In order to understand the natural world we need to measure it. Access to scientific machinery has been historically limited to formal academic institutions and research laboratories. Now, we can all build measurement tools to conduct our own science experiments!
In its simplest form, a colorimeter passes visible light of known wavelength and intensity through a liquid sample and measures the change in the light as it passes through. Comparing this value to a substance of known characteristics (control) we can infer particular differences between the two.
Typically, colorimeters produce wavelengths of light in the visible spectrum and often include components native to human vision. The human eye has evolved to process intensities of red, green, and blue light and thus our colorimeter utilizes the same wavelengths to reliably match what is seen with what is measured. By calculating the amount of each particular color present in the sample, we can compute the apparent color.
Colorimeters are widely used to monitor the growth of a bacterial or yeast culture. They're commonly employed to measure and monitor the color in various foods and beverages, including vegetable products and sugar. Aside from general research laboratories, colorimeters have many practical applications such as testing water quality by screening chemicals such as chlorine, fluoride, cyanide, dissolved oxygen, iron, molybdenum, zinc and hydrazine. They are also used to determine the concentrations of plant nutrients such as ammonia, nitrate and phosphorus in soil or hemoglobin in blood. More applications of colorimetry can be seen in color printing, textile manufacturing and paint manufacturing for precise quality inspection. But this tool tends to be expensive. A minimalist and affordable version of a transmission based colorimeter is proposed here. This colorimeter is in its first version and is open for beta testing. The goal of this was to reduce the cost of the device to as low as possible and not use access prohibitive technologies. We first presented it during our workshop at Maker Faire Rome 2017. It ain't pretty (yet) but its data will be sexy! ;) As they say, never judge a book by its cover or a colorimeter by its shabby looks! Version #2 will definitely see a substantial diminishment of indoor mounting tape! :D Stay tuned for more updates!
Overview and Background
Theory Of Operation: What's A Colorimeter?
A colorimeter is a light-sensitive device used for measuring the transmittance and absorbance of light passing through a liquid sample. The device measures the intensity or concentration of the color that develops upon introducing a specific reagent into a solution.
The three main components of a colorimeter are a light source (of Red, Green and Blue), a cuvette containing the sample solution and a photocell for detecting the light passed through the solution.
The colorimeter is based on Beer-Lambert's law, which tells us that the absorption of light transmitted through the medium is directly proportional to the medium concentration. In a colorimeter, a beam of light with a specific wavelength (meaning a specific color) is passed through a solution all the way to the measuring device. This analyzes the color (RGB) compared to an existing standard (blank). A microprocessor then calculates the absorbance or percent transmittance. If the concentration of the solution is greater, more light will be absorbed, which can be identified by measuring the difference between the amount of light at its origin and that after passing the solution.
To determine the concentration of an unknown sample, we first prepare and test several sample solutions of a known concentration. These concentrations are then plotted on a graph against absorbance, generating a calibration curve. Afterwards, the results of the unknown sample are compared to those of the known sample on the curve to measure the concentration.
For more information on how to set up your own experiments and laboratory, or for further theoretical contexts, questions and/or proposals, visit us at binomicalabs.org or feel free to contact me directly! :)
An example of experimental cuvettes in various tests. | PLOS Biology | https://doi.org/10.1371/journal.pbio.2001413 (Published March 21, 2017)
Lesson Plan and Activity
Time To Build!
We'll assume you already have some familiarity with soldering and using Arduino. If not, which is totally fine and gives you a chance to learn so much more, check out Adafruit's guide to soldering (https://learn.adafruit.com/adafruit-guide-excellent-soldering/tools) and the following guide for Arduino Pro Mini users: https://learn.sparkfun.com/tutorials/using-the-arduino-pro-mini-33v :)
Step 1. Start by soldering the Arduino headers that come with it. Solder them as seen in the closeup image in Step 9. Be sure to add two header pins to the SDA and SCL pins too. Afterwards, upload the code to the Arduino Pro Mini. You'll find the colorimeter's code as a .txt in the downloaded zip file. Copy and paste it in your Arduino IDE.
Overview of the parts, after soldering.
Step 2. The OLED screen requires four wires. We'll keep the standard of black being GND and white being VCC. Then solder a purple wire for SDA and a brown one for SCL.
Step 3. Solder the 200 Ohm potentiometer to the anode (+) of the photodiode, in order to adjust the sensitivity of the light sensor and thus calibrate the range so the data values are mapped to the full response curve of the photodiode.
Step 4. Solder the two push button switches, RGB LED, and GND fork as shown. Be sure to make solder bridges across the pin headers so the entire segment becomes electrically connected. We call this the GND fork (a better image of the GND fork is seen in Step 22).
