Adding a relay to detect when a switch tied to ground is open.

[PaulDay]

I'd like to add individual footwell lighting and have the lighting come on when each door is opened individually. I've found the wire locations and looking at the wiring diagram, I not certain which way to wire a relay to properly detect when the switch is open to trigger the LED light to come one.

Here the wiring diagram. I'm interested in the "Door ajar" switch, which is normally closed. When open, I'd like a relay to energize and send power to the LED lighting. It looks like the system is monitoring the ground and when it's not there, sends a door open signal to the dash.




Any help would be greatly apricated.

Have a good day.

 

[ElectroChem]

Couple of possibilities.

You could use the door ajar switch to provide the 0V/gnd to a 12v relay, this would be held energised whenever the door is closed. Using a NC contact on this relay would then give you a light whenever the door is opened. This would introduce an extra static drain on the battery when the car is stopped, but given that the average relay draws <2W and a car battery is ~600Wh, you should be ok.

Other option, depending on how the input impedance and current rating of the door ajar input works is that you could have a relay coil between the input and ground. When the door is closed, the door ajar will hold the input at ground so relay off. When door open, if the input current is high enough and the impedance low enough, the +12V at the input could maybe be enough to pull up the relay and energise. Or at least maybe enough to pull up a solid state relay?

 

[PaulDay]

Couple of possibilities.

You could use the door ajar switch to provide the 0V/gnd to a 12v relay, this would be held energised whenever the door is closed. Using a NC contact on this relay would then give you a light whenever the door is opened. This would introduce an extra static drain on the battery when the car is stopped, but given that the average relay draws <2W and a car battery is ~600Wh, you should be ok.

Other option, depending on how the input impedance and current rating of the door ajar input works is that you could have a relay coil between the input and ground. When the door is closed, the door ajar will hold the input at ground so relay off. When door open, if the input current is high enough and the impedance low enough, the +12V at the input could maybe be enough to pull up the relay and energise. Or at least maybe enough to pull up a solid state relay?

Thanks for the reply.

I'll be checking 4 doors, so don't think I want that kind of electrical draw all the time that the first suggestion points out.

I believe that the second option might be the solution. I plan on using these miniature board mounted relays:

https://www.amazon.com/gp/product/B07WP7C8ZD/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

which have a selectable low level trigger and mounting them in these hobby boxes:

https://www.amazon.com/gp/product/B07S7RJGSR/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I'll be putting 3 relays in one box for the two front doors and for the dome light. Two relays in another box for the two back doors. One 12V fused input going into each box for power and ground, to the relays, along with separate leads for each input and LED output with male connectors to the I/O's.

I'm hoping that it's a low voltage line to ground that this relay can detect and latch down until the door opens without setting off the door ajar light on the dash..

This is for a show car, so I'll probably be ordering Ford connectors. Of course, all wiring will be covered with wire loom and dressed.

Any thoughts greatly apricated.

Have a good day.

 

[ElectroChem]

My field is industrial automation, not auto electrics, so I'm chucking ideas into the breeze a bit here.
I had another thought, do you have access to the actual door micro-switch? The wiring diagram shows it as a 3-terminal changeover switch. If you can get a third wire to it and swap the existing two, then when the door opens you will supply GND to the third terminal and could use that for a relay.

 

[PaulDay]

Unfortunately, the switch is built into the door latch itself. It's funny that the connector has 6 wires, with #2 marked "Not Used". Wondering if #2 really connects to the unused pin on the switch. I'll have to verify when the time comes. Thanks for the help.

Maybe someone with auto electrical experience can chime in.

Have a good day.

 

[StephenRowley]

What about the switch for the normal interior lighting?

 

[PaulDay]

What about the switch for the normal interior lighting?

Interior lighting is controlled by the common "door ajar" switch in each door, triggered whenever any door latch is not fully closed. With Ford all you get on the dash is a "door jar", regardless of which door is open. Cheap!

Nissan and Toyota have separate ground points that connect to each door and the dome light. Dash shows which door is ajar. Pull power for your LED and ground it at the appropriate door ground and you're all set. You're LED will light up when only that door is opened. Great for individual footwell lighting. Adds a custom look.

I found these on DHGate:



I wrote to the seller asking if they light up for each door individually, and he said yes. but won't provide documentation on the wiring before sale, so not sure if they actually understand the question. I don't feel like buying them, first I have no use for them and also not sure if they can be wired as I'd like. I've also looked to see if I could find someone who's already installed these, but no luck.

The Ford Mustang version of the LED sill is wired to the dome light, per Ford installation instructions sheet, both coming on whenever any door is opened. Kind of lame if you ask me!

Thanks for the input.

Have a good day.