How Do Photocells Work?

Ever wondered how photocells take in light and turn on/turn off a light source? They’re actually really cool devices once you understand how they function.

Photocells, or photovoltaic cells, are devices that convert light into electricity. They work in a way similar to solar panels – photovoltaic cells use the sun’s energy to produce an electric current. The difference between photovoltaic cells and solar panels is that photovoltaic cells can be used indoors or outdoors, while solar panels only work when they’re outside. Also, solar panels take in photons and develop a voltage to operate a load, whereas photo-cells take in light and remove a voltage, to seize the operation of a load.

Photocells are pretty simple devices when you look at them from the outside. They consist of a photo eye that takes in light, a photovoltaic cell that converts photon energy into electrical energy, and a switchable main circuit that allows and/or disallows current to flow through the device, to the load. They’re essentially a backward solar panel hooked up to a switch.

The photo-eye is a lens that takes in light and has a photovoltaic cell inside that converts energy from incoming photons to electrical energy in the semiconductor circuit. A lot of times when photocells age, or are exposed to prolonged moisture, the photo-eye becomes dirty and will no longer allow enough photon energy in to trigger the photocell to work. If the eye cannot be cleaned off a new photocell will be required.

A photoelectric eye takes in light from incoming photons

The photovoltaic cell converts the incoming light into electrical energy. The cell is made up of semiconducting material such as silicon that transforms photons from sunlight into free electrons via the photovoltaic effect. The photovoltaic cell typically converts wavelengths of about 700nm, but can also convert infrared and ultraviolet radiation. 

In the photovoltaic cell, photons with a certain wavelength can excite electrons in the cell, which are then collected by electric conductors or electrodes on either side of the semiconductor device to form a difference of potential. This difference of potential creates a voltage which is essentially the “control circuit” that causes the main circuit to open or close – similar to how a contactor or relay works. Since photocells work to open a circuit when photons are coming in, they’re considered an N.C. (Normally-Closed) device. The buildup of potential that occurs with photonic energy transfer actually opens the circuit rather than closing it. 

One thing to note is that after prolonged heat changes, or corrosive environmental conditions these cells can wear and become faulty. When this occurs, the only thing to do is replace the device.

120-volt photocell – Source: chadwellsupply.com

The control circuit is essentially an internal switch that controls whether the main circuit is open or closed. If the photovoltaic cell is not taking in energy from photons, causing electrons to move and form a difference of potential, the control circuit will remain closed, and power will flow through the device to turn a light on. To get the photocell to operate, it needs a light source to pump photons into the photo-eye. Doing so the photons begin to excite the electrons in the material of the semiconductor, then once a large enough difference in potential exists the photocell will open the main circuit, disallowing current to flow and the light will turn off.

The way we electricians test photocells in the daytime is by covering the photo-eye a few times with black electrical tape to simulate nighttime. At night we’re able to do the opposite – shine a flashlight in the photo-eye to simulate daytime. This is one tip/trick to test a photocell to see if it is faulty. A photocell will always fail in one condition, so it will not open and close when you simulate either daytime or night. Sometimes it will fail open, other times it will fail closed.

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