On an automobile, Twilight Sentinel is a device on General Motors' cars that senses outside light and turns the exterior lights on and off depending on lighting conditions.
In addition, Twilight Sentinel also allows the driver to set a timer (located with the headlight switch) that delays shutting off the headlights for a specified time period, usually up to three minutes. The driver can park the vehicle, shut off the car's engine and exit the automobile after dark, then use the headlights to light the path into a nearby building, such as a house, before the system shuts the headlights off automatically.
An electric photocell located in the dashboard detected lighting conditions and activated the headlights as appropriate. Usually, this would mean turning on the headlights at dusk and shutting them off after sunrise. During daytime driving, the photocell would be able to keep the headlights off when driving under a bridge, through a short tunnel, etc.
However, Twilight Sentinel does not automatically shut off the headlights if the driver manually turns the headlights on (they must manually turn the headlights off), nor does it turn on the exterior lights for driving conditions such as heavy rain or fog (however in some newer versions of the system, Twilight Sentinel is linked to GM's RainSense automatic windshield wiper system to turn lights on automatically when the wipers are on).
Twilight Sentinel was first offered exclusively on 1960 Buick LeSabres, Invictas, Electras and Electra 225s.
A related feature offered on many General Motors models during the 1950s and 1960s was the Autronic Eye headlight dimming system (which did just that). Cadillac later referred to its automatic dimming system as "Guidematic Headlamp Control" which encompassed Twilight Sentinel and Dimming Sentinel controls. (See Discussion Page)
Ford Motor Company offers a nearly identical exterior lights on/off/delay shut-off system called "AutoLamp". The Ford automatic headlamp system could be had with automatic dimming on Lincoln models . Chrysler offered Twilight Sentinel on Imperial model from 1967-1975, other full-size C-body Chrysler products had it as an option from 1965 or 1966 through 1975 as well.