Automatic headlight sensors are photoelectric sensors that respond to the amount of ambient light and turn on headlights when they reach preset levels. Most systems retain the conventional headlight switches, which allow drivers to turn lights on or off and bypass the sensors.
Because the sensor detects light, it cannot distinguish if the darkness level is due to sunset, overcast conditions or driving into a garage. This causes the lights to illuminate when a storm blocks the sun or when the driver is parking.
Most manufacturers wire the automatic headlights through the ignition switches of their cars. When the driver starts the vehicle and light levels are low, the headlights illuminate automatically. When he switches off the key, the system may turn off the lights immediately or have a delay function that gives the driver and passengers some time to exit the vehicle and walk to safety.
Drivers with concerns about battery drain due to starting their vehicles with the lights on have the option of manually switching them off when starting and then turning the switches to the automatic settings. This causes the lights to illuminate when the sensor determines they are necessary, and it shut them off automatically.