Growing Something Great with Grow Lights


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If I had to vote for a hot topic of the moment, my vote would go towards horticultural lighting. The decline of price in Light Emitting Diodes (LEDs), an upswing in maker market mentality along with legislative change of mind has brought in a flood of “How do I do this?” and “Where do I begin?” type of questions into our engineering department. As a Minnesota Master Gardener and a technician at Digi-Key, this seems to be one of the more personally exciting topics I’ve chosen to research. By all means, I am not a botanist, nor an electrical engineer with a specialty in optoelectronics; but I do feel I have a good handle on topics within these realms. Hopefully this post and test results from my endeavors will point you in the right direction into the thought process to create appropriate lighting for your plants.

The Science Behind Light and Plant Response

You may recall from your high school science class the basics of chlorophyll, a compound in photosynthesis that allows plants to take up sunlight and convert it to energy to allow the plant to grow, prosper and produce.

There are two subdivisions of chlorophyll, named simply Chlorophyll a and Chlorophyll b . Chlorophyll b is optimized to capture sunlight spectrums at 460nm whereas Chlorophyll a is an expert at capturing energy at the 650nm spectrum.


Source: Osram White Paper: LEDs for horticultural lighting applications

Of course, there is more science to plants than just chlorophyll absorption. Different spectrums can induce different botanical reactions such as flowering, fruiting and plant development. Duration of exposure to light – called photoperiodism - can also make a difference. For example, a poinsettia requires 13 hours of uninterrupted darkness and 11 hours of bright light to produce those bright red leaves.

The advantage of utilizing LEDs instead of a wide spectrum light source is that LEDs allow for a custom recipe of spectrums to induce desired plant reactions. (Micromanagers, rejoice!) You’ll need to answer the question of what exactly you seek to accomplish with your lights as this answer will impact which LEDs you include into your design.

LEDs for horticultural lighting applications by Osram is a fantastic white paper and features a section detailing light spectrums for specific plant outcomes. Look for “Typical Lighting Recipes” on page 8.

LEDs are also knighted to be more efficient and have longer lifetimes than other light providing sources such as fluorescents and high pressure sodium lights.

Brief introduction to other lighting sources:

Fluorescent light fixtures

Fluorescent light fixtures are suitable sources of light for hobby gardeners mainly because of cost and ease. Hang the fixture, add the bulbs and it’s a done deal. These bulbs cascade a white light, which is composed of different wavelengths and therefore covers a wide spectrum:

500kfluor
Source: Color Temperature from Wikipedia: https://en.wikipedia.org/wiki/Color_temperature

Fluorescent lights are referred to as T12, T8 and T5. The letter T means it’s a tubular shape, and the number indicates its diameter in eighths of an inch (T5 = 5/8 inch; T8 = 8/8 or 1 inch and T12 = 12/8 or 1 ½ inch).

For many years, I have used two dual-bulb shop light fixtures along with a cheap mechanical timer to overwinter geraniums, succulents and start seeds. But for the commercial gardener other solutions need to be sought, mainly because fluorescent lights aren’t a great way to provide lighting over a wide area and are not as efficient.

High Pressure Sodium (HPS) lights

Used in larger greenhouse applications, HPS lights deliver high light intensity but no energy savings. They can be large, expensive and have other requirements such as warm up and cool down periods.

hps_lamp_in_gh
Source: http://horteng.envsci.rutgers.edu/conspics.htm

Designing your own LED grow light fixture

Starting from Scratch

The majority of what we have for this type of application are going to be surface mount LEDs that are located in our LED Lighting – Color category. For starters, you’ll need to select your preferred wavelength for your application. If you’re looking to create a general purpose grow light, aim to go with the chlorophyll producing spectrums 460 and 650nm.

But I don’t want to create a PCB…

Another option is to utilize a module that has the LED soldered directly to it. These types of modules will be in our LED Lighting - COBs, Engines, Modules section. Again, you’ll need to select the wavelengths that would be appropriate for your desired plant reaction.


LST1-01F06-RYL1-00 (Digi-Key # 1672-1131-ND) by Opulent Americas features a 455nm Osram OSLON LED Q65112A0793 (Digi-Key # 475-3273-1-ND) mounted on a starboard

These types of LEDs may be easier to work with, but you’ll still need to develop a schematic and pick an appropriate LED driver to your design. You’ll also have to keep thermal considerations in mind – when in doubt, heatsink the LEDs as much as possible.

Do you have anything more finished?

Another option that is a step closer to a finished product is a PCB LED board in the Digi-Key system marketed for a grow light application (part # 1121-1640-ND). This PCB is 12” long and includes 12x 660nm LEDs and 7x 460nm LEDs. The datasheet recommends driver 1121-1048-ND if using only one LED board, and 1121-1087-ND for driving two LED boards in parallel.


132541 (Digi-Key # 1121-1640-ND) by Thomas Research Products

Example Grow Light Design

I created a small grow light fixture using starboards along with a driver and an AC input cable, this covers a small 5” x 5” area and would be good for a single plant:

1 x 1866-1114-ND: LED Driver – 9-36V, 350mA $8
2 x 1672-1127-ND : 450nm COB starboard $7 each
2 x 1672-1129-ND : 660nm COB starboard $8 each
1 x Q114-ND : AC cable, 6’ $3

Total cost: $41 in parts

Additional Helpful information

Got Questions? Suggestions? Please comment below.


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Great heat sinking for 1121-1640-ND LED light bar is below.

Two fans of the 50mm size keep the heat sink cool. Since fans always blow toward the sticker side, it works well to mount each fan on both sides of the heat sink with a piece of VHB Tape The fans will pull external air and blow it down through the fins of the heat sink keeping it the proper temperature. Since the fans are 12VDC, a separate 12VDC power supply is recommended. Below is an example list of parts:

Click here for 987-1302-ND | 364-1254-ND | 3M9825-ND | 1121-1640-ND | 1121-1221-ND | ATS2189-ND | MB50151V1-000U-A99