Dome Light Mod to Trigger Other Lights

In my ceiling electrical rough-in post (See here), I described the ceiling wiring. Among the circuits, I mentioned the original dome light, and moving it over the slider. I wanted to take this 3-position switched light and have it optionally trigger the dome lights in the rear and the front. Today's post will cover how I made that work.

Original Dome Light

dome switch innards

Let's start with a review of the original light and switch. This switch is some basic stuff. There are 2 wire tabs near the switch and one at the other end of the housing for connecting to ground. The 2 tabs correlate to the wire that connects to the switch on the dash (if you still have it) and the switches in the doors (again, if you still have them). The door switched side of my dome light never worked, and I removed the switch from my dash so I could reuse the hole for the heater fan switch. But, I digress; there are 2 tabs. The tabs relate to either side of the 3-position switch, with the middle position for all-off.

Inside the light housing, how the switch works is more apparent. Each wiring tab has a corresponding little tang that hangs or runs down towards the lens on either side of the plastic switch. Outside of each tang runs a metal arm that connects to the positive side of the bulb. So, when the plastic switch is moved from the middle / all-off position, it pushes the little tang against the metal arm, closing the circuit. When 12V is sent to the corresponding tab, the bulb lights up.

Experimenting

So, with that context, we consider how do we tie into the switch to close a circuit elsewhere. The wiring to the other bulbs has been strung; they just need 12V sent when the switch is flipped. We cannot just attach to the metal arm, because then the remote lights would fire up regardless of which position the switch was in. If that's your goal, then that's great, and less difficult. We cannot just tie into the wiring tab because then the remote lights would be on all the time. We need to somehow tie into the movement of the switch so that when the tang is pressed outward, we can pick it up for the extra lights.

experimenting

I noticed that the little hang-down tangs are not the same length, the amount of exposed brass related to each circuit is not equal and the wiring tabs are 90* offset from each other. I do not know how much these details relate to one another, but the longer length of the one hang-down tang provide a way to tie in. The longer one is attached to the side with less exposed brass overall and with a wiring tab that is parallel with the lens. Again, not terribly relevant.

I conducted some experiments with the switch and a bare-metal female wire connector (picture on the right), as opposed to one that's wrapped with a plastic housing. I was able to set a connector fairly close to the housing, but not touching any brass, unless the switch was moved in that direction. Then, the female connector would make contact with the end of the hang-down tang. It was a great fit inside the lens, and did not prevent the circuit connecting with the metal arm. Similar experiments with a wire connector that had a plastic housing produced different results: did not work. The housing was too thick to fit in the space, even if I cut it down so there was metal exposed for the tang. I tested connectivity with a basic multi-meter to confirm that the switch would create a connection when engaged in the one position, but not at any other point.

After spending the better part of a weekend morning fiddling with this plan, I had to change it. I glued the female wire connector onto the lens and discovered that during the experiments, the connector moved slightly as I activated the switch. So, when I had it completely stationary, it jutted out just enough to prevent the tang from touching the arm: the connectivity on the multi-meter was infinite. So, that bulb would not power up had I left it that way. The lesson here, I guess, was to vice-grip the female wire connector in-place during the experiments so it would absolutely not move.

Tying In

bailing wire tang

So, I considered something that was smaller, and more like the little tang I was trying to interact with: a short stretch of bailing wire. With the tip of my needle-nose pliers, I shaped a hook-end and then a twist to create a footing I could adhere to the lens. Learning from my experimenting mistake, I set the bailing wire tang in place with vice-grips and repeated my experiments. In retrospect, something more pliable for the tang, like some thin copper or even a short stretch of solid 20ga wire probably would have been more effective because it would have more flex/give when the switch was flipped. Still, after a few cycles of set-test-reset-retest, I was able to find the spot when both the original arm and the new bit of bailing wire demonstrated connectivity with the wiring tab when the switch was in position.

I soldiered a stretch of insulated 18ga wire to the bailing wire tang, and then a-fixed the tang to the lens with superglue. Once the glue set up, I repeated my experiments to make sure that I could consistently find connectivity from the wiring tab to the end of the 18ga wire as well as the on the original light.

Triggered

Reasonably or not, I considered the amp draw through that little connection between the tang and the bailing wire, and assumed that expecting 2 lights to get powered through it is asking too much. So, to protect against overheating that connection, I'm adding a basic 4-pin relay. Recall the basics of a relay: when there is a sufficient voltage difference between pins 85 and 86, a connection is made between pins 30 and 87. So, we connect the inbound juice (that will be attached to the tab on the light) to pin 30 and ground pin 85. Pin 87 connects to the wires supporting the remote lights and pin 86 connects to the bailing wire tang I added to the light fixture. Because of where I am putting the light over the slider, there is considerable room in the ceiling for the relay and the extra wire.

Install Awaits

one switch. 3 lights

Parts of this could be installed immediately, but so much of the interior is still in disarray. The sound suppression steps are still ongoing, the remote lights are still just bare wires. I thought about stuffing the relay up into the ceiling, but decided that I could just as well wait and install the whole unit at once. Before setting it aside, I confirmed the concept, though. I pulled out a car battery, and set the 3 light fixtures on my kitchen table. I ran wiring between all the lights, and the relay, and simply played with the switches (note the fuse next on the positive side of the battery on the right edge of the picture). Yes, I did use clothespins to hold the wires together; I didn't want to stress the wires by twisting them together or waste wire connectors. When the original dome light is in one position, all 3 lights fire. When it is in the other position only that light comes on. Perfection. While I had it all set up, I played with the other (remote) lights, just cuz. I will, of course, have to retest everything when it gets installed, but seeing things light-up gives me some confidence that the concept, and maybe even the implementation, will work.

That's it for today. Thanks, as always, for following along-