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I am wiring some automotive LED lights (underglow), and currently have them wired from the 12v ACC power (on when ignition is on), with a 5-pin illuminated on/off rocker switch on the dash. It works fine as is, such that I can turn them on and off with the switch when the car is on, but they will always go off when the car is turned off, even if the switch is left on.

What I would like to accomplish is to have them also come on when the doors are unlocked, like entry/courtesy/dome lights, and go off when the doors are locked, independent of the rocker switch.

In other words, with the rocker switch in the OFF position, I would like the underglow to come on when I unlock the doors, and go off when the doors lock. With the rocker switch in the ON position, I would like the lights to come on when I unlock the doors, but stay on when the ignition is on and the doors are locked (while driving).

Basically, I want the underglow to act like entry lighting all the time, but also be able to turn them on or off with the switch while I'm driving.

I was planning on tapping into the interior lighting circuit for the entry lighting effect, which is on when the doors unlock and off when they lock, and as I said I have already tapped into the ACC power for rocker switch control when the ignition is on. I'm pretty sure I will need to use diodes or relays (or something else?) to regulate current for the situation when the rocker switch is on, the ignition is on, and the doors are unlocked (i.e. when both circuits are energized and trying to simultaneously power the LEDs).

I'm certainly not an electrical engineer, but I'm quite familiar with 12v automotive wiring and basic circuits, so I should be able to follow instructions and understand basic wiring diagrams.

Apologies for the crude diagrams, I'm not well-versed in circuit drawing... this is basically what I have now:

Diagram 1

Assuming both circuits are in fact 12v DC and not PWM, could it be as simple as this: Diagram 2

I cannot seem to find much data on the LED product, but it is the Mictuning N8 underglow kit, model MIC08N8-RGBW.

EDIT: I have searched everywhere for a constant 12v DC source that is on with doors unlocked and off with doors locked, to no avail. The dome light behavior is exactly what I would like to mimic, however this signal is in fact PWM.

When I wired the LEDs to the dome light ground wire, they come on for a half second then go off. What I'm thinking I need to do is use the PWM signal to trigger a relay for the lights, but have very little idea how to do that. I know a mechanical relay will not work with a direct PWM signal, so I will need a low pass filter to convert it to a DC voltage that will be lower than 12v, but hopefully enough to trigger a relay coil that can then turn on a 12v DC signal from the battery.

Edit: I have managed to measure the PWM signal from the dome light and it has a frequency of 120Hz and a duty cycle of 60% at 12v peak.

Any help figuring out how to use this circuit to power my underglow LEDs would be much appreciated!

EDIT (Again...):

Proposed schematic for what I think might work and what I can build:

Schematic

EDIT (11/30): After finding wiring diagrams of the dome light from my vehicle, I'm becoming convinced that the PWM signal is generated on the ground side through the multiplex control unit, as someone else previously suggested.

I'm not sure tapping into the wiring anywhere along the path will allow me to use that PWM signal, as the low-side ground wire from the light itself before the MCU carries a constant always-on 12V signal when I test it with any other ground source (bypassing the MCU), and isolating the ground connection in that circuit after the MCU is not going to be easy or even feasible I think.

I'm making progress on possibly finding an alternate 12V source that is active with door unlock and inactive with doors locked. I'll update if i find a solution. As always, any other ideas are welcome!

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    \$\begingroup\$ More and more cars use PWM (Pulse Width Modulation) to control more and more lights. Are you sure that both of your 12 V DC sources really are DC?? If so, then diodes might in fact be all you need. What resources do you have to "look at" the DC sources? \$\endgroup\$
    – AnalogKid
    Commented Nov 17, 2023 at 2:21
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    \$\begingroup\$ feed it from the dome light (or whatever) through a diode. They might be slightly dimmer but you could paint that as a feature. That's safe, and it's even safe without the diode, but if you omit the diode, your dome lights would come on when you lit the underside with the rocker; the diode prevents such "backfeeding". \$\endgroup\$
    – dandavis
    Commented Nov 17, 2023 at 4:11
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    \$\begingroup\$ A relay powered by dome lights might be nice to provide one feature: switching the existing setup's power source between raw batt+ and ACC. This lets the rocker switch work all the time, so you could turn off the under-lights even with the door open if you needed to sneak around. You don't need a diode in that case, just one relay. \$\endgroup\$
    – dandavis
    Commented Nov 17, 2023 at 4:17
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    \$\begingroup\$ In general, automotive and LED = low side switching. You need to measure yours to be sure. \$\endgroup\$
    – winny
    Commented Nov 17, 2023 at 14:28
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    \$\begingroup\$ So the dome light behavior I want to mimic is a PWM signal after all. Have not been able to figure out the frequency or duty cycle though. Is it possible to convert this signal to a constant DC signal that can power a relay? See edited OP above. \$\endgroup\$
    – GoleeMD
    Commented Nov 20, 2023 at 2:03

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EDIT: Here is a much better solution I worked out with help from a guy on an automotive forum.

Basically we switched everything to the low side, wired the LED controller positive directly to the battery and the controller ground to the COM terminal of the relay. Then wired the NO side of the relay to the dash switch and the other side of the switch to battery ground. The NC side of the relay is then wired to connect to the ground wire of the dome light circuit. The relay is then triggered with the ACC circuit to switch ground from the dome light circuit while car is off to the dash ground switch when ACC is on.

New wiring schematic

New video showing updated wiring and effect.

Original answer for context: Thanks for everyone's input. I decided to post my own answer since the solution was a combination of multiple different answers.

Indeed, separating the two circuits with diodes was part of the solution, but finding an adequate signal wire that would either power the underglow directly or trigger a relay for the underglow was the tricky part.

As the dome light behavior is what I wanted to mimic, I tried tapping into that circuit only to find it is a constantly on 12V signal whether the light is on or off, being dimmed and then turned off on the ground side by the multiplex control unit (MCU), producing a temporary PWM signal until off. Tapping into the ground wire between the dome light and the MCU and then grounding that tap elsewhere bypasses the MCU ground and keeps both my tapped circuit and the dome light always on whenever the car is on.

I was able to find a connector in the A-pillar that has a wire that is off when the car is locked, produces a 50mV signal whenever the dome light is triggered by the car unlocking or door opening and ACC still off, and 12V when car is on. I then found a 12V relay with a high and low trigger setting that when set to low, would trigger the relay only when the 50mV signal was present and would go off when either 12V or 0V were supplied, and used that relay to power the underglow from a constant 12V battery source.

Here is a video I made that shows what I did.

The only minor issue I would like to fix now is the quick temporary buzzing of the relay as it turns on and off, as it cycles through that PWM signal, but I'll ask that in a separate question. Thanks again everyone!

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    \$\begingroup\$ +1. Thanks for reporting back, and updating your answer, including schematic. I would +5 if I could. \$\endgroup\$ Commented Dec 10, 2023 at 2:26

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