Introduction: Dome Light Triggered LED Footwell Lighting for a 2010 Honda Fit

This instructable details the installation of red-green-blue (RGB) LED footwell lighting in a 2010 Honda Fit. For this type of application, RGB LED kits and controllers are readily available for purchase on sites like I chose not to use a commercial kit for two reasons. First, that's not any fun. Second, most kits either lacked features or had features I wanted to avoid, like remote controls. For my design, I wanted the following features:

  1. Absolute control over RGB components.
  2. Adjustable from driver's seat.
  3. Manual controls = no remote.
  4. RGB settings are not forgotten when car is turned off.
  5. Minimally destructive to the vehicle trim.
  6. Operation follows dome light.

The last point was the most challenging requirement. In the Fit, the dome light comes on for various events, like opening the door or pulling the key from the ignition. It turns off automatically and when it does, it fades out slowly. This fade effect is probably produced by attenuating the current through the dome light bulb. This project requires that the LED controller have a separate power supply than the LED's themselves. That is, you can't just wire the controller to the dome light circuit, since dimming the controller would be undesirable. And, I'd like the LED's and controller on a different fuse than the dome light. The unique aspect of this instructable is the of rewiring the RGB LED controller to separate these three circuits (dome light, controller, and LED) but allow them to operate in unison.

Step 1: Required Materials

First, of course, you need the vehicle. This instructable was made considering only a 2010 Honda fit, but should be applicable to any number of cars and trucks. Basic soldering skills are required. The main components of this project are:

  1. 3 channel RGB LED dimmer/controller (
  2. Strip of 12 V RGB LED's (5050) (
  3. Add-a-fuse (fuse tap), ATM or "mini" size (
  4. 10A ATM (mini) fuse
  5. Extra cable, 4 wires for red, green, blue, and common (
  6. Some sort of terminal/jack and appropriate connector
  7. T-tap connector (
  8. Blade connector (
  9. Electrical tape
  10. Diode
  11. Sticky-backed velcro

The RGB LED dimmer/controller should have analog controls (potentiometer knobs), one for each channel. It's also important that the common output match the polarity of the common rail of the LED strips. The controller used here has an individual ground for each RGB channel and outputs a common +12 V. The LED strips are also "common anode", meaning they have one +12 V connection and three ground connections, one for each RGB channel.

The LED strips I was able to find are all 5 m in length which is way more than will be required for this project. The cost is not severe though. I paid about $13 shipped. I recommend finding a strip with 5050 LED's as these are brighter than the 3528's. The other big difference is that on a strip of 5050's, each chip has three LED's, a red, green, and blue. On a strip of 3528's, each chip is either a red, green or blue. The colors from a 5050 strip will be better blended since the component LED's are spaced much more closely together. Also be sure the supplier sends you the right part. The LED chips of true RGB 5050 LED's should be white. If they are yellow, the supplier has sent you a strip of white LED's - you will not be able to change the colors. You will have the option of waterproof or non-waterproof. Here I used a non-waterproof strip but waterproof might be a better option and should work equally well.

The add-a-fuse (fuse tap) should be ATM or "mini" size. The 2010 Honda fit uses low profile mini fuses. A low profile fuse tap is difficult to find, but an ATM (mini) fuse tap will fit. We will be tapping an unused socket in the Fit's fuse box, so we will only need a single ATM (mini) fuse for our new circuit. If you are modding a different vehicle, be sure to get a fuse tap that fits in your fuse panel and a corresponding fuse.

We need 4-wire cable to run from the LED strips to the dimmer/controller. We will also use some of this wire to rewire the RGB dimmer/controller, etc. We will need to connect a new wire to the dimmer/controller box. We want that wire to be easily disconnected so you will need some sort of wire termination or plug and a corresponding terminal or jack. I used a connector I ripped from some old circuit board.

To tap into the dome light circuit, a t-tap is handy. This will bite into the dome light circuit's ground wire without requiring you to cut the wire. These t-tap connectors accept blade type wire terminators so you'll need at least one of those.

