You're right Lightbringer ,
it is similar with 4-20mA industrial process loops controllers in analog systems , and along with the 0-10v controllers they are the standard process controls .The difference is that with 4-20mA is far more complicated to control PWM than with 0-10V , where by , just injecting on the inverting input of an comparator the 0-10V signal ( and having on the non inverting input the tooth saw signal from the oscillator ) , you can vary the PWM , at the output...
The idea with the 9V battery is good , and could be the second option if the one with the simple potentiometer does not work ! In fact ,is brilliant !
@ VofB :about the switch from your link...
As you know there are on the market led bulbs that are dimmable and some non dimmable.., I didn't have the chance , yet (!) , to open a dimmable one and see what is the solution for doing this , is a challenge I'm willing to accept as soon as I'll have one of those in my hand...
In the description of the product , the manufacturer are stating that is suitable for led and incandescent bulb !!!
If , for incandescent is easy to do it ( a triac , a diac and few more components...) , for led drivers are much more complicated , because of the following :
For incandescent loads the form of the wave is not critical , they will accept a square wave form with no problem and the armonics , created by the triac's switching ,will not affect the load ( bulb) .., except maybe , a small flickering at the lowest level...
For led bulbs with standard drives ( drivers acting like a switching power supply ) the wave form is mandatory sinusoidal , due to the diode bridge rectifier at their input.., so , using that kind of dimmer is not acceptable !
Also , I have to highlight the fact that , in the last years, the manufacturers managed to make dc-ac invertors with pure sinusoidal wave form.., so is possible , with proper miniaturization to obtain dimmers suitable for led dimming..!!!
I've tried to get a schematic diagram of those but , they protect the intellectual property ( for a good reason , I guess...). What I can assume is that at the price of ~ 14 $ , these dimmers suppose to be fitted for led dimming , as your panels appear to be. I do think that the 14$ dimmers will work ( if the first solution with the potentiometer , that is also the cheapest , would fail...)
Good luck , and tell us the story...