Light Sensor: Automatic headlights are equipped with a light sensor usually located on the dashboard or near the windshield. This sensor measures the ambient light levels outside the vehicle. Threshold Setting: The vehicle’s manufacturer sets a threshold value that determines when the headlights should turn on. 

This threshold is typically based on the amount of natural light present, such as during dusk or when entering a tunnel. When the light sensor detects that the ambient light has fallen below the threshold value, it sends a signal to the vehicle’s lighting control module. The lighting control module receives the signal and activates the headlights. Depending on the system, it may also control other lighting elements like the taillights and dashboard illumination. 

Many vehicles with automatic headlights allow drivers to manually control the headlights as well. This is usually done through the headlight switch, which provides options like “auto,” “off,” and “on.” When set to “auto,” the automatic mode is active. As the ambient light increases and exceeds the threshold value, the light sensor signals the lighting control module to turn off the headlights.