Light Meter To Measure Plant Growth Conditions

Synopsis

This project is an investigation into the effect of light intensity on the health of plants and people, or more specifically, gardens and gardeners.

I have spent my life moving back and forth between the north of the UK and the south, and even as far as France, and I have always been puzzled about the incredible effect that geographical location has on plant growth in my gardens, and on the physical health of myself and my friends.

As a child, I moved from Glasgow to Paris and back before the age of 9 and was acutely aware of the changes that I saw my own physiology and on the plant life around me. This same observation held every time I visited Scotland from East Anglia as an adult, through various seasons and changes of weather, and then reversed as I returned to work in the south. In recent years, as my long term friends and I have matured, I have become aware that certain more serious health problems are more common in the north, and that these are almost certainly brought on partly by lack of sun exposure. At the same time, I have now become a garden owner myself. I now struggle daily with very different gardening challenges in the south, from those of my garden-owning friends in the north.

Throughout this process, I have noticed that gardeners and dog walkers seem disproportionately insulated from the health problems associated with northern living. This is presumably because their hobbies force them outside every day, regardless of the weather, though there are many possible reasons for the correlation.

I know that the difference in human health and plant life, between north and south, is almost certainly to do with light levels. However, I am constantly frustrated by my own inability to accurately measure light intensity with my own eyes. I feel that if I could properly judge how much sunlight I have had in each day, I could make sure to get enough. I would then also be far better able to judge which spots would give a good home to which plant species in my garden. However, this is not an easy call to make.

The difficulty in judging light intensity is caused by the accommodation of the pupil of the eye. The pupils open wide to let more light in when it is dark, and close down to let less light in when it is bright. As a consequence, the light levels always feel about the same to us, and usually just about the right amount to let us see properly. However, just because there is enough to see, doesn't mean there is enough to make us feel good, especially in winter.

My hope in this project is to build a light meter that will enable me to accurately measure how much light there is in my environment in a range of different situations, and at different times of day and year. My hope is that this will better enable me to manage my own physiology and also my own garden, with much less guesswork.

Hardware

Visit the hardware summary page to find out about the construction of the light meter.

5 jumper leads, male to female

• One light sensor BH1750
• One Arduino Uno
• One Grove-LCD RGB Backlight v4.0
• Battery holder + wire to power the Arduino.
• AA batteries ideal, as rechargeables are easy to obtain. 6 x AA batteries are needed to give 9V. Four AAs is not enough to power the screen. In the end, I switched to using a rechargeable 9V battery.

The light sensor measures from 0 to about 55,000 lux, which is a very good range for measuring from the darkest room in a house to the brightest day outside.

Here are some initial images of the hardware.

Software

Visit the software summary page to find the code that runs on the light meter.

Installation, Maintenance and Testing Guide

Using the machine is very easy. Just plug it all together as shown in the hardware summary page. Plug into the computer with the Arduino's USB cable. Run the Arduino software and paste the code above into the code window. Then press the button (top left) to upload the code to the Arduino. Then unplug from the computer and plug in the battery and walk outside. The machine should automatically start displaying the light intensity on the LCD screen. The table below gives some example readings.

Maps of test areas

License

I have no claim on this design. The parts are all commercially available, specifically to do this, and I am just using them for my own fun.

Results

This project produced some very interesting information.

To cut a long story short, in Cambridge on an overcast day, it's just about 1900 lux outside, and only 35 lux inside at my desk.

That's compared to about 100,000 lux in high summer, and a recommended
level of 300 lux for screen work in offices.

There are a few things we can do:

1) Buy an SAD lamp and use it.

2) Move the desk close to as big a window as possible.

3) One of the major problems with getting outside for sunshine every day in Cambridge is that it's really cold. So it really makes a lot of sense to buy proper warm clothes, and make sure to get out every day.

Thanks again to the OpenPlant Fund. They provided the electronics kit as part of the 2017 Biomaker competition, and it was that box of kit that spurred me on to try this experiment.

Now roll on spring...