Arduino and car's 12V switch

Question

I'm trying to control my headlights with my Arduino and a LDR. For this I will use the Arduino as a middle man between the lights switch and the actual lights. The switch has three positions OFF / DAYTIME LIGHTS / HEADLIGHTS. The switch itself is a potentiometer and has 3 cables: 12V, GND and the signal one. The Arduino then would read which setting is chosen in the switch, decide what to to do, and send the signal thru this cable.

The problem comes here. The Arduino, as far as I know, can only handle <= 5V. So how am I supposed to read the switch, and then send a signal back to the car? (I suppose that these signals would be around 12V or so).

Sorry if I'm not explaining so well. Not very experienced in electronics.

EDIT

I have measured the voltages across the three cables (WHITE/GREEN/BLACK), assuming black as ground.

When engine is off:

White = 23-25V

When engine is on:

• Switch == off

White = 28.5V

Green = 30.3V

• Switch = daylights

White = 0V Green = 30.3V

• Switch = headlights

White and green = 0V

From this I wouldn't know what to do. Thought the switch worked like a potentiometer but I think I am wrong.

Also I am trying to find a logic answer of why both white and green have 0V when the lights are switched on.

EDIT 2

I found the part online, and its name is "WELLS SW6250", a combination switch. I thought that, because of the white and green cables going 0V when the lights are turned off, that there might be another cable coming from the square part of the knob, where there are other connections.

Answer 1

The situation may be a little more complex than you imagine.
Since your switch (potentiometer) is an input device to the car ECU to set intent, you would need to disconnect the signal wire from the ECU so you can read the switch position. Then you need to generate a voltage into the ECU to provide control from the Arduino.

It's highly likely that the switch is NOT on a regulated power supply, so the voltage may vary from say 10 V (starting) through almost 15 V (charging). This will be coped with by the ECU (it knows what the battery voltage is). There may also be fault sensing to discern if the switch has failed, so the current sourced or sunk from the ECU may be important.

You can deal with the high input voltage with a simple resistive divider, though it's worth having overvoltage protection.

Perhaps something like this:

^{simulate this circuit – Schematic created using CircuitLab}