Why not put pots on all of them? Adjust accordingly, then the differences when operating the "master" potentiometer will likely be minimal.

Good idea. But I have a bunch of resistors laying around and they are cheaper. If I can figure out what values would work, I would rather use them. This project has become a Pandora’s box already.

 

Nevermind, that was a terrible suggestion...it could take all day to get them all "in synch".

[EDIT]
Verified in simulator. Not only is the adjustment finicky, but tuning the main pot demonstrates that LEDs with different voltage drops will not necessarily dim/brighten "in concert".
[/EDIT]

 

Thank you for your help, everyone. I will try these ideas out now. I appreciate it.

 

I will try these ideas out now.

Except for that last one anyway. =p

 

The Circuit I provided can be built for around ~$20-bucks,
it simply replaces the Pot in your Schematic,
and controls all LEDs simultaneously,
and in a perfectly balanced manner,
by switching the Ground of the LEDs on and off about ~1000 times per second.

No other parts are required except for a small Perf-Board, and a small plastic-Box to put it in.

It can control a virtually unlimited number of LEDs at the same time, with one Pot.

The Pot can be remote mounted anywhere You can fit it.

A nice looking Knob for the Pot is the most expensive individual part, running around ~$6.oo or so.
.
.
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To keep the project simple, and inexpensive, and not taking up a lot of space, using a resistor for each LED will be the way to go.
You will need a resistance meter (ohm meter) to measure the resistance of your pot after you adjust the brightness of each LED to what you want. The test circuit will be like what you have shown us, but with only one LED connected at a time. After setting the desired brightness, disconnect and measure the resistance that was in series with the LED. You will discover that it is different for each color.
Applying the full 12 volts to an LED that does not have an internal current limiting scheme will destroy it in just a very few seconds.

 

Plus, from your description, you are using an audio pot. This resistance will change faster at one end of its travel than the other. You may need a linear tapered pot. Brightness still will not be linear, but more so than with an audio taper.

With a pot, you are varying the current the LEDs see. LEDs are current controlled devices; not voltage controlled. Different colored LEDs require different currents to operate.

Resistors are a simple but adequate way of controlling current. Since different LEDs require different current to operate, you’ll need individual resistors to set the current for each LED. Then your pot can reduce the overall current so the LEDs dim.

Here is a “typical” graph showing how current affects brightness. Usually, one would check the current (and forward voltage; you’ll need this value to calculate your individual resistances) plus a specific brightness graph.

Given the supply voltage (Vs), forward voltage (Vf) and maximum current (I, always use less than the maximum; say 80%), calculate the approximate resistance (R) required this way:

​

R = (Vs - Vf) / I​

Since resistors come in standard values, use the closest value to your calculation.

Then again, some people just use a 1k resistor, but your LEDs brightness will not be the same across different colors.

EDIT: MisterBill2 has provided a technique that requires less math.

 

Once the values were determined that was going to be my next suggestion.

 

The ground symbols were a shortcut. They all actually go to various circuits in the car. e.g. Accessory 1, accessory 2, ignition, radio/data interface, and headlights/night light circuit.

That doesn't make sense because cars are negative ground these days.

 

What is the easiest way to make a schematic for here? The pot is wired center and tapered side of an audio taper. I had the pot handy so that’s why I used it. The LEDs are from eBay: “Chanzon 8MM indicator light kit BA0023x10 8MM mounting hole”. That’s all the package says. There is no indication that the LEDs are automotive, but it does say 3-12V. I don’t see any resistors. Could they be too small to see? I didn’t think this would become complicated.

Is this what you got:
https://www.amazon.com/Chanzon-Diffused-Frosted-Universal-Indicator/dp/B08G4X5WBT?th=1

If so, you can see the resistor under the heat shrink up near the LED. The description says that it's 560 Ω. Assuming ~2 V drop for the LED, then at 3 V you would expect ~2 mA and at 12 V you would expect ~20 mA.

Since each one has it's own current-limiting resistor, you should be able to put a single resistor in series with them and get it to work reasonably well, but the perceived brightness of an LED by the human eye is complicated.

