A Guide to LED lighting for Plants

This post is meant as a rough guide to building your own high powered LED lights. These lights are fairly versatile, not too complicated and provide a reasonably cheap option for a number of different plant growing situations.

This guide will focus on a light fixture that has three strings of LED’s: Blue, Red and a combo of White. They will be on/off with no dimming available – but I am planning to write up a guide to dimmable LED’s shortly (I’m waiting on new LED drivers to be delivered.).

Once built, we can use these lights by adding a main switch and turning on all the LEDs, make a switch for each string of LEDs or hook all or individual strings up to a set of relays connected to a microcontroller like an arduino. In the next lighting guide, we’ll get into the wiring and code to connect the lighting fixture to an ESP32 for PWM (Pulse Width Modulation) dimming.

WARNING:
This project will require you to work with mains voltage – which is the power that comes from your home wall outlets. In North America, that is 110-120 V. This can kill or seriously harm you.

Being careful, triple check your work, use a functional multimeter, and not drinking BL Limes while working will save you a lot of pain. Please be careful.

What you will need:

  • 1,3 or 5w LED mounted on starboard aluminum heat sinks
    • I purchased 3w LED’s in 4500k (warm white), 6500k (Cool White), Deep Red, and Blue. I purchased them from AliExpress. 20 of each came to about $25 CAD.
  • 3 Constant Current Drivers of various forward voltages.
  • 22 AWG Hook-up wire
  • Thermal Paste
  • Nylon screws, nuts and rubber washers
  • Aluminum angle stock
  • Hacksaw
  • 1/2″ bolts + nuts
  • Soldering iron, solder and associated tools
  • Drill with bits for metal work

Concepts to know:

  1. Your LED’s should have a datasheet available somewhere. For me, I found it on the AliExpress page where I purchased the LED’s.
    Check the datasheet for two important numbers:

    1. The current requirements. It should be something like 600mA or 1000mA. This is the maximum amount of current that can be handled by the LED. The higher the number the brighter the light.
    2. The forward voltage. This is often expressed as something like 3.4vdc.
  2. Drivers are specialized power units that ‘drive’ the LED’s. When selecting a driver we need to balance our voltage needs with the current needs. Our LED’s want a constant current, so that means that the driver will be constantly compensating the voltage in order to supply a consistent current across an unspecified number of LEDs.
    1. We should look for a Constant Current driver with a current rating appropriate for our LEDs. My LED’s are rated for 700mA, so I purchased a driver that was good for up to 600mA, but 700mA would have worked.
    2. Secondly, the driver will have a max voltage rating. Let’s say the max voltage is 60Vdc – well, we don’t want to run it at full power because that’s unsafe, so let’s say 55vdc. Now we can divide 55vdc by the forward voltage of the individual LED, which for the warm white, cool white and blue LED’s is 3.4vdc: 55/3.4 = 16.18
    3. So we can have 16 LED’s running on one driver.
    4. The Red LED’s have a lower forward voltage, their vdc is 2.4, so we can again divide 55 by 2.4 for a total of 22.91, so 22 LEDs. This is why you need to check the datasheet of the LED’s.
  3. Running LEDs in series vs parallel.
    1. We are going to be running our LED’s in a series. This means that we feed the positive lead to the positive side of the LED, the negative side of the LED then goes to the positive contact of the next LED. The last negative contact of the last LED in the string will connect to the negative lead of the driver. It should make one big loop starting and ending at the driver.
  4. Heat Diffusion:
    1. LED’s are very efficient, but they will lose energy through heat. The extent to which they generate heat depends on a few factors, but regardless – you should figure out a way to dissipate the heat because this is a main reason why LED’s burn out or operate at low efficiencies.
    2. This is why we are going to use aluminum angle stock – aluminum is a good conductor of heat. We’re also going to use thermal paste to make a highly conductive bridge between the LED and the Aluminum.
    3. You can also buy aluminum or copper heat sinks that have numerous skinny tines to dissipate the heat more effectively but I found these prohibitively expensive and now, having built the fixture, realize they were unnecessary anyway. More LED’s, a tight fitting enclosure or other factors may necessitate a proper heat sink.
      1. I am however, constantly on the look-out now for scrap aluminum or copper that could be used as a heat sink.

Plant Science:

  • In the past, aquarists used watts per gallon to figure out their lighting needs. As technology has progressed, this has become more and more outdated and ineffective. For one, lighting has become more efficient, so 1 watt of light from an incandescent bulb literally pales in comparison to a watt of light from an LED. For two, wattage only tells how much power is running through the light – not what spectrum of light you are getting. An older incandescent bulb will burn warm yellow, while some florescents will appear clinical blue/white. Not only will the amount of visible light differ between these technologies, the light spectrum being outputted will also differ.

  • Many people now focus on PAR, which is an acronym for Photosynthetic Available Radiation – which means, how much light is available for plants to use in photosynthesis. You can purchase a PAR sensor, but they are very expensive. We can also use Lumens as a unit – lumens should be included on the LED datasheet.
  • Plants use light to photosynthesize energy to grow. For most of history, plants grew outside and under the sun. Our sun gives off a massive amount of light…obviously. It is full spectrum white light – which means, it is comprised of all of the colours of the visible light spectrum, but to various amounts. Plants use different parts of the light spectrum for different things and use some parts more than others. This also changes depending on species.
    • For instance – plants tend to use a lot of the blue and red parts of the spectrum. They use very little green – this is why a lot of foliage is perceived as green. Because the leaves don’t absorb much of the green spectrum, it is bounced back and absorbed by the cones in our eyes.
    • This is also why high tech indoor growing facilities often use fusia or pink-ish lights (a combination of blue and red spectrums). Since the cost of energy is such a big factor in indoor growing, they prioritize the light that has the most impact on plant grow and leave out parts of the spectrum that add little benefit.
  • In our light fixture, we’re going to use red and blue LED’s to make sure these spectrums are strongly present, while using warm white and cool white to round out the spectrum and provide something that doesn’t look like a grade 12 acid trip disco lounge.

TO BE CONTINUED …