I've just viewed a few of the circuits you mention and just building one would require me purchasing multiple components. But, I see why a self resonant circuit makes sense opposed to building a circuit to one frequency when too many other variables would effect the piezo and the frequency I "dialed" in.
It appears, from what I read square waves are also used in these applications. This is more of a learning process

 

Here is wimpier ones that may demand less power (2W):

----> https://www.ebay.com/i/332377870429?chn=ps

I just now got back to messing about with this project (life happens), and I ordered pretty much what you suggested- except I got it off Aliexpress. Two of them for dirt cheap:
https://www.aliexpress.com/item/20m...Accessories-PCB-Module-D20mm/32858785372.html

I have to say I'm very impressed with these little things. I'd recommend to anyone who needs to make some mist on the cheap. They'll run on 3.7V to 12V. Supposedly they need at least 1.5W optimal, but I can tell you I'm no where near that. I doubt I'm hitting a single watt. I just took a mini solar panel (that claimed 2watts-never came close to 1) and now my tree is going to be happy in direct sunlight with some humidity.

 

I ordered a couple of those same boards off AliExpress and they work pretty well, so thanks for posting this. My only complaint is that it has a tiny barrel jack to connect power and a tactile push button switch, neither of which make much sense for a development board. But it's not hard to get the power with soldering and the switch by adding in a relay.

I did however just find someone who has reverse engineered that very board and posted his schematic here: https://www.instructables.com/id/Make-Your-Own-Super-Simple-Ultrasonic-Mist-Maker/

I haven't tried this build yet myself but plan to do so soon.

 

I ordered a couple of those same boards off AliExpress and they work pretty well, so thanks for posting this. My only complaint is that it has a tiny barrel jack to connect power and a tactile push button switch, neither of which make much sense for a development board. But it's not hard to get the power with soldering and the switch by adding in a relay.

I did however just find someone who has reverse engineered that very board and posted his schematic here: https://www.instructables.com/id/Make-Your-Own-Super-Simple-Ultrasonic-Mist-Maker/

I haven't tried this build yet myself but plan to do so soon.

Hey, that's fantastic. Thanks for the link! He does some good builds and experiments. Look forward to checking it out.
Like you said, power is an easy work around. In my case I wasn't using a dev board though. No use for the jack or push button. The solar panels (ending up putting more than one in parallel for a bit more current) are hard wired directly into the board. Keep in mind the disc needs to rest on the surface of the water and not be too submerged. I used an old, thin cork coaster -I cut a hole from the center- mounted the disc inside. Now it floats perfectly on top and as the water level decreases the whole thing rests on the surface and just follows the water level down nicely. A few small pieces of putty on the edges balance everything. Very nice little plant humidifier!

 

Hey, that's fantastic. Thanks for the link! He does some good builds and experiments. Look forward to checking it out.
Like you said, power is an easy work around. In my case I wasn't using a dev board though. No use for the jack or push button. The solar panels (ending up putting more than one in parallel for a bit more current) are hard wired directly into the board. Keep in mind the disc needs to rest on the surface of the water and not be too submerged. I used an old, thin cork coaster -I cut a hole from the center- mounted the disc inside. Now it floats perfectly on top and as the water level decreases the whole thing rests on the surface and just follows the water level down nicely. A few small pieces of putty on the edges balance everything. Very nice little plant humidifier!

Love the cork board idea! Thanks for sharing that.