I’m going to describe how I built the LED light pictured at the bottom of this post.
I have gone through a couple of iterations of this design….this bumbling provides the brain food that inspires me to evolve. Besides, every once in awhile you make a comment that helps grow my point of view. I thank you for that. Your knowledge and wish to share that knowledge inspire me.
Thanks to Those That Went Before
- The “Greenhouse and Horticultural Lighting Summer 2016” extension course offered by Michigan State University. The course gave me insight into how plants use light, what wavelengths are “best” for plant growth, and how much light plants need to grow.
- Chris Gammell’s Contextual Electronics course. I had absolutely no experience or background before participating in Chris’s course. I found I learned quite well through Chris and his approach to learning electronics.
Here is the BoM for the LED lamp:
|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|
|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|
|DC-DC Buck Converter||DROK||B00BYTEHQO||1||7.99||7.99||7.99||7.99||7.99||7.99||https://www.amazon.com/gp/product/B00BYTEHQO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1|
|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|
|Power Supply||eBay||(Used/HP)0957-2271 32V||1||12.05||12.05||12.05||12.05||12.05||12.05|
|Thermal Heat Sink Tape||Amazon||11||0.01||0.10||0.01||0.10||0.01||0.10|
I didn’t spend time trying to get the “best” prices. Or find the “best” part for the job at hand. Rather, I cobbled together a bunch of parts that so far works ok. Couple of points:
- I’m reusing what I have on hand. The CPU heat sink I actually used was equivalent to the one on the list. I had bought the one I used several years ago. It is no longer sold. I have several Meanwell LDD-700H LED drivers with throughole pins. I already had thermal heat sink tape.
- The power supply is one used by many of the HP printers. I got a used one from eBay.
- The LED lamp needs power for both the CPU heat sink’s fan as well as the LEDs. I use a DC-DC buck converter so the 32V power supply can provide 12V of power to the CPU heat sink.
LEDs emit at different wavelengths. As I discussed in a previous post, red and blue wavelengths provide the most efficient energy for plant growth. I discussed factors that go into determining how much red versus blue light is needed. In that post, I decided that 100% red light would be my choice. Since then, I decided to add some blue with the red, somewhere between 90R:10B and 75R:25B. I decided to use 3W Cree XP-E2 red and blue LEDs.
- They are designed with horticulture in mind. There is a data sheet on the quality of the light generated by the LED SMDs from a horticulture point of view.
- They are known to be high quality.
- They are available at Digikey. I am a very happy customer of Digikey.
- I wanted control of the location and number of LEDs used.
Quantity – Daily Light Integral
The number of LEDs to use is dependent on the DLI.
I liked the grower’s perspective of DLI given in this post: The grower can think of DLI as the plant’s daily “dose” of light… I’ll modify this perspective: The grower can think of DLI as the plant’s daily “dose” of PAR. In his article, Light Wavebands & Their Effects on Plants, Dr. Runkle explains the important light wavebands and their effects on a plant’s growth. There’s a few other concepts to know about before we figure out the DLI we’ll need:
- PPF: The quantity of PAR emitted by an LED per second. The PPF is useful to give an idea of how much PAR is being pumped out by an LED. However, the amount of PAR that smacks into a plant leaf is depended on the distance the plant leaf is from the LED. I am grateful to Cree for making the info on PPF available in this document.
- PPFD: Becomes the important quantity number for determining DLI with LEDs because it is the actual measurement of how many photons that are within the PAR wavelength range smack into the plant leaf.
DLI Value To Be Used
Based on the recommendations made in this document, I decided the DLI should be around 10 mol·m-2·d-1. As we have been discussing, DLI is the cumulative amount of photons within the PAR wavelength. How much should the LEDs be on? 24 hours? Something else? In the Michigan State class, we learned the time period the light is on (i.e.: the photoperiod) can be a factor in the healthy growth and/or when flowering happens. I’ve decided on a 20 hour on / 4 hour off time cycle. After some experimentation, I decided 9 red LEDs and 2 blue LEDs will work with the bucket I am using to hold the nutrients.
Here is an image of me measuring PAR:
The PAR meter was about 10” from the LEDs.
Formula for Calculating DLI
Spectrum gives us the formula to calculate the DLI when the PAR is fixed:DLI = PAR reading x (3,600 * photoperiod)/1,000,000
- The PAR reading’s unit is μmol/m2/s. The DLI’s unit is mol/m2d. This means the calculation has to go from µmols to mols and from seconds to days:
- 3,600 seconds in an hour
- 1,000,000 µmol / mol
- photoperiod = 20 hours
These are the (instantaneous) PAR readings and DLI values using a 20 hour photoperiod:
As expected, PAR/DLI readings fall off as the measurements are made away from the center.
The fan on the CPU heat sink needs 12V. The LEDs will be connected together in a series. The power requirements = 9*FV of Red LEDs + 2*FV of Blue LEDs. I’ll be running the LEDs with a constant current of 700mA. From the Cree XP-E2 data sheet:
- blue ~= 3.3V
- red ~= 2.4V
Putting the Heat Sink and LEDs Together
I soldered each LED onto an 8mm LED PCB then mounted them onto the heat sink using thermal tape. I explored making a PCB – and did one to test. However, I decided to use the 8mm PCBs so that I could easily arrange them onto a heat sink – as many and in whatever pattern I wanted to try. I used a hot air gun and solder glue when soldering the LED PCB.
I arranged the red LEDs in a 3 x 3 matrix. The blue LEDs are placed within the 3 x 3 red matrix. I then used a soldering iron to solder silicon wire between the LEDs. I used silicon wire because it can withstand the heat that is generated by the LEDs.
The 8mm LED PCBs have indicators which end is the cathode and which is the anode.
Footprint in XP-E2 Data Sheet
Front and back of Red XP-E2
Turn on the light
The LEDs are connected the the LED driver. The LED driver is connected to the HP power source. The CPU fan keeps the temperature down around the LEDs. To run the 12V fan from the 32V power source, the power source is connected to the DC-DC buck converter that adjusts the voltage to the CPU fan plugged into the other end down to 12V..
and here’s the light:
Sure is ugly :-)! But…it has been working. There is certainly many places for improvement.
Thanks for reading this far. Please find many things to smile about.