How many LEDs are you using? Let's call that N.

To get down to ~2 mA with a 12 V source, you would need something in the 5 kΩ range.

Another alternative is to use an adjustable voltage regulator, such as the LM317, to adjust the voltage across the whole thing.

You could also put a transistor on each one and use pot to control the base voltage.

 

To keep the project simple, and inexpensive, and not taking up a lot of space, using a resistor for each LED will be the way to go.
You will need a resistance meter (ohm meter) to measure the resistance of your pot after you adjust the brightness of each LED to what you want. The test circuit will be like what you have shown us, but with only one LED connected at a time. After setting the desired brightness, disconnect and measure the resistance that was in series with the LED. You will discover that it is different for each color.
Applying the full 12 volts to an LED that does not have an internal current limiting scheme will destroy it in just a very few seconds.

Great idea. Although I don’t see any limiter, I am applying 12 volts and they are holding up. (About an hour or two so far.) Thank you. Edit: I just found out they do have resistors.

 

That doesn't make sense because cars are negative ground these days.

With due respect, I don’t understand why it doesn’t make sense. I have a lead from the positive side of the battery to the switch, then to the various circuits, then to negative ground. The LEDs connect in parallel to the circuits. Please clarify what doesn’t make sense.

 

The Circuit I provided can be built for around ~$20-bucks,
it simply replaces the Pot in your Schematic,
and controls all LEDs simultaneously,
and in a perfectly balanced manner

Driving the LEDs in parallel with a PWM (Pulse Width Modulated) signal is probably the only way to get close to what you are looking for. It sounds very much like your LEDs have resistors in series with them so that the current is equal when 12V is applied. So for a red LED which maybe drops 2.0V compared to blue LED which drops 4.0V (not exactly but I’m keeping the maths simple) the series resistors would be 500 and 400 ohms respectively to have both LEDs seeing 20mA. Now if you drop the voltage to 6V the red LED will see 4.0V across the 500 ohm resistor, so 8mA but the blue LED will see 2.0V across the 400 ohm resistor, so 5mA.

With PWM the pot is adjusting the mark/space ratio of the voltage applied to the parallel LED bank. Either they are turned on (probably at 20mA) or off so the average current through all of them is the same.

Using 555 IC is a nice way to make an astable with controllable mark/space ratio and examples are widely shown and explained on the web

Rather than a pot you may get away with a single resistor switched in and out to adjust the mark/space between two discrete values.

 

I am using multiple LEDs (20mA 3-12V) in parallel to indicate active circuits in a car. They are too bright at night, so I put a 250K potentiometer to their collective ground. At less resistance, some LEDs are brighter than others. At greater resistance, the brightest LEDs become the dimmest. 1. How do I make them respond equally? 2. Will they eventually burn out when set at least resistance at the pot? 3. Why is this happening?

I would strongly suggest that you obtain a multimeter. (You can get them starting at about $10.00.). Along with this tool, familiarize yourself with Ohms Law. With these 2 tools you can find out quite easily how much current each LED requires to obtain your desired brightness. There is excellent coverage of this on this website.

I would be careful when measuring current as you can blow the fuse current limiter in the meter. A safer way would be to put a low ohm resistor (10 to 100 ohms) in series with the LED that you are evaluating. The value of this resistor is not critical as if you know the value (measured with the meter) and you know the voltage across it, you know how much current is flowing thru it. This current is the same as the current flowing thru the LED as the resistor and the LED are in series with each other

Connect your variable resistor pot , (measure with you multimeter to insure it is at its max value before applying 12V. adjust it until you obtain the desired LED brightness. ( I would suggest using a 1K (1000) to a 5K (5000) Ohm pot

If E (voltage) = I(current) divided by R(resistance) then I must equal E/R. For example if the resistor you pick is 110 Ohms and you measure 2 volts across it, current flowing thru it is 2 volts / 110 ohms or 18.2 mA. (0.0182 Amps)

12VDC----5k Pot -----110 ohm-----LED---0v