What are Photocells and How Do They Work?

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Most electricians, heck even most PEOPLE, understand that actuating a light switch will result in the lights turning On and Off! That works great for inside of a room, but what about parking lots or the entire exterior of a building? In the latest episode of Electrician U, Dustin talks about Photocells- what they are and how they work.

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In essence, a photocell is a light switch, just with more moving parts. When you buy it off the shelf, a photocell is in a Normally Closed State. Normally closed means that with no external help, the circuit within it is closed and electron flow can happen. For reference, with a Normally Open device, without any external influence, the circuit is open and electron flow CANOT happen. With both of these, the control circuit operates a coil, that when energized, will change the state of the device to the opposite of what it is when it’s just sitting there. For most photocells, there are 3 wires- a black, a white, and a red. The white is the neutral for the control circuit, the black being the incoming hot of the control circuit, and the red being the switch leg.
The photons from the sunlight act as the catalyst here. During daylight hours, the photons change the resistor inside the photocell and it provides so much resistance that current cannot flow and the circuit stays open and the lights off. When the sun goes down, the resistance lessens in the resistor, current can flow and the lights come on.
Something to be aware of is that photocells have ratings- both to voltage and amperage. This means if you are attempting to have the photocell directly control a light fixture(s) that you have to match it with the circuit it is attempting to control. If you are controlling several fixtures (say a parking lot full of pole lights or an entire building exterior), having the photocell control a contactor may be necessary. Contactors generally have a much larger amperage rating than a photocell alone. In this type of scenario, the photocell is still controlling something, but it is now controlling the coil of the contactor. Same result, just more fixtures are able to be controlled.
There are also several different types of photocells to choose from. Your standard photocell, a photocell that has a knuckle so it can be precisely aimed, even small photocells that can be mounted IN a fixture (or right next to it in a bell box) and are hardly noticeable. Since photocells have “moving parts” they are susceptible to failure over time (much like GFCI receptacles or motion sensors). A great tip to troubleshoot a lighting circuit to see what the issue is would be to trick the photocell into thinking its dark outside forcing the lights to come on. To do this, simply use black electrical tape and completely cover the sensing eye of the photocell. If the lights come on, then it may be something other than the photocell being the problem. If the lights stay off, then the photocell is probably your issue. Another way to sort of test this theory would be to bypass the photocell from the circuit and see if the lights come on. If they do, then its probably a photocell issue.
Photocells must also be aimed properly in order to operate correctly. The sensing eye should be in full ambient sunlight (maybe not directly pointed at the sun!) during most of the day so those photons will continue to bombard the photocell and keep the lights off. So, watch out for shady areas, bushes/trees, etc. and move the photocell if needed.
We hope this has been an insightful look into photocells and their usage. Is there a topic you would like to see discussed here on Electrician U? Leave us a comment in the comment section and let us know. please continue to follow Dustin and Electrician U as we are constantly updating our content to assist our followers in becoming the best electricians that they can be.

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