Ways to Face Photocells

By Amy Rodriguez

Photocells control automatic light fixtures.
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Many devices have photocells for controlling a light fixture. Automatic daytime and nighttime running lights on cars are regulated by photocells embedded in the vehicle's dashboard. Other photocells regulate outside home lighting fixtures, automatically illuminating the fixture when night falls. However, photocells will not work correctly unless you face them in the proper direction.

Photocell Features

A basic photocell is an electronic component called a resistor. The photocell's resistive value changes as light passes across its surface electrodes. Resistance reduces as the photocell has more light passed across it. Complete darkness will allow the photocell to display its highest resistive value, commonly 10 megaohms. This change in resistance allows the photocell component to serve as an indicator of light or dark to control a light fixture.

Natural Light

You must place photocells strategically for the best light fixture control. Outside, south-facing photocells will pick up too much natural midday sun, diminishing the component's effectiveness. The photocell should face north, away from direct sunlight. Alternatively, face the photocell to the west or east, if a north position is not possible. You can also face the component downward.

Artificial Light

After nightfall, the photocell should not face any artificial light sources, such as a lamp shining through a window. The artificial light will deceive the photocell, prompting the component to shut the light fixture off since it is sensing the light as daylight. Make sure no light reflections strike the photocell. Intermittent light across the photocell's surface can cause the component to switch the light fixture on and off repeatedly.


You may want to test a photocell if it seems to be malfunctioning. Turn the photocell's power on and wait approximately five minutes for the photocell to react. You can also cover the photocell with black tape to simulate nighttime. Allow five minutes to pass to verify that the photocell is reacting correctly.