Photocells and Photosensors
Commercial LED light fixtures manufactured by Jarvis Lighting are typically compatible with a variety of controls and sensors, including photocells, photosensors, motion sensors and dimming controls.
Introduction to Photocells and Photosensors
Photocells are one of the most common lighting control methods. The most common photocell functions like a switch. The photocell is installed on the same circuit as the light fixture. Photocells are often mounted directly to a light fixture, mounting through a knockout or hole created in the luminaire’s housing. Power to the circuit is left on. When the photocell detects sunlight, the photocell switches the light fixture off. When no sunlight is detected, the photocell turns on the light fixture.
Voltage Compatibility for Photocells and Photosensors
When selecting a photocell, the most critical specification is voltage. The correct voltage is determined by the existing circuitry, not the light fixture. For example, if a building’s power runs at 277V and the LED light fixture is rated at 120-277V, a photocell rated for 120V cannot be used. In this instance, a photocell rated for 277V (including photocells rated for 120-277V) would be correct for this application.
Button Photocells vs. Stem Photocells vs. Locking Type Photocells
The three main varieties of photocells are button, stem and locking. These differences are mostly mechanical. Button style and stem style can often be used interchangeably and can be installed on most light fixtures or into a weatherproof box on the same circuit. Locking type photocells only work with light fixtures which have a receptacle specifically designed for a locking photocell.
Button photocells mount on the inside of a light fixture or on the inside of a weatherproof box. The photocells mount through a through-hole or knockout. They feature a short stem which is exposed to outside. This stem is how the button photocell senses surrounding light levels. Benefits of using a button photocell include a clean, flush installation appearance and lower production costs due to their simple design. Button photocells are not waterproof and must be mounted inside a light fixture or weatherproof box. Button photocells are recommended to only switch one light per photocell.
Stem photocells mount to the outside of a light fixture or weatherproof box. The photocells feature a threaded stem and mount through a through-hole or knockout. The photocell’s circuitry, including sensor, is enclosed and located entirely on the outside of the light fixture or weatherproof box. Stem photocells are typically slightly larger than button photocells. Benefits of using a stem photocell include increased durability (due to increased size). Some varieties of stem photocells feature an adjustable sensor so they turn off earlier or later. Stem photocells can typically be used to control several light fixtures and ensure that all luminaires switch on simultaneously.
Locking photocells typically install by twist-locking into a special receptacle located on the top of a light fixture. This style is used typically for street lights and certain dusk-to-dawn lights. Locking photocells typically have a large physical size then button or stem photocells. Excluding municipal lighting applications, this style photosensor is the least common. Benefits of locking style photosensors include increased durability and ease of serviceability.
Information about Dimming Technology
Before LED technology, old technology dimmers operated a bit differently than LED dimmers. There are a variety of older designs that operated in a few different manners but the most common setup was the simple resistance dimmer. These older dimmers limited the power going to the entire light fixture, which in turn dimmed the light level. You could take a regular bulb (hooked up to a regular switch) and then change the switch to a dimming switch and it would now dim. There were no communication wires and nothing was ‘talking’ to each other.
LEDs operate differently. There are a few different dimming styles but commercial lighting in general (and all 0-10V dimmable Jarvis luminaires) operate on what’s called ‘0-10V Dimming’. For purposes of this explanation, we are talking about 0-10V dimming.
The key difference between a non-dimming and dimming LED light fixture is the power supply. With both types of lights, electricity is not limited (via a resistance dimmer) to the fixture. The fixture (and power supply) receive full power when switched on. A non-dimming power supply then provides full power to the LEDs. A dimming power supply, when dimming, limits power to the LEDs so that the LEDs dim.
Before getting into the differences of a dimmable fixture, it is important to go over some basics about non-dimming LED light fixtures. A regular LED fixture has a power supply and the power supply is hooked up to its electricity source. This power supply has 4 wires (and sometimes a ground wire bringing the total to 5). Two are the input wires (getting power from the circuit panel) and two are the output wires, which send power to the LEDs.
0-10V Dimming – Purple and Grey Wires
A 0-10V dimming power supply typically has 6 wires (sometimes 7 if you include the ground the ground.) It has the two input wires, two output wires, and two dimming communication wires, which are purple and grey. Inside the power supply, there is control that adjusts the level of power going to the LEDs. This control senses how much power is coming into the dimming communication wires and then adjusts power accordingly.
The dimming input is simple but not obvious. The 0-10V input is backwards – lower voltage equals more brightness. If 0V is going into the dimming communication wires, then the light fixture lights up 100%. If 2V is going into the dimming communication wires, then the light fixture dims approximately 20%. If 8V is going into the dimming communication wires, then the light fixture dims approximately 80%. The one exception is 10V. 10V just means ‘maximum dim’ which sometimes is 100% (completely off) and sometimes is about 85-90% dim. This is because large LEDs become hard to control at low brightness levels and different power supplies are either better or worse at controlling them.
The light fixture needs a dimming controller to create the 0-10V signal. This is done in two main ways.
Option 1 – 0-10V Dimming switch
When a light fixture is installed with a dimming switch, there are 4 wires coming from the dimming switch to the light fixture. There are two ‘input’ wires which are only switched on/off. This dimming switch does not limit any power to the whole light fixture. There are also the purple and grey dimming communication wires, sometimes known as the 0-10V wires.
This option is easy for new construction – just run 2 extra wires. For retrofits, it becomes difficult because you have to run two extra wires.
Option 2 – Motion sensor with dimming and light-sensing built in
This is used for both new construction and retrofits. Either inside or next to the light, a ‘smart’ motion sensor is installed. Note – there are a ton of ‘smart’ motion sensors and not all of them work this way but many of them do, so it is important to make sure that the correct type is being used.
This sensor is hooked up to a power source (which is typically the same power source the light fixture uses.) The sensor detects motion activity and outside light levels and then sends a 0-10V signal to the LED power supply. The LED power supply then adjusts its brightness level based on the 0-10V signal. This is easier for retrofits because it eliminates the need to run wires far (or at all if the sensor is inside the light.)
These sensors can typically be programmed so the dimming and motion sensing is optimized. A typical setup for an LED parking garage installation includes light sensing and motion sensing and can be described as follows. The sensor detects outside light during the day and switches the light fixture 100% off. When outside light is not detected at night but no motion is detected (because nobody is around) the light fixture lights up approximately 30% of full brightness. When motion is detected, the light fixture increases brightness to 100%. When motion is no longer detected, the light fixture would then dim down to the 30% brightness level. Some sensors can be programmed to dwell for a period of time after motion is no longer detected.
Different sensors have different capabilities. Contact a Jarvis Lighting representative for more information about specific sensor operation or sensor compatibility.
Notes about this Information:
This information is provided to be a practical guide. Some of this information is simplified to make it easier to explain. While the information provided is generally accurate, it should not be relied upon for any specific application without consulting Jarvis Lighting. Please contact our team at (800) 363-1075