A twilight switch is an electronic component that allows the automatic activation and deactivation of a lighting circuit when natural light decreases or increases in a given environment. Among its numerous uses, the most common is to enable automatic lighting of streets, paths, highways, roads, gardens, patios and other spaces, thus turning on the lighting when the intensity of sunlight drops below a set level, and turning it off when the same sunlight exceeds said level, as occurs during twilight, hence its name.[1]​.

A circuit built with a twilight switch requires, in some cases, other components, such as relays or contactors, when you want to control greater electrical power (lights, electrical appliances, among others).

A light intensity sensor is required (photoresistor, photodiode, phototransistor, etc.) that detects the amount of light that illuminates an environment, triggers an electrical circuit that opens or closes the contacts of a mechanical or solid state relay (power transistor, thyristor, triac, etc.), which activates the lighting system. Generally, natural lighting is used that falls directly on a photoresistor, obtaining the effect of a lamp that turns on automatically at dusk and turns off, always automatically, at the first light of dawn. Thanks to this system, a wide range of use examples is created, from lighting both public and private spaces to presence simulation, where the twilight switch provides the intermittent operation of a lighting circuit to simulate the presence of people who are not physically present.[2]​.

There are increasingly innovative models that allow greater sensitivity to sunlight in which the threshold can be adjusted so that the switch is triggered at a certain level of darkness, thus establishing a delay for turning on and off with respect to the level of ambient light. There are even models that are not activated by artificial light and distinguish it from natural light, although in some cases it may be convenient to combine it with time programming systems.[3]​.

The most used twilight switches are of the electromechanical type, which differ from the electronic ones by the use of piloted relays already integrated into the circuit itself, and which allow small loads to be directly connected (for example, a single lamp). Twilight switches are sold in different shapes to suit all needs. In fact, they can vary from the shape of a lamp holder to that of a separate box (cylindrical, square, etc.).[4]​.