Hello, this seems like such a trivial question, but I can't find the answer. I have an LED panel that works on a 3V battery switch. Needs more current than the Arduino gives. I would like to control the voltage from 0V-3V via the Arduino. What do I use for that? Not a relay, want to be able to control brightness via voltage control.
Since LEDs are current operated devices, you need to control the current to change the brightness. You do that by changing the series resistance that controls the current.
Use PWM. The quote between "pulse on" and the total period makes the illumination vary.
Also note reply #2.
Controlling the voltage is not a good or easy way to control the brightness with LEDs. Is there a reason you want to do it that way? As mentioned, using PWM is the usual way. For this you need a transistor. What model of transistor depends on the current that your panel draws. Measure that and we can recommend a transistor.
How much current do you need?
Are there any other electronic parts on the panel other than LEDs?
Which Arduino, some can run on 3V.
How many LEDs? Does the switch turn all LEDs ON or OFF at the same time?
Hi, @tkuudog
Welcome to the forum.
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
How many LEDs?
Thanks.. Tom.... 
What is the recipe for Lobster Thermidor?
Heh. Can't upload attachments. ¯_(ツ)_/¯. Anyway, the LED panel is Parallel EL Panel 14" Circle - Large , Ellumiglow
Want to ramp the "on" from 0-12V in a controlled fashion. Thought about a relay capacitor, but surely there is a chip that does this.
Excuse me but an EL, elecrtoluminicent, panel is NOT LEDs. An EL light contains zero LEDs. They operate on an internally generated high AC voltage.
Welcome! To the Arduino Forum.
There are several ways to accomplish what you want. Since you haven't provided enough details, I am assuming you are using an Arduino UNO operating at 5V.
The first circuit uses a 2N3055 NPN transistor, but many general-purpose NPN transistors would work. Just make sure the transistor has sufficient current gain, collector current capacity, and power dissipation capability for your load. You could also use an NPN Darlington transistor, either as a single device or constructed from two discrete transistors.
Keep in mind that an emitter-follower circuit loses approximately 0.7V across the base-emitter junction of a silicon transistor. A Darlington configuration loses approximately 1.2V to 1.4V. The actual voltage drop depends on load current and temperature.
Also, be aware that if the pass transistor develops a collector-to-emitter short, the full supply voltage, 5V in this example, will appear at the output. If that could damage your load, additional protection should be considered.
The second circuit is a simple digital-to-analog converter (DAC) arrangement using an Arduino PWM output and a low-pass filter formed by C1. The capacitor smooths the PWM pulses to produce an approximate analog voltage.
The filtered Vout signal can be used to drive your output transistor. Depending on the required load current, you may need a Darlington transistor, either discrete or integrated, to provide sufficient current gain.
Remember that this is not a true DAC, but a filtered PWM output. Its accuracy, ripple, and response time depend on the PWM frequency, filter components, and load. A buffer or driver stage may be necessary to prevent the transistor's base current from loading the filter.
Neither circuit provides closed-loop voltage regulation. The output voltage will change somewhat with load current and transistor temperature. If accurate output voltage is required, voltage feedback should be added.
Hi, @tkuudog
Welcome to the forum.
Can you please post the part number and links to data of your display?
Thanks.. Tom....
That will NOT work. Forget about it
Buy the dimming inverter.