How do Dusk-to-Dawn Photocells Work?


As the push to cut energy usage continues, one method is to eliminate the use of light sources when they are not needed. To this end, most quality outdoor LED fixtures now come equipped with a factory-installed photocell which tells the fixture when it should operate. This is primarily used to turn exterior lights off at dawn and back on when the sun sets.

There are numerous types of photocells in the market but the technology behind them is all the same, as they utilize semiconductors to control the electric current. When the semiconductor is exposed to a certain level of light, current starts to flow and the fixture will be shut off. Some photocell sensors come adjustable, allowing you to choose the light level that will activate the semiconductor. As the light fades during the evening, the flow stops and the fixture is powered on. This allows for “Dusk-to-Dawn” light output.

Being that photocells sense ambient light levels, they will automatically adhere to seasonal changes and are not affected by daylight-savings time. This results in maintenance-free energy efficiency in addition to providing the security of having lights on in the evening to deter intruders.


Photocells are most commonly hard-wired via 120-277V line voltage to control an entire electrical circuit that operates outdoor LED fixtures. But there are also photocells that screw into light bulb sockets, allowing nearly any lamp or fixture to become a dusk-to-dawn lighting system. The difference being that a photocell has to be installed into each individual socket. There are also photocells that work with a standard wall outlet and control a pass-through plug.

Timers and motion sensors are other types of controls that are sometimes used in conjunction with (or to replace) photocells. Timers are useful for on-demand lighting controls, for example to turn off lights at a work site during set times. More complex timers are set up to allow input of multiple time configurations. But they lack the ability to work well with environmental factors. Local sunrise and sunset patterns change over time and would have to be manually set, whereas a photocell will sense this automatically. Timers also won’t be able to react to a rainy or cloudy day like a photocell would.

Ultimately, the needs of your particular application will decide which controls make sense but photocell sensors have proven to be essential components for many outdoor lighting systems.

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2 Responses to “How do Dusk-to-Dawn Photocells Work?”

  1. WarrenMarch 8, 2019 at 11:50 am #

    With dusk to dawn switch fixture comes on at dusk 2 or 3 nights and then will not come on 2-3 nights. Why

    • PremierMarch 8, 2019 at 11:57 am #

      The sensor could be defective or not installed correctly

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