connecting LED lights to a door trigger circuit which is normally closed

Thread Starter

samsepi0l

Joined Jul 29, 2017
15
I am looking to install "puddle lights" (LED strips) under the inner edge of my doors on my 2012 Mazda 3. The goal is to have the lights come on when you open the door and go off when you close the door.

On this vehicle the computer (body control module) monitors the door latch switch to know if the doors are open or closed.

When the doors are closed, the circuit is closed and it has a ground. When the doors are open, the circuit is open and I measure 12V at the latch on one side (obviously ground on the other all of the time).

I assume this goes through some high resistance in the computer as I am reading 12V because there is no current flow (no current flow = no voltage drop).

I am trying to find the most elegant, and simple way of using this input to trigger my LED lights. They will draw no more than 250mA when they are lit.


Here you can see what I am after. I've done this before with great success on circuits that are normally open.
PICT3538.jpg
 

Thread Starter

samsepi0l

Joined Jul 29, 2017
15
I might need a little help (sorry, I'm a mechanical engineer- not an electrical one!)

Here is what I found online searching "nfet"
RtzAu.png

I assume I would put battery voltage on the "D" (drain), Full time ground on the "S" (source) and the switching wire on the "G" (gate).

I would need to put my LED strip inline with the polarity correct between my DRAIN and SOURCE right?
This way when the transistor "switch closes" it will pull he 12V through my LED strip and to ground completing the circuit.

How does this sound?
 

IMP002017

Joined Jan 28, 2017
192
I am little lost? So you found a Wire that when you have door closed it is showing a Ground at BCM. Then when you open the door it shows 12v+ at that wire at the BCM? Is that right so far?

If that is right then why not use a Relay and wire your Puddle lights as such...

Relay
Pin 85 Ground to Body
Pin 86 12v+ input from wire at BCM Diode Isolate it from Relay
Pin 30 Output to Puddle LIghts
Pin 87 12v+ from battery or other High Current area add fuse

Puddle lights
Ground from Body
12v+ from Pin 30 on Relay
 

Thread Starter

samsepi0l

Joined Jul 29, 2017
15
Thank you for your reply, but I want to keep this 100% with no moving parts.

One other issue I have with this solution is that I want to minimize the current draw when the vehicle is off and the doors are closed. With your solution the relay coil itself will have to be energized in order to keep the puddle lights off. This vehicle has 4 doors so I will need to do this 4 times. With these coils being energized it will certainly add consistent draw and shorten my battery life of the car.

Even if I decided to go this route it would need to be a relay with a super-high coil resistance, typically its good to have less than 50mA draw from the battery when all of the modules on a car are "asleep" and it is parked and off for some time. Any more than that is considered a high enough draw to cause battery drain problems.
 

IMP002017

Joined Jan 28, 2017
192
Um no. the Coil don't see Volts when there is no power going to the relay from wire you said that shows 12v+ when door is open... 12v from battery is just like any other things at 87 there doing nothing. No power draw until you open door...

However I will let you work it out. I have ran many many things without power drain issues using relays. I wouldn't set you up with something that was causing a constant drain. I am not that mean...

Good Luck
 

Thread Starter

samsepi0l

Joined Jul 29, 2017
15
Oh wait- maybe my assumption was incorrect (I don't know my relay pin numbers).

I thought you meant to use the ground that is supplied when the door is closed as the control for the relay. That would energize the coil and break the contact lighting the lights (making them stay out)

You want it to work the other way, where the 12V shown with the door open will energize the coil. The problem here is that there is very high resistance on that circuit. Not enough to even support the flow through a coil. I could purposely short it out with an amp-meter and I bet it would read as only a milliamp or two.
 

Dodgydave

Joined Jun 22, 2012
9,936
That's kind of what I was thinking after reading the first nfet suggestion. The way you drew the transistor looks a little foreign to me though. I think this is what you mean... View attachment 131930

The strip I'm using is the same style I've used for years. They are designed for 10-14V.
Yes, the Gate goes to the door wire, the Source to ground, the Drain to your leds Cathode... You don't need a gate resistor.


Try a IRLZ14 Mosfet
 
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Thread Starter

samsepi0l

Joined Jul 29, 2017
15
Thanks- just an update: I found the wiring schematic for the door latches. I opened the door (opening the circuit) and took a measurement and I get 10.8V.

When I shut the door and take a measurement at the pin on the BCM it drops to 0V just as expected.

I also connected a second meter (as an ammeter) and used it to pull the pin to ground when the door was open. I wanted to see how much current flow. Current flow = .001 amps.
 

Thread Starter

samsepi0l

Joined Jul 29, 2017
15
So, my parts came today and I tested it with jumper wires and alligator clips in the car. Everything seems to work perfect. I see no current draw what-so-ever on both the gate, or the LEDs when the door is shut. Even when the door is open I can't measure any current draw on the gate, but it behaves just as I wanted it too. Thank you all so much for your help. It will be a while before I get the chance to tear everything open and install the parts the "right way" but when I do, I'll be sure to post photos and video.
 
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