To attach the LED strips to the underside of the vehicle's interior trim, I opted for electrical tape. The strips usually have a sticky 3M backing but I find it lacks adhesive strength. Many people opt to drill holes and use wire ties. I needed to avoid altering the trim in any way, so electrical tape it is.

Step 2: Altering the RGB Dimmer/Controller

The RGB dimmer/controller used in this project uses pulse-width modulation in order to control the intensity of the LED's. Since LED's can't be dimmed by attenuating their supply voltage, it is necessary to maintain the supply voltage (here 12 V), pulse the supply, and use the pulse interval to control the observed intensity. The potentiometers on the dimmer control the pulse width or how long each pulse is applied. To switch the LED circuits on/off during/after the pulse, MOSFET's are used.

A MOSFET has three terminals: the gate, source, and drain. For the type of MOSFET found in this controller, when a voltage is applied between the gate (+) and source (-), current flows between the drain (+) and source (-). The controller logic applies a voltage at the gate and source to turn on the LED's. In the original configuration, both the controller logic and the LED's share a +12V supply and ground. In our design, the LED's will share a ground with the dome light. The controller logic cannot share the dome light ground because it would be undesirable to dim/fade the logic circuit. Imagine dimming the power to your computer. The results would not be good and could possibly damage the digital components.

To decouple the LED circuit, we will unsolder the source pins of the MOSFET's and wire them to a new ground input connector. To this connector we will later attach the dome light's ground wire. In the photos you can see that I've unsoldered each source pin and attached a small wire to each. All three wires are soldered to a connector that I've hot-glued through a hole in the controller housing.

In another photo you can see a short grey wire connected to this new ground pin. When that wire is in turn connected to the ground input of the controller logic circuit, The dimmer/controller is restored to its original design/operation. This is NOT the where this wire will be attached when the project is completed but is a good way to test that you haven't destroyed the dimmer/controller. When operated in this way, with the entire 5 m strip connected and the levels for each channel at their maximum, my strip pulled less than 1 A.

Step 3: Dome Light and Power Connections

To supply power to our controller, we will tap an unused +12 V socket of the fuse box. The empty socket is #4. It should have a constant +12 V supply even when the car is off and the key is not in the ignition. The dome light circuit has the same type of supply. This way the light can illuminate when you first open the door, before you get in and turn the car on. We could have tapped the +12 V line that feeds to the dome light. This will work. Some of you may opt to do this since then you wouldn't need to purchase a fuse tap and fuse. I wanted my LED's to be on a different fuse/circuit than the dome light.

Tapping the socket is easy. First the new fuse is put into the top slot of the fuse tap. The fuse tap wire should have a crimp splice connector at its end. A short piece of wire is crimped to it to extend its length. Then the fuse tap is pushed into the empty socket. The dangling wire will be connected to the +12 V input of the controller which is one of the green screw terminals on the left side.

The controller will need it's own ground connection. For this it's simple enough to connect a wire to the ground input of the controller (green screw terminal on left side) and run it to a spot on the car chassis. I found a screw nearby to which I connected the ground wire.

Finally, we need to connect the dome light's ground wire to the ground terminal for the LED's, the one we added to the controller housing. This is where we'll use the t-tap. The dome light wires can be found at the bottom of the fuse panel. Two wires, one pink, one cyan, should be the only two wires in a small green connector. Unplug the connector for easier access. Clamp the t-tap connector somewhere on the cyan wire. To one end of a short piece of wire, crimp a blade connector. On the other end, attach the plug or terminator that will mate with the jack or connector you installed to the controller housing. Then plug-in the wire to make this connection.

With this step completed, you should see the red power indicator LED of the controller is always illuminated. The controller is always on, but the LED's will only come on when the dome light ground connection is switch closed. This action is controlled by the car's microcontroller and is triggered by several events like opening the door or removing the key from the ignition. I've measured that the controller circuit in this always-on state draws about 17 mA from the battery. Given that car batteries can supply more than 10,000 mAH, there is no danger of this circuit draining the battery in any practical amount of time.

