Supplementary Grow Light for Carnivorous Plants

I started growing a Venus flytrap about 6 months ago. In the space from then to now, my collection has grown to include 2 VFTs, a few sundews, and a tropical pitcher plant. Unfortunately for me, I live in a large city on an apartment in that faces away from the sun; thus the plants cannot actually get any direct sunlight.

The plants are doing well in terms of growth and development, but the lack of direct light from the sun is causing them to stay green, and not develop the striking colors they are known for. So about a week ago, I decided to get a grow light for them. Unfortunately, the ready-made grow lights I've seen are either too large or too expensive for my purposes as the plants still do get natural light. This is when I made the decision to make my own!

The main things I needed for the grow light were

1. It needed to be sufficiently bright to illuminate the plants with at least a quarter of the the intensity of direct sunlight.
2. It needed to be compact, so it can fit in my small home office.

3. Bonus feature: It has to be powered by mains AC without being converted to 24V DC or lower. (either through an external or internal driver) This should reduce the waste heat from resistive losses.

It would be beneficial to understand how this grow light functions in more detail, so in the next section, the science behind the build is explained first.

TL:DR at the bottom of this section.

Venus flytraps and many sundews (genus Drosera) need plenty of sunlight to develop properly, and hence look nice. The tropical pitcher plant (genus Nepenthes) does not need as much light, but apparently is tolerant of the levels the other plants need.

Since the plants need a lot of light, it is a good idea to determine the illuminance of light falling onto the plants. Illuminance is usually measured in lux. To make this easier to understand, the illuminance of light that one wax candle produces, viewed a foot (30 cm) away, is approximately 10 lux.

In order to determine the correct lighting illuminance, I took an average of several readings of the illuminance in various situations. I used a lux measuring tool that can be downloaded onto a smartphone. Here are the average results:

• direct sunlight at noon: 50,000 lux
• direct sunlight on a horizontal surface, at 9 am or 3 pm: 12,000 lux
• building shade: 5000 lux
• my window with indirect sunlight: 4000 lux

Basing from these numbers, I decided to use 12,000 lux as the total illuminance needed. Since the illuminance on my window was about 4000 lux, an additional light source of 8000 lux was needed. Furthermore, we do not need an exact value because the illuminance of natural sunshine varies naturally.

Unfortunately, lamps are usually sold not with a lux rating but a lumen rating. This is an easy obstacle to overcome, as the lux is defined as the number of lumens falling per unit area, i.e. 1 lux = 1 lumen per square meter. In other words, the area where the light will fall needs to be calculated.

In this case, the plants are placed in a tray, so the surface area of the tray is a useful value to determine. The tray is 25 cm by 35 cm, which gives a working area of 0.0875 square meters.

The intensity of the light can now be calculated to be

(8000 lux) x (0.0875 sq m) = 700 lumens.

This is a minimum value, as this assumes that all of the light from the source falls onto the plants. Since some light will inevitably fall elsewhere, a light source of about 1000 lumens will suffice.

One more important idea: the light source must be one where the light travels in a limited direction, somewhat like a spotlight. This is to ensure that as much of the light produced actually goes into illuminating the plants, which decreases the amount of wasted light, i.e. electricity.

When I looked at my local hardware store, the best lamp I found was an outdoors waterproof floodlight that cost around 5 dollars, which was way cheaper than a professional grow light and has all the properties I needed.

TL;DR: Artificial sunlight needed to work with natural light. 

The needed lumen rating of the artificial light can be calculated by: 

Lumen rating = 8000 lux times the area of illumination needed in square meters 

OR 

Lumen rating = (8 lux times the area of illumination needed in square cm) / 10.

Parts (with photos)

• A cheap, thin aluminum baking pan
• 10W, 900-1000 lumen outdoor floodlight
• An AC power cord
• Some heat-shrink electrical tubing
• 8 or 12 pcs 6-mm wooden dowels, cut to size
• 3d printed corner pieces to hold on to the dowels
• Appropriate size screws
• Some string (if 8 dowels are used)

Tools

• Utility knife
• Screwdriver and Allen wrenches
• Drill bits and Dremel/rotary tool
• Pencil
• Ruler
• Soldering iron and solder or wire connectors.

Build Instructions.

1. Mark the center of the pan.
2. Open up the floodlight by removing the bolts and screws holding it together.
3. Use the floodlight's front bezel to mark on the pan where to put the holes for the bolts and the opening for the light.
   1. The opening in the tray should be the same size as the opening for the light.
4. Drill the holes for the bolts to go through, and cut out the opening for the light with the utility knife. The pan should be thin enough to cut with the knife.
5. Position the lamp onto the tray so that the bezel is inside the tray, and the main body is on the other side.
6. The bolts that kept the lamp together are now too short to reassemble the lamp. Use slightly longer but otherwise same-sized bolts to attach the lamp with the tray.
7. Connect the AC power cord to the lamp with either the soldering iron and heat-shrink tubing, or with the connector you are using.
8. Place the corner pieces on the tray and use them to determine the appropriate length of the wooden dowels. These will form the base of the grow light.
   1. OPTIONAL: Make a second base using these instructions. This second base will form the top of the grow light, where the tray will be put on.
9. Connect the side pieces (4 dowels) onto the base. These dowels can be cut to size so that the light illuminates the tray with minimal light lost to the sides.
   1. The top piece can be put at this stage as well.
   2. if a top piece is not built, the tray can be kept stable with some string. See the pictures to see how to attach the string.
10. Voila! All done!

Now, let's see if it works according to plan.

As mentioned in the Science section, the overall illuminance of the light on the plants must be at least 12,000 lux.

Using the Arduino Science Journal app, I measured the illuminance on the plants at 9 points on the tray to get an average value, as well as see whether there are spots where the illuminance is too low. The results were promising!

The data shows the lux levels at approximately 7 am. As can be seen the lowest reading of the illuminance with the supplementary light on is 11,000 lux, which is already almost equal to our calculated minimum. The average value is almost 15,000 lux, and the maximum (right at the middle of the tray) is 19,000 lux. In other words, the light we can provide for the plants is enough to simulate sunlight.

Please look at the images for the raw data.

Since the results are promising, this project is deemed a success. Nevertheless, there are still are a few possible modifications that can be made to improve the build:

• adjustable legs
• the use of reflectors and stronger lights for a purely indoor setup (no sunlight coming in)
• the use of a timer to sync the light with the rising and setting of the sun

If you'd like to test out these modifications, please let me know how it went!

It is also worth mentioning that this idea is scalable to a degree. Once a certain size is reached, a ready-made grow light might be the better option. If you are just growing the plants on a tabletop though, this DIY project is a good and fun alternative.

Thanks for reading this Instructable! if you have any questions or suggestions, I'm all ears (or eyes)!