Introduction: 24 Watt LED Grow Light With Brightness Control

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About: Autistic person who's interests include utility cycling, recreational cycling, cycling safety, electronics, gardening, Arduino, and LEDs.

Food growing is one of my favourite hobbies because I am a big fan of organic foods and healthy eating. This Instructable will show you how to build an LED grow light with red/blue brightness controls to suit your growing needs and allow you to experiment.

LED grow lights are a relatively new method of growing plants. They are very efficient because they produce only the wavelengths needed for photosynthesis and very little heat. Most grow lights produce a lot of green light, which is reflected by the leaves. It costs less than $40 to make and doesn't cost much to run. Since it produces little heat, it's safer for your plants.

In this Instructable you will learn:

  • How the colour of light sources affects the growth rate of plants
  • How to make a grow light system with high power LEDs, PC heatsink, and other electronic components
  • Why it's crucial to drive high powered LEDs properly
  • Why a light meter is essential when designing a grow light system
  • How LED performance can be optimized
  • How to create an LED grow light system so that plants receive more light

Step 1: Things Needed

Greenhouse

  • Aluminum foil
  • Large box
  • Plastic board

Grow lamp

  • 2 x 3W royal blue LED (445nm)
  • 6 x 3W deep red LED (660 nm)
  • Heatsink with fan
  • Thermal paste
  • Epoxy
  • Solder (lead-free when growing edibles)

Note: You can get LEDs at e-Bay for as little as $2 each when you purchase them wholesale.

LED Driver

  • 1A fuse with clips
  • Resistors (0.33, 0.56, 1, and 100k ohm)
  • N-channel MOSFET (e.g. IRF540N) with heatsink
  • General-purpose NPN transistor (e.g. 2N3904)
  • Enclosure
  • Switches
  • 1A adapters (see below)
  • DC adapter socket
  • Wires

Finding the right voltage adapter
You can find adapters at a low price at used computer stores, second-hand stores, electronic stores, and eBay.

Tools that you may need:

  • Multimeter capable of measuring several amps
  • Light meter
  • Electric timer

Step 2: Grow Light

Use a Current Regulator
High power LEDs require a constant current source so that they last a long time. That means they'll need an LED driver. I used the MOSFET regulator below. Before soldering, you should test the circuit on a breadboard. The second diagram included the brightness settings. I used an on-off-on two-pole switch.

Mount to a Heatsink
These LEDs also require a heatsink, or they will get extremely hot. I attached them with epoxy. The heatsink I used can hold a maximum of 8 LEDs. You can strain relief the wires with hot glue. With the heatsink fan, the heatsink doesn't get hot.

Tie the Wires Together

Step 3: Housing for Plants

I used an enclosure to hold the grow light and increase the lighting with aluminum foil.

Step 4: Cost and Power Consumption

Watts Used by the System (High Setting)
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:
P=IV.

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
LEDs: $16
MOSFET with heatsink: $7
NPN transistors (per package): $1
Resistors (4 packs): $2
PCB: $0.75
16V laptop charger: Free
12V adapter: $3
9V adapter: $3
Switches: $2.50
Fuses: $0.60
Fuse clips: $1

Total: $37.30

Step 5: How Much Light Does This Put Out?

Measurement of Light Output
You can use a horticultural light meter to measure the light output objectively in the useful range.

Ways to Increase Performance
Use lenses
Use low footprint LEDs to pack more power into the same space.

Step 6: Other Tips

Adjust Length of Day
With an electric timer, you can actually "change the seasons" by adjusting the length of the dark period. Cool-season vegetables prefer approximately 12 hours of sunlight. Warm-season plants like roots and most fruits and flowers require longer days, like 16 hours. Please feel free to experiment.

Use a Heating Mat
Since grow LEDs produce very little heat, heating mats are useful in cold 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 were to red light for a shorter period than actual.

Step 7: Results With Photos

Step 8: Results II