24 Watt LED Grow Light With Brightness Control




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

Food growing is one of my favorite 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 fairly 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 is 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.

In this Instructable you will learn:

How the color of light sources affects the growth rate of plants
How a grow light system can be made from high power LEDs, PC heatsink, and other electronic components
Why it is important to properly drive high powered LEDs
Why a light meter is essential when designing a grow light system
How LED performance can be optimized
How an LED grow light system can be designed so that plants receive more light

Step 1: Things Needed

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
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.

LED Driver
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).
For example:
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)

Step 2: Grow Light

Use a Current Regulator
High power LEDs require a constant current source so that they last a long time. This means they cannot be connected directly to a power source. There are many options for LED drivers. 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. They were mounted 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. 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:

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
A light meter can be used. They measure wavelengths which are useful for plant growth.

Ways to Increase Light Output
Use lenses
Use SMD LEDs which take up less space on heatsink

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. 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.

Step 7: Results With Photos

Step 8: Results II



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36 Discussions


2 years ago

What is "overhead voltage"? I have tried searching the term and can't
find a definition anywhere. Google doesn't even recognize the term for
some reason and only gives results involving overhead power lines.


4 years ago on Introduction

How do I calculate the current passing through the LED? for example,

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

why the current for Red LED is 0.68 and for blue LED is 0.64

1 reply

Reply 3 years ago on Introduction

Me, confuse too, 3W red/blue led consume 600-700 mA from datasheet. math does not match


4 years ago on Introduction

Not only the instructable but the conversation that it is going on down here. Excelent!!!! Thanks Everyone!


6 years ago on Introduction

What can you suggest for growing a citrus tree indoors during the late fall to early spring?
Is 24 Watts enough and is your current configuration acceptable?

3 replies

Reply 6 years ago on Introduction

Citrus trees require full sun. In full sun, my light meter reads G to H but you will need lenses for similar lighting more than a few inches from the LEDs. Narrow angle lenses provide greater light penetration. For a small area, 24 watts might be enough. What kind of citrus tree are you growing?


Reply 6 years ago on Introduction

I have one meyer lemon tree which will grow its first fruit this winter and a persian lime that hasn't flowered yet despite being taller than the lemon


Reply 5 years ago on Introduction

Sure no problem.
Royal blue LED:

Deep red LED:


6 years ago on Introduction

Interesting review. Respect. But ... This is not the full range! My plants live a long time. They do not see the light from the window - only the LED. I use the 3 wavelengths, 445, 620 and 660 Nm. Important - the right combination of range, or a great result for the start and the finish of the collapse. You can use different combinations of range and get different results. I grow pelargonium, a caprice plant is very sensitive. However, 90% I was able to give the right light for the plants.
If you are interested, I can show my picture plants that all life under the LEDs.

Alex 77

6 years ago on Introduction

Why did you use 4 deep red (660nm?) + 2 bright red (630nm?)?


7 years ago on Introduction

does anyone know why when you use a power supply with higher or low voltage why the amps go up with higher voltage and down with lower voltage. It just makes it hard to get the amperage right.

3 replies

Reply 7 years ago on Introduction

In my grow light, changing the voltage had little or no effect on the current. You may want to check if you wired it correctly. I tested the circuit with a breadboard and measured the current before soldering.


Reply 7 years ago on Introduction

The N-channel MOSFET used here has a different pin configuration from that of the NPN transistor.


7 years ago on Introduction

do you know Apollo led grow lights from cidly? i think it is better than this.


7 years ago on Introduction

Thank you hanlin_y for the help. one more question in the schematic the led are wired in series would it be ok to do the led in parallel?