Dusk to Dawn Feature
Many times we are asked about the meaning of the "Dusk to Dawn" feature of motion lights. This is a very nice feature that you will find on most motion sensor lights, so understanding this will help you make the best choice when choosing the light that is right for you. In short, the dusk-to-dawn feature of a light will cause your motion light to only activate when it is dark. In daylight hours when existing and ambient light is present, the light will not activiate when motion is detected. This feature uses a small photocell like the one pictured above to detect the presence of available and existing light. If light is detected, the motion light will not trigger. If light is not detected, then the light will turn on when motion is detected.
What is a Photocell?
Technically, a Photocell is a light sensor that varies its resistance between its two terminals, based on the amount of light received by the photocell. In other words, it is a sensor that detects light, and turns your light fixture on as it senses less available light (usually around dusk) and turns the light fixture off when it senses more light (usually around dawn) -- as a result, the feature is called a 'dusk to dawn' feature.
By using both a photocell and a motion detector built into a motion light, the system will turn on only when both (a) motion is detected by the main motion sensor AND (b) the photocell detects that less light is available (dusk). Then, when the photocell detects that more light is available (dawn), the light will not operate, even when motion is detected.