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LED grow lights are a fairly new method of growing plants. They are very efficient because they produce only the wavelengths needed for photosynthesis and very little heat. The wavelengths used are red and blue. Most grow lights produce a lot of green light which gets reflected by the leaves.This grow light costs up to $40 to make and doesn't cost much to run. It won't burn your plants even if you put them very close to it.
Plastic corrugated board
4-3W royal blue LED (445nm)
2-3W deep red LED (660 nm)
2-3W bright red LED (630 nm)
Heatsink with fan
Solder (lead free when growing edibles)
Note: You can get LEDs at e-Bay for as little as $2 each when you purchase them whole sale.
1A fuse with clips
Resistors (0.33, 0.56, 1, and 100k ohm)
N-channel MOSFET (eg. IRF540N) with heatsink
General purpose NPN transistor (eg. 2N3904)
1A adapters (see below)
DC adapter socket
26 gauge wires (Correct sizing for current ratings can be found here)
Finding the right voltage adapter
You can find adapters at a low price at used computer stores, second hand stores, electronic stores, and ebay. To supply enough voltage, you add up the LEDs' forward voltage with the required overhead voltage (0.6V here).
Supply voltage for red LEDs=6*2.5V+0.6V=15.6V
Supply voltage for blue LEDs=2*3.8V+0.6V=8.2V
Note: Fuses and wires may cause a small drop of voltage so you may want an overhead voltage of at least 1V. The voltage difference is larger with thinner or longer wires. It can be significant if the current is high..
Multimeter capable of measuring several amps
Light meter (optional)
Electric timer (optional)
High power LEDs need a constant current source so they won't burn out. As long as you are giving them the right amount of current, they can last for 50,000+ hours. Too much and they will fry instantly. If you vary the voltage by 10%, the current may vary by 2 fold! As temperature increases, their forward voltage decreases and current increases. This means they cannot be connected directly to a power source. With an ideal current regulator, the output current will not change regardless of the input voltage.
There are many options for LED drivers. I used the one in the diagram below because it is cheap and needs about 0.6V of overhead voltage. Most popular IC linear regulators like the LM317 requires a few volts overhead but they are simpler to use. Buck or boost converters are great but they can be pricey.
This project requires some electronic skills. Before soldering, you should use a breadboard and measure the current. Expect the current to increase a bit after soldering since soldered parts have less resistance. For 3W LEDs, the recommended current is usually 700 mA. For 1W LEDs, 350 mA.
The second diagram was used for my grow light with high/low settings set to 0.68A and 0.3 A. I used an on-off-on two pole switch and I labelled the photo of the switch to make it easier to follow. From light meter readings, 0.3A was considered shady for plants and you can simply lower your plants from the LEDs. A fuse was also included in case something goes wrong which is unlikely. 16 volts was used for the red LED string and 9 volts for the blue LED string.
Mount to a Heatsink
These LEDs also require a heatsink or they will get extremely hot. They were mounted with nylon screws, thermal paste and epoxy. If the screws are too loose, you can glue them into the holes. The heatsink I used can hold a maximum of 8 LEDs. The wires can be glued to the heatsink with a glue gun to keep them from breaking. With the heatsink and fan, the thermal pad remains cool and the heatsink rarely exceeds 24°C at room temperature.
Tie the Wires Together
Make sure your transformers are not overloaded or overheated. I personally turn off the grow lights when I leave the house for a long time. If you smell smoke or if it doesn't work, immediately turn it off and check the circuit again. Do not look at the LEDs while they are on.
The grow light had been mounted to a detachable plastic board using bolts, nuts, and washers so you can make changes easily if you ever want to. Disposable chopsticks or bamboo stakes may be used to support it. Twist ties were tied to the heatsink and plastic board to keep it from collapsing. To keep the circuit board enclosure from skidding, you can use adhesive velcro.
