led’s to courtsey light, to +?

Ok, it has been several years since my install days so I'm feeling a bit out of the loop. I did look across the forum but couldn't find the exact answer I needed.

Just got a 2010 CX-7 and I want to connect some LED scuff plates, kick lighting, and under door LED strips but I can't figure out how with my dome trigger. When the lights are on it is a 0.00 "meeter" and when they fade out it goes to +12+. I thought about a relay but I want the led's to fade the same way my current lights do. I know there must be a simple answer that is slapping me in the face but I can't seem to get it! Old age... :)

Thanks in advance!
Shone

Connect the LEDs from +12V to the dome trigger lead (that goes 0V to 12V). [ie: LED anode to +12V, (K)Cathode to trigger]
That should work...

Ok, I think you lost me... If I connect the + lead from the led's to a 12v source and the - lead from the led's to the dome trigger the led's will be on when the dome is off and go off when the dome comes on.

If I do it the other way around I'll end up with the same result.

Why would the LEDs be on when the dome light is off?
Have you tried it?

Isn't the dome trigger grounded, and gradually goes high (to +12V)?

The dome trigger ramps up to +12v as the dome dims out. So when the light is on it is at +.08 and goes to +12 as it goes out. So it basically has a constant + feed and the trigger gradually changes the grounded side to a + to fade out the light.

I was hoping to get a definitive answer before I start testing. The BCM is to expensive to replace if I back feed into it...

If you connect the LEDs positive wire to a constant 12 volt source, and connect the LEDs negative lead to  the wire in the car that fades up to 12 volts when the door is closed, they will work just as you want them to.  !2 volts constant and then 12 volts on the other wire = 0 difference in voltage.  12 volts on one leg, 12 volts on the other leg = 0 volts across the device.  0 volts = off.  When the door is open, there is ground on that wire.  That will turn the LEDs on.  The LEDs will be seeing the same fading voltage as your domelight, but you should not expect as smooth of a fade out.  When the domelight begins to fade, it may have to go about half way to off before you even begin to notice a difference on the LEDs.

Thanks Ima,

Rowley's definitive answer is that the LEDs start off with 12V across them. This gradually reduces to 0V.

In this case it's just that the negative/cathode side of the LEDs ramps up to +12V. Most are used to thinking of the +12V ramping down or disappearing (to become 0V)

Great, thank you guys... Now...

How will that work with a voltage regulator in-line? I want to use a 12v Radio Shack $2.49 special in line to prevent premature burn-out. Should I wire it the same or will the +12 cause an issue coming in on 2 lines. The regulator has +feed, ground, line-out. How will that work with 2 +'s as the feeds?

A regulator?
To prevent premature burnout?
Of a 12V string of LEDs?

Maybe you can explain WHY to me?


Have you tried connecting the LEDs as we have suggested?

If it does not work as we have said, we may be able to figure out why.


If you want to know why it will work, it's because the LEDs (strings) will have 12V across them.
12 Volts is 12 Volts - ie doesn't matter of it's between -1,234V & -1,246V or 36V & 48V - it is still a potential difference of 12 Volts.


Do NOT use a voltage regulator. Resistors are all that is required to protect LEDs (in general).
Many don't even bother with resistors when in series for 12V applications (typically about 4-8 series LEDs depending on type).

PS - you might use the VoltReg in current-limit mode, but a normal transistor current limiter would be more robust.
Besides, think of the poor regulator trying to keep up output voltage as its input drops! (It might be a way to turn off instead of dimming the LEDs, but it's still cruel.)
And you only have one +ve supply - not two.

Just an FYI because I don't believe I've seen this mentioned yet, LED's don't "dim" in the same manner as a bulb does. With a normal filament type bulb, you can simply reduce the voltage to reduce brightness. This does not work with LED's

While there will be a slight difference in the brightness of an LED if you reduce the supplied voltage, LED's are more of an ON or OFF type device. To properly simulate an LED dimming, you have to pulse it on and off rapidly. The faster the pulses, the brighter it appears, the slower the pulses the dimmer it appears. This is going to make your circuit a LOT more complicated if you really want this feature. You'll have to use a 555 timer or some sort of microcontroller circuit to pulse the LED.

Anyway, hope this helps, you might get lucky and get the LED's to dim enough for your liking, but if it doesn't work quite like you thought it would, then that's why!

I hope after all this that that was understood from the start.... I presumed it was from the original writings - it sounded like a basic test had been conducted....

But you are quite right....
In fact someone has just written about it at www.mp3car.com/vbulletin/lcd-display/137810-changing-cold-cathode-backlighting-leds-auto-brightness-control-4.html#post1365910

oldspark wrote:

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I hope after all this that that was understood from the start.... I presumed it was from the original writings - it sounded like a basic test had been conducted....

But you are quite right....
In fact someone has just written about it at www.mp3car.com/vbulletin/lcd-display/137810-changing-cold-cathode-backlighting-leds-auto-brightness-control-4.html#post1365910

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I thought he was just looking for a solution to convert the voltage output of 0v (increased to +12v) when the door was open, and instead have it as 12v (fade to 0v) when the door was open. Not a method of pulsing the LED's so that they appear to fade... But maybe I read something wrong.

Yes - mtrhd0024 was just pointing out that LEDs do not dim like lights.

As I understand it, Rowley was merely looking at a power source to drive the LEDs which he has - ie, the 12V when the courtesy lights are on, decaying to 0V as they extinguish.

I took his LEDs to "fade the same way my current lights" meaning off the same supply etc - but not matching luminosities (since the LEDs will extinguish before bulbs).

mtrhd0024 just clarified that last point.

Is saw the issue as merely being one of confusion of LED ground voltage rising from 0V to 12V as opposed to the more conventional(?) LED supply dropping from 12V to 0V. (Both are the same as far as the LEDs are concerned.)


If Rowley wants matched decay - that's easy. The 555 PWM circuit (in my earlier link) controlled by an courtesy-light mounted LDR (Light Dependedent Resistor). No switching necessary!
It needs appropriate calibration - eg courtesy lights almost off may equate be a 20% LED duty cycle (though I think will be similarly close to 0%).
Since the LDR value is proportional to courtesy lamp intensity, and LED intensity proportional to duty cycle, they should drop with similar intensity (electrically speaking!).
(Alternatively maybe a LED could used as an LDR for a better match.... It's been years since I used a LED as a LDR....)