Hi 
The short version is that I’m trying to work out how to dim a new 12v LED lighting circuit in proportion to an existing low voltage circuit.
The longer version is that my car has dimmable LED dashboard lights. I plan to add some additional gauges and switches that all contain ‘12v’ LED illumination, but I want to dim them using the existing dash light circuit as the control.
Is there a component or a known circuit that will accomplish this? What am I looking for?!
Hey 
Welcome to the forums
You might be able to drive them directly from the dash circuit. Alternatively, you’ll need something (probably a microcontroller) that can read whatever the Dash LED’s are doing, then use that measured value to control your second circuit.
Exactly what you’ll need to do depends on exactly what control signals you have available. I’d say there’s a good to fair chance that if you can get to your dimmer knob that it’s just a potentiometer and you might be able to read that control signal.
What kind of car is it? Do you have a wiring schematic?
find the wire that controls the output to the lighting…im assuming that its +tve so tapp into it…
drain of a feed wire put an 1n4004 in the line and feed it to the base of a power transistor say something like an 2n3055...and feed 12 volts to the collector.
the positive output vie the emitter to your other lights connected in parallel to -tve…
also depends on the circuit weather the negative or positive is the actual original feed wire control..I.E is it controlling -tve or +tve...
its a basic concept but simple is often best …
Hi guys, thanks for the replies and sorry for the delay getting back.
The car is a Holden Rodeo, nothing as simple as a pot unfortunately it has a pushbutton on the instrument cluster that cycles through 6 levels of brightness. There is (inconveniently) a separate pot for the other lights in the cockpit which are currently all filament globes, but these don’t respond to the headlight status the way the instrument lights do.
I’m replacing all the globes with LEDs and also adding a few meters and accessory switches that are all LED lit too. I want to delete the old pot and have everything respond to the instrument lights. It will be far too much of a load to just wire it all directly into the instrument cluster, so I need to control a separate circuit.
I have established that the instrument LEDs are connected across the battery voltage (with a 1k resistor) and dimmed on the ground side by a ULN2003A chip, so I have essentially a 12v PWM signal.
I will look into the suggestions posted above and let you know how I go. Thanks again!
well each of the 8 drivers are capable of 500mA 50v d.c so all you need to do is use my circuit as described above and feed each of the the outputs independently with one extra transistor …i would redesign my above with a mosfet though…shore one of the guys here could draw u a cct…easy to increase current drive as to give more load capability… per pin …there darlington outputs on the 20003 chip though this word u may not know or even understand…it matters not basically…its 2 transistors in series parallell type setup to boost output gain…
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