Step 4: Running Cables and Mounting Hardware

You will need to choose a location for mounting the LED strips. I mounted a strip on each side, driver and passenger. I stuck them to the underside of the vehicle trim where the black plastic was mostly flat across the width of the footwell. Once you have your location selected, measure how long your strips will need to be and then cut appropriate size strips from your 5 m RGB LED spool. The strip I used is able to be cut every 3 chips, so every 2 inches. When you cut the strip you will do so down the middle of the bare copper connectors located between each segment. My two strips ended up being 14 inches and 16 inches long.

From the spare 4-wire cable cut a length for each of the two LED strips. The strip on the driver's side will need a much shorter piece of cable than that on the passenger's side. In order to measure, run the cable along the planned route it take to the LED strip. A little extra length is a good idea. I used a 24 inch piece of 4-wire cable to run to the driver's side LED strip and a 48 inch piece to run to the passenger's side strip.

You'll want to solder the RGB cables to the strips before trying to run them through the car. Then, feed the cables behind trim and panels to all the way to the fuse box cavity where the controller/dimmer will reside. Next, it's a good idea to clean the plastic where you will stick your LED strips with a little soap and water or alcohol to remove any dirt or oil. Then remove the plastic backing of the LED strips to expose the adhesive and press them into place. You will need to secure them using your chosen mounting method. I opted to use electrical tape to hold the LED strips firmly to the plastic. If you also choose to use tape, make sure it is a high quality tape that won't come un-stuck.

With all of the necessary cables and wires now run, You can connect all to the appropriate terminals of the dimmer/controller. This in turn will be mounted to the underside of the driver's side's far left cup holder. If you reach in the fuse box cavity at the top you should be able to feel the underside of the cup holder. This provides a flat surface to which the dimmer/controller can be affixed. Cut a piece of velcro (hook or loop, it doesn't matter) and stick it to the underside of the cup holder. Stick another piece of the same length but opposite closure type to the back side of the dimmer/controller housing. It should be positioned all the way to the left side of the housing. Now the dimmer/controller is mounted (upside-down) by simply pressing the two velcro surfaces together.

Step 5: Modifying the Dome Light

At this point, the RGB LED's should be operational. They should come on when the door is opened and fade out with the dome light when the door is closed. The Fit's dome light has three settings, OFF, DOOR, and ON. In the ON position, the dome light remains constantly illuminated. Because this setting uses a different ground connection than the DOOR setting, the installed LED strips will not be illuminated in this position.

To get the strips to come on when the dome light switch is in the ON position, a small diode is wired to the dome light between the DOOR ground and the lamp's negative terminal. It is necessary to use a diode so that current cannot flow from the lamp to the DOOR ground through this extra connection, or else the dome light will be operate in DOOR mode even when switched OFF. The diode will only let current flow one way. Therefore, it's important to orient it in the right direction when installed.

To get access to the dome light, the clear plastic cover is first pried off with a knife or screwdriver. The entire housing can then be removed from the ceiling by undoing two screws and unplugging the wire connector. In the photo you can see where to attach the diode. Unfortunately, it is not possible to solder to the metal contacts with normal solder. I happened to have some conductive silver epoxy on hand and used that to attach the diode leads. Notice the direction of the diode. The white line that runs the circumference of the diode is positioned toward the right side. Make sure to choose an appropriate sized diode. I robbed this one from another old circuit board. It is rated at 20 A and 50 V, far above what we require here.

With the diode attached, simply reinstall the dome light housing to the car ceiling.

Step 6: Completed Project

That completes the project. Once everything is wired you should find that the LED's match the operation of the dome light. They will come on when the door is opened and fade out when the door is closed. When the dome light is switched to the always ON position, the LED's will likewise come on. One caveat: as described, there is no way to turn off the LED function. Even with the dome light switched to the OFF position, the LED's will operate with the door. In order to match the OFF operation of the dome light, it would be necessary to cut wires which I was unwilling to do. A separate on/off switch could be wired for the LED dimmer/controller, but then you need a convenient spot for the switch. I couldn't see a place to mount a switch without drilling or cutting holes in the trim.

In order to set the color of the LED strips, it's necessary to open the fuse box cavity to access the knobs of the dimmer/controller. I expect that once set to a nice color, I'll have little reason to change the color frequently, so this extra step is a minor inconvenience.