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Can the individual dimmer (rotary) controls of 2 identical prefab PWM LED light dimmer units be removed and replaced with a single dual gang potentiometer to crossfade between them for the blending of 2 different color led circuits?

The PWM unit I am referencing is this one.

I cannot confirm the nature or values of the rotary control mechanism but as it comes in a pack of 3, they would likely all be identical, and they are almost certain to be some form of potentiometer. The manufacturer specifies one color of LED light per PWM module.

I hope to learn if there is any valid reason not to do this in the manner proposed. If after addressing the main question, you have a very simple suggestion that is more efficient, cost effective, space saving, and easier to implement for a person (me) who is mostly inexperienced with circuit building then please feel free to abstract away.

I would really like to find a dual gang potentiometer which has a log / anti-log profile but after a bit of searching, they apparently no longer exist. While I am certain many intelligent people would accomplish this same (log / anti log) feat using technologies in which I am not well versed, just for the sake of obtainability and realistic execution let's assume the only pots available to me commercially will be linear dual gang.

To be clear, my interest is in if the desired results can be achieved with / is it safe to accomplish this with a single dual gang potentiometer, 2 prefab PWM dimmer units, and 2 different colored LED lighting circuits which fall within the manufacturer's supported tolerances.

Thanks for any help, expertise, or patience exerted on the part of the reader.

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  • \$\begingroup\$ personally I'd be looking to modify the second dimmer to operate as the inverse of the first, can you add pictures of the inside of these devices? \$\endgroup\$
    – Jasen Слава Україні
    Commented Dec 7, 2022 at 19:06
  • \$\begingroup\$ video of internals here: youtube.com/watch?v=JGSzoa18_gY it uses a 555 to make a sawtooth and then (half) an lm358 to make PWM from that. \$\endgroup\$
    – Jasen Слава Україні
    Commented Dec 7, 2022 at 19:22
  • \$\begingroup\$ Waiting to purchase them until I have a better handle on this, Jasen, so no pics. Could you elaborate a bit? my goal is to use one knob to shift between the 2 with the middle (preferably) being both are off; however, I'm settling for a blend of the lights in the middle (might be a cool effect) and I'll switch it off at the source. With that in mind, what would be the reason to invert the second? unless you're implying, I should wire them both to the same single gang pot & send the inverse value to one side and regular to the other? but in my mind, my proposed method sounds easier, doesn't it? \$\endgroup\$
    – shigginpit
    Commented Dec 7, 2022 at 19:30

1 Answer 1

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absolutely you can use a dual potentiometer for this, pick one with a linear (aka B) taper and the same resistance.

but you can probably modify them by interconnecting them so that the second unit acts as the opposite of the first.

Here blue lines are wires (you'll need to scrape away some of the green to solder to the copper under it) and the red lines are cuts you'll need to make in the copper of the second board.

also connect the power and ground on both units. enter image description here

However seeing as you want dark on the middle setting the dual potentiometer is probably the easiest way.

schematic

simulate this circuit – Schematic created using CircuitLab

Here out is out is the centre potentiometer terminal, 0V is the negative supply and +5 is the output of the L7805 regulator chip. there will be two resistor associated with the existing potentiometer, one of them will be from 0V to the potentiometer and the other from +5 to the potentiometer, the 0V one will can be bypassed (or replaced with a piece of wire) the +5 one can be swapped for a 27K (or sommething approximately half the resistance of the potentiometer - if your replacement potentiometer has twice the resistance of the original this part will probably not need to be changed)

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  • \$\begingroup\$ Thanks, do you know of an easy way to determine an alternative value for the pot which will give some fade on the ends but at center both would be effectively off? I'm wondering how far is too far to stray from the original pot values, I don't want it to be on/off or I'd just use a switch and save $10, but a dual linear with a center detent of higher value seems like it might work. Any opinion? \$\endgroup\$
    – shigginpit
    Commented Dec 7, 2022 at 19:44
  • \$\begingroup\$ for dimming the active range is 1.66 to 3.33 volts produced by the potentiometer, so different end resistors would be needed so that with the dual potentiometer centered both outputs were outside that range. \$\endgroup\$
    – Jasen Слава Україні
    Commented Dec 7, 2022 at 20:28
  • \$\begingroup\$ Thanks again for all the information. Just so that I'm clear, what you've detailed is interconnection of both units and inversed the function of one so that both sides of a single pot act as a balance control, is that correct? so does the pot need to be removed from (let's call them unit A and unit B) unit B, and leave control to unit A? Also, I'm wondering if there is a way to add a switched resistor across the the pot terminals, switched meaning that when it's circuit is open the center acts as "off" for both sides, and when closed, the pot blends at center. Would that get too hot? \$\endgroup\$
    – shigginpit
    Commented Dec 7, 2022 at 20:47
  • \$\begingroup\$ cutting the traces shown is all that's needed. disconnecting the potentiometer would have the same effect as cutting the large track. you still need to cut the small track \$\endgroup\$
    – Jasen Слава Україні
    Commented Dec 7, 2022 at 22:46
  • \$\begingroup\$ this is super interesting; it makes me want to try it just to find out how it works. thank you, Jasen, do you think it's a bad idea to attempt to add additional resistance on a switched basis to try and get both behaviors (center dark / center mix) out of the circuit? my main concern would be something melts or goes poof. I should probably pick one for simplicity. if I went with the center mix only, and switched the circuit on/off via switch from source, do you know if I could add a large capacitor between source and the dimmer to get the lights to turn on smoothly, like a swelling up effect? \$\endgroup\$
    – shigginpit
    Commented Dec 7, 2022 at 23:29

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