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I got feedback from my earlier LED lamp prototype asking why not just use a “white” LED instead of monochromatic red/blue. After all – I was told – the efficiency of white LEDs have improved over the years.  

The Goal

The goal of this post is to describe my latest LED lamp prototype that is based on using a fuller spectrum LED instead of just using monochromatic LEDs of red and blue on the plants.

Thanks To Those That Went Before

I am learning a lot about growing agriculture in a controlled environment by taking Dr. Kubota’s course: Greenhouse Plant Physiology and Technology.  Thank you for the fascinating and useful knowledge you are guiding us to learn.

I enjoy and learn from the hydro subreddit.  A special thank you to SAG for sharing his (I assume his because G is for “Guy”) knowledge.

Challenges with Using Just Red and Blue

Here is an image of the red and blue hydro build I made for growing basil:

BasilUnderRedAndBlueLED

Eerie looking, right?   The light that is emitted is not a very welcome site for the family to gather and talk around.  On the other hand, the red/blue combo makes sense from an efficiency perspective – particularly if a plant’s use of light considers only chlorophyll.

Chlorophyll

And as noted by Dr. Runkle in his article, Red Light and Plant Growth,

Red Light and Plant Growth

The primary reasons why LED fixtures emit a lot of red are 1) red LEDs are among the most efficient at converting electricity into photosynthetic photons, 2) chlorophyll strongly absorbs red light, thus it is effective at photosynthesis, and 3) red LEDs are relatively inexpensive. 

Dr. Runkle goes on to explain why blue LEDs are sprinkled in with the red LEDs:

Many plants grown under only red light, such as plants grown indoors under only red LEDs, have a stretched, elongated appearance; the leaves are thin and large and plants become tall. In most cases, plants grown under only red light do not have desirable growth characteristics.  However, when a relatively small amount of blue light is added to red light, extension growth of plants is inhibited. Therefore, plants grown indoors with 80 to 90 percent red light and 10 to 20 percent blue light are quite compact, with smaller leaves and shorter stems. 

Here is an image I took of the basil plant I am growing using my current red/blue LED setup:

LargeBasilLeafMostlyRedLED

It is the biggest basil leaf I have grown…although I am new to paying attention to my gardening results in any detail.

The biggest pros of my red/blue setup:

  • I have tight control on the exact wavelengths (quality) and quantity of red/blue LED lighting.  This tight control allows me to see for myself how the red/blue affects plants.
  • I learned a lot building it.  From figuring out which SMT LEDs to use, to getting the tiny MCPCBs (Metal Core Printed Circuit Board), to using a CPU heat sink, etc…gathering the parts and putting the pieces together is an accomplishment I am proud of.
  • I absorbed more about using LEDs as the sole source of light for plant growth.  
  • An abundant amount of herbs and veggies can be grown with just red/blue LED lights.

My biggest challenges include:

  • The combination of red/blue is not pleasing to our eyes.  I’m growing plants in my kitchen/home.
  • While my DIY efforts work, I am concerned with replicating even in small batches for the rest of my family.  Besides potential robustness problems, they are not aesthetically pleasing.  
  • It takes me several hours to put the LED lamp together.  In some ways the time spent is relaxing, but I question if it is the best use of my time.
  • I listed out all components to make the red/blue LED here.  In the BoM below, I took out components for the lamp as well as the buck converter.  It turns out the CPU heat sink is “good enough” without needing a fan.  The price for just the red/blue LED is $47.40.
Red/Blue LED BoM:
Type MFG Part Quantity 1 Piece Price 1 Piece SKU 100 Piece Price 100 Piece SKU 1,000 Piece Price 1,000 Piece SKU Link
Red LED Cree XPEBRD-L1-R250-00601 9 1.9 17.1 1.679 15.111 1.679 15.111 http://www.digikey.com/product-detail/en/XPEBRD-L1-R250-00601/XPEBRD-L1-R250-00601CT-ND/4111303
Blue LED Cree XPEBRY-L1-0000-00J01 2 1.72 3.44 1.518 3.036 1.254 2.508 http://www.digikey.com/product-detail/en/XPEBRY-L1-0000-00J01/XPEBRY-L1-0000-00J01CT-ND/5806152
CPU Heat sink aliexpress Shenzhen JuHome Tech. Co., Ltd. 1 7.98 7.98 7.98 7.98 7.98 7.98 https://www.aliexpress.com/item/Free-shipping-gift-CoolerMaster-A73-Intel-Socket-478-cpu-radiator-cooling-fan-CPU-Fans/754035046.html?spm=2114.10010108.1000014.8.NKtiQK&scm=1007.13338.47797.0&pvid=c5106c1d-87f3-4f22-926d-928d14a84bf5&tpp=1
Heat Lamp Woods B001KDVY42 1 12.16 12.16 12.16 12.16 12.16 12.16 https://www.amazon.com/gp/product/B001KDVY42/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
Silicon wire eBay hdjkhdjk1 0.05 1.62 0.08 1.62 0.081 1.62 0.081 http://www.ebay.com/itm/282110938768
8mm PCBs Micro Commercial Co 1N4448W-TP 11 0.14 1.54 0.0816 0.8976 0.02719 0.29909 http://www.ebay.com/itm/282110938768
LED Driver Meanwell LDD-700H 1 4.99 4.99 4.24 4.24 3.992 3.99 http://www.ledsupply.com/led-drivers/mean-well-ldd-h-series-cc-step-down-mode
Thermal Heat Sink Tape Amazon   11 0.01 0.10 0.01 0.10 0.01 0.10  
TOTAL         47.40   43.61   42.24  
 
Indeed, not only is the 3000K LED pre-assembled, at $16, it costs far less.
 
3000K LED BoM:
Type MFG Part Quantity 1 Piece Price
3000K 1,500 lumens flood light Cree 810048029488 1 $16.00
TOTAL       $16.00

My Latest LED Lamp Prototype

 

 For my latest prototype, I am using a flood light that has a 3000K light quality.  

 I chose the 3000K light quality because the spectrum quality of light appears to be a pretty good match to the McCree curve.

The McCree Curve

Yes, red and blue are very efficient….but note photosynthetically active radiation includes not just red and blue – there are other activities going on within a plant leaf that is triggered by light.  McCree’s work brilliantly shows this. McCree showed if a single leaf is analyzed as a whole system (i.e.: besides chlorophyll, there are chloroplasts and carotenoids…as well as other actions I am not that familiar with…), the entire PAR range is used.

Flood Light

I got the most appropriate light from my local Home Depot – the Cree 810048029488 PAR 38 flood light.  

Cree90wfloodlight

Once I got home I had a chance to look at reviews of this bulb.  The bulb gets low reviews because too many have burnt out after several months.  It should work fine for the prototype.

The packaging states the LED uses 18W and has a light output of 1,500 lumens.  The cost was $16.  The screw fitting (E26 medium) is the most common size used on bulbs in the US.  This makes it easy to use many different lamp bases.

Lamp

I found a desk lamp that was no longer used and made a clamp for it with my 3D printer.  

3DPrintedClamp

Talk about a YIPPEE! moment…I thought of using the 3000K LED light and sometime later the same day I was taking the PAR value and starting to use it on some tomato seedlings. 

Using my PAR meter,

TakingPAR ReadingOfFloodLED

 I took the following readings (μmol/m2/s):

PAR Values for Flood Light

 

 

Whoa..the PAR values exceed what I was getting with my first LED setup.

 

Now it is time to grow plants with this light.  I’m starting with some tomato seedlings.  I may switch over to growing a leafy herb or vegetable.

 

 

Thanks for reading this far.  Please find many things to smile about.

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