Step 5. Solder a white female jumper to one of the side terminals of the switch. Solder another white female jumper to the middle terminal of the switch but cut off the actual female header and strip the wire for later use. Lay the switch and wires across the exposed tape such that the switch is just at the edge of the battery holder. Add another piece of tape of equal length on top so it makes a sandwich that stabilizes the switch.
Step 6. Cut off the original connector to the battery pack. Solder a black female jumper to the negative terminal post. Solder one of the white switch wires (one without jumper) directly to the positive post. Connect the black jumper to GND and the white switch jumper to RAW.
Step 7. Grab your 3D printed cuvette holder body (left). Notice this is the back of the body.
Step 8. Front view of the cuvette holder body. Apply a half width piece of tape near the bottom, as shown. This will be an anchor point for the OLED screen to rest at a 45 degree angle later.
Step 9. Ensure the Arduino is firmly secured to the tape via strong downward pressure.
Step 10. Peel off the protective film from all the tape surfaces on the battery holder unit.
Step 11. Place the cuvette holder body close against the Arduino and apply even pressure to make a firm connection with the tape.
Step 12. Place the OLED screen as seen above at an approximate 45 degree angle (for adequate viewing). Ensure the screen adheres to the tape and the purple wire is facing upwards.
Note the screen is using both the vertical tape strip and the bottom tape surface as anchors.
Step 13. Peel off the front facing protective film from the tape squares.
Step 14. Adhere the buttons as seen above. Press firmly to ensure a proper connection.
Step 15. Turn the device around so you can see the pins and easily route the wires.
Step 16. Insert the RGB LED into the cuvette holder body as seen in the image above. Make sure the red lead is on top.
Step 17. Insert the photodiode with the blue potentiometer facing upwards.
Step 18. Bend back the leads of the photodiode and RGB LED carefully, such that no two exposed leads make contact. Try to not bend the leads back and forth too many times else it will break off.
It should look like this. Once all the components are seated where they need to be, we can begin routing and installing the wires into the appropriate pins.
Step 19. First we connect the RGB LED leads to the digital pins 9 (red), 8 (green), and 7 (blue).
Step 20. Connect the top wire of each button to the adjacent pins. Orange goes to pin 6. Yellow goes to pin 5.
Step 21. Connect the white lead of the photodiode to pin 4. Remove the tape backing from the rear strip and get your fork ready (not for food though, I'm sorry if this disappoints you).
Step 22. Press the GND fork onto the top central border of the tape strip. Be sure to leave the top pins sticking out so you can add wires to them later. A good method is to align the plastic part of the pin fork a millimeter below the top edge of the tape.
Step 23. Connect the GND fork to the other available GND (which is two pins away from the white photodiode wire). The GND fork will now act as a wire bus for the remaining ground connections.
Step 24. Connect the gray photodiode wire to the analog pin 0. Ensure it is A0 not the digital 0 pin. Or just make your life easier and count four pins down from the front left, now plug it into the fifth pin header and call it a day!
Step 25. This part is a little tricky and requires some dexterity. Connect the purple LCD screen wire to SDA and the brown wire to the SCL pin. These are the two isolated male pin headers above the main row of pins closest to the GND fork. Let purple and brown be forever alone, together! (You came for the colorimeter, but all you get is bad jokes!)
Things should look like this. Double check your wiring of the OLED screen so that you are not on the main row of pins, rather the two isolated pins behind them.
Step 26. Connect the white OLED screen wire to the VCC pin. (Count the pins!)
A bird's eye-view of the wiring. You weren't expecting birds here, were you?
Step 27. Add the black OLED wire and the yellow button bottom wire to the GND fork.
Step 28. Add the remaining ground wires that are still unconnected to the GND fork.
Closeup of the completed fork.
Step 29. Flip the switch to turn on the device and see if the system works! Fingers crossed!!
Step 30. Play with each of the buttons to see if they cause the machine to BLANK or READ. Yellow blanks, orange reads. (The colorimeter shown in the picture above has both its button wirings in yellow, merely because I don't like orange). All three LED colors should flash whenever either button is pressed. Check your wiring if the colors do not change or either of the three colors do not light up. Always start by blanking. Make sure the clear faces of the cuvette are in the light path. In order to show the LED lighting, these pictures were taken while using the colorimeter without its cap on. You'll find the .stl for the cap in your zip file. (The true reason behind the total lack of pictures showing the colorimeter's cap is that I forgot where mine was, so I needed to print a new one while taking pictures for this tutorial. Sorry!)
This will be your screen after blanking: RGB values, Transmittance (T) and Absorbance (A).
Step 31. Proceed with the reading (orange-wired button).
This will be your screen after reading: RGB values, Transmittance (T) and Absorbance (A).
Look at me still talking when there's science to do! You now have a functional and adorably tiny colorimeter ready to use. Congratulations! Have fun and remember to provide feedback!