Have you ever wondered how street lamps seem to know exactly when to turn on? They’re never too early or too late… they light up just as the sun sets and the sky blackens.
Of course, they aren’t operated manually. But surely they don’t work on a timer either since they switch on and off at slightly different times every day?
The answer is photocells, also known as dusk to dawn sensors.
The founding principle behind photocells has been around for longer than a millennium, so does it work with modern LED bulbs?
Standard photocells require a small current to operate during the daytime when the light is off. Because LEDs run at such a low voltage, this small current causes interference, flickering, and premature degradation. Therefore, LEDs require a different type of photocell than conventional bulbs.
If you’re the type of person who likes the lights to be on when you arrive home from work in the evening, this blog post is for you.
Just because you use LED bulbs does not mean you have to miss out on the convenience of dusk to dawn sensors.
Keep reading to find out everything you need to know!
What Is Photocell?
Photocells, dusk to dawn sensors, light-dependent resistors, whatever you call them, they all do the same thing.
Photocells are light-sensitive modules used to determine the on/off status of lighting based on ambient light levels. In simple terms, they are light detectors that automatically turn lights on at dusk and off at dawn.
They are most commonly found outdoors in parking, roadway, and security lighting.
They’re a great way of reducing energy consumption since they’re designed to turn lights off during the day when artificial lighting isn’t necessary.
But how do they work?
There are tons of different photocells on the market, but they all rely on the same principle. Much like LEDs, photocells are made from semiconductors, primarily cadmium sulfide.
They’re designed to respond to visible light. When the semiconductor is exposed to a certain level of light, an electric current is created, and the fixture is turned off.
But as the sun sets and light levels deplete the electric current stops, and the fixture is turned back on.
Make sense so far?
Good, then let’s delve a little deeper.
In essence, photocells are a subsidiary of the photoconductive effect, which is the idea that electricity can be produced using a beam of light. This principle was first discovered in 1887 by Heinrich Hertz.
Photocells work because when they are not near a light, they have a high electrical resistance of around 20 million Ω (ohms). Electrical resistance is a measure of opposition to the flow of electric current.
High resistance means that the flow of electric current is wholeheartedly rejected by the photocell, so the LED can work to its full potential.
Conversely, during daylight, the photocell has a low resistance of around 100 Ω. The sensor is buzzing with electricity, but the start-up initiator of the LED bulb gets nothing, preventing it from turning on.
Generally speaking, there are two main types of photocells. With plug-in photocells, the sensor is inside the occupied space, i.e., the bulb.
With line voltage photocells, the sensor is part of a larger circuit of fixtures and is likely to be in a different location to the bulb itself.
Are LED Compatible With Dusk To Dawn Sensors?
Let’s lay down the facts.
When photocells were first invented several decades ago, they were designed to work specifically with tungsten (incandescent and halogen) and ballasted (fluorescent and HID) bulbs.
It wasn’t until LEDs became popular in the early 2000s that people realized traditional photocells are incompatible with energy-efficient bulbs.
Due to how they work, photocells generate a small amount of electric current during the daytime when the lights are turned off.
Traditional bulbs operate at high voltages, so this small current has no impact, and the light stays off.
But LEDs require much less energy, so this small current causes interference and acts as a capacitive load.
Ultimately, this leak through voltage harms the electronics inside the LED. If paired with a traditional photocell, the LED is likely to start behaving erratically, flickering on and off, or prematurely burn out.
Thankfully, bulb manufacturers soon caught up and designed a photocell system that directs the small current away from the LED.
To summarise, old dusk to dawn sensors will probably be incompatible with LEDs, but newer models will be more flexible.
Can LED Lights Flicker With Incompatible Photocell?
If you’ve been paying attention, you’ll already know the answer to this question.
But to reiterate: yes, LED lights will flicker if paired with incompatible photocells.
This is because the low current of the photocell will interfere with the LED.
But there are also several other reasons your dusk to dawn LED lights may be flickering.
I’ve seen many people complain that their lights flicker during the twilight period before sunrise and sunset. This is when the lights are in the process of turning on or off.
Unfortunately, there’s not a lot you can do about this. If your photocells are adjustable, you could try playing with the parameters of light required to trigger the lights.
But once it becomes completely dark or completely light, the flickering will stop on its own.
Another thing to mention is that photocells are triggered by any light source – be it natural, artificial, or reflected.
If your photosensor is picking up on its own light, it may start flickering on and off. The easiest way to counteract this is to cover the photocell so that it can only be reached by natural daylight.
How To Select Photocell Sensors For Outdoor LED Lighting
I’m sure you can see that choosing a LED-compatible photocell is no easy job.
You can’t just go to your local hardware store and purchase the first one you get your hands on, a more considered purchase is required.
So what kind of things do you need to be looking out for?
First things first, you need to make sure that the sensor utilizes the modern photocell technology explicitly designed for LEDs.
You can check this by reading the sensor’s product specification sheet. This is usually listed under the ‘Rated Loading’ section.
Regardless of the type, photocells have three wires. In some instances there can be be the additional wire if the photocell comes with the timer.
Then, you need to check the voltage of the photocell matches the installation voltage of the LED. This 120-volt photocell from Amazon needs to be paired with a 120 volt LED bulb, for instance.
That’s a wrap! That’s everything you need to know about photocells and their relationship with LEDs.
You no longer have to worry about tripping over your garden ornaments when you arrive home after an evening with friends.
I suspect that the traditional photocell design will slowly be phased out over the coming years, in line with the decline of incandescent and halogen bulbs.
Did you know about the science behind photocells? Will you be purchasing dusk to dawn sensor for your LED lights? It would be great to hear from you, leave a comment below.