Red LEDs: 14.55V x 0.68A = 9.89W
Red LEDs with driver:16.13V x 0.68A 10.97W
Blue LEDs: 6.98V x 0.64A = 4.47W
Blue LEDs with driver: 10.24V x 0.64A = 6.55W
Watts used by grow light: 17.5 W*
Cost to run the grow light: 17.5W x (1kW/1000kW) x $0.10 per kWh x 16 hours x 365 days per year =$10.22 per year
Watts Used by System (Low Setting)
Red LEDs: 13.13V x 0.32A = 4.20W
Red LEDs with driver = 16.19V x 0.32A = 5.18W
Blue LEDs: 6.28V x 0.31A = 1.95W
Blue LEDs with driver:10.64V x 0.31A = 3.30W
Watts used by grow light: 8.48 W*
Cost to run the grow light: 8.48W x (1kW/1000kW) x $0.10 per kWh x 16 hours x 365 days per year =$4.95 per year
*Power supplies are excluded.
For conventional 250W grow lights, the cost is about $146 with similar schedules.
Measuring Power Consumption
The power consumption can be calculated by measuring the voltage across the circuit and measuring the current across the switch while the LEDs are off and solving the equation:
If you want to know the LED's voltage drop, measure the voltage across the LED's. The current across the LED(s) is similar to the current across the entire circuit because the resistance of R1 is very high. Note that the power dissipated by LEDs is not always equal to the labelled voltage.
Cost of the Grow Light
MOSFET with heatsink: $7
NPN transistors (per package): $1
Resistors (4 packs): $2
16V laptop charger: Free
12V adapter: $3
9V adapter: $3
Fuse clips: $1
A grow light meter can be a good investment. You don't need to buy an expensive one. Our eyes are not very reliable at predicting how bright a light source is because their sensitivity peak is 555 nm and chlorophylls peak is at 450 and 660. Based on the light meter readings, if you are growing sun loving plants, they should be half a foot or closer, or further away for shade loving plants. Try measuring other light sources with a light meter and compare it with their wattage ratings.
Light Meter Readings at a Given Distance
1 inch: G
12 inches: C
Ways to Increase Light Output
LED's light output can also be improved by keeping them cool. Longer wavelength LEDs tend to be very sensitive to heat and produces less light at higher temperatures which means they produce even more heat. Shorter wavelength LEDs are more resistant to heat.
Most 3W LEDs are driven at 700 mA. Some are rated for 2A of pulsed current. If you want to get more light, you can drive them at higher current if they are properly heatsinked. The main advantage of doing this is that fewer parts are used, lowering the cost and the use of space. The lifespan and efficiency may be somewhat reduced. Before doing this, check the specifications. Pushing it further can be very tempting but it will not produce more light but will produce too much heat and shorten the lifespan.
Lenses can also be used to concentrate the light. If your plants are close to the LEDs, try using 60 or 90 degree lenses.
If you want to add more LEDs to your heatsink, you can use surface mounted LEDs since they take up less space. This would be useful if you have larger growing spaces or denser plants.
In the pictures below, there are charts of eye response spectrum, photosynthesis rate vs. spectrum, output spectrum of common grow lights, and Cree's LED output vs temperature. There is also a chart showing the luminous flux vs current grouped by temperature.
With an electric timer, you can actually "change the seasons" by adjusting the length of the dark period. For cool season vegetables, 12 hours of sunlight is recommended. Warm season plants like roots and most fruits and flowers require longer days like 16 hours of sunlight. Feel free to experiment.
Use a Heating Mat
Since grow LEDs produce very little heat, heating mats are useful in cool conditions.
Use Far Red (730 nm) LEDs
If you can find far red LEDs, you can try using them to control flowering. When exposed to far red at the end of the day, plants would think they have been exposed to red light for a shorter period than actual.
Give Plants Direct Sunlight
Certain vegetables like red lettuce are actually known to produce more antioxidants in response to UV radiation. Shortly before harvesting, you can bring them out in the sun to increase the nutritional value.
Mount the Regulator to a Larger Heatsink
Even with a small heatsink attached, the regulator can easily exceed 85°C when there is too much overhead voltage. This is hot enough to damage wires. The regulators can share the heatsink with the LEDs without electrical insulation if it is large enough and if they do not share the same power supply. You need to do this only if it gets too hot. If you want to skip this, you can add a 1 or 2 ohm power resistor in series with the LEDs or direct the heatsink fan at the regulators. The voltage dropped by the resistor is V=IR.