The circuit design to replicate the function of the Honda S2000 20th Anniversary footwell lights requires a standard 5-pin automotive relay and a power resistor. The system relies on the S2000's negative door trigger system, where the door switch provides a ground connection when the door is opened.

The circuit operates based on two inputs: the headlight positive trigger (+12V when headlights are on) and the door negative trigger (Ground when the door is open).
Required Components

• 1x 5-pin automotive relay (SPDT - Single Pole Double Throw): This relay has pins 30, 85, 86, 87, and 87a.
• 1x Fuse holder and appropriate fuse: Use a fuse rated slightly higher than the maximum current draw of your lights (e.g., 2A or 5A fuse for LEDs).
• 1x Power Resistor: The value (Ohms) and wattage of the resistor must be selected based on your specific LED lights to achieve the 50% brightness (see the "Resistor Calculation" section).
• LED Footwell Lights: The lights you wish to install.
• Wiring and connectors.

Circuit Diagram and Explanation
Here is the wiring plan:

1. Power Input (Fused +12V): Connect a fused power source that is active only when the headlights or parking lights are turned on to Pin 30 of the relay. This ensures the entire circuit is off when the headlights are off.
2. Relay Coil Power:
   • Connect Pin 86 of the relay to the same fused +12V headlight source as Pin 30.
   • Connect Pin 85 of the relay to the S2000's door trigger wire (this wire connects to ground when the door opens).
3. Light Circuit:
   • Connect the positive (+) wire of your LED footwell lights to Pin 87 (Normally Open - NO) of the relay.
   • Connect one end of your power resistor to Pin 87a (Normally Closed - NC) of the relay. Connect the other end of the resistor to the positive (+) wire of your LED footwell lights (at the same point where Pin 87 connects).
   • Connect the negative (-) wire of your LED footwell lights to a solid chassis ground point in the car.

Operation

• Headlights OFF: No power reaches Pin 30 or Pin 86. The lights are off (0% brightness).
• Headlights ON, Door Closed: Power reaches Pin 30 and Pin 86. The door trigger wire (Pin 85) is an open circuit (not grounded). The relay coil is de-energized, so Pin 30 connects to Pin 87a (NC). Current flows through the resistor to the lights, resulting in 50% brightness.
• Headlights ON, Door Open: Power reaches Pin 30 and Pin 86. The door trigger wire (Pin 85) connects to ground. The relay coil energizes, switching the connection from Pin 87a to Pin 87 (NO). Current flows directly from Pin 30 to Pin 87, bypassing the resistor, resulting in 100% brightness.

Resistor Calculation
To determine the correct resistor value for approximately 50% brightness (assuming standard 12V LEDs):

1. Measure the current draw (
   Code:

   I_full

   ) of your LED lights at full 12V brightness using a multimeter.
2. For a simple resistor dimming, you generally want to drop the voltage to about 6-8V for 50% brightness visually, or reduce the current by half. This can be trial and error with resistors, or if using LEDs, it's safer to use a PWM dimmer module. The resistor method is an estimate and less efficient.

A simple resistor can work for basic LED strips or incandescent bulbs. If you use a simple resistor, common values might be in the range of 50 to 200 Ohms, with a high power rating (at least 5-10 Watts) to dissipate heat.
Online Design Tools and Resources
For designing and simulating circuits, you can use:

• The12volt.com: An excellent resource specifically for automotive wiring, including many pre-made relay diagrams for various functions like illuminated entry. You can find generic relay circuit diagrams there.
• Falstad Circuit Simulator: A free, browser-based general circuit simulator where you can design and visualize the current flow in your proposed circuit.
• Tinkercad Circuits: A free, easy-to-use online simulator by Autodesk, suitable for beginners.