FeAtHEr-Cm spectrometer

If you’ve been following (and I know one or two of you are), then you know that FeAtHEr-Cm is my Adafruit Feather microcontroller-based approach to building scientific instrumentation for the chemistry teaching laboratory. Not only does the platform allow for inexpensive instruments to be distributed throughout a classroom (at under $50/unit, each student in an analytical chemistry lab could have their own potentiostat), but the instruments are designed so that students can understand what makes them tick.

The btm100-alpha version

Joining the team is the btm100 which is a spectroscopic instrument designed to perform turbidity and nephelometry experiments. These techniques help scientists explore heterogenous solutions by measuring their cloudiness, and the techniques are used widely in fields such as environmental analysis. As an added bonus, the response from turbidity/nephelometry measurements mimics that of absorption/fluorescence measurements which are commonly covered early in the chemistry curriculum, so we have a fine opportunity to build on fundamental concepts (Beer’s Law) while expanding the suite of tools students are exposed to.

So what does the btm100 have to offer? Here’s the annotated version:

btm100-alpha with the components highlighted. The Adafruit Feather M4 Express is the microcontroller underneath.

At its core is a dual op amp (the MCP602) which serves two purposes: to convert current from a photodiode into a voltage and then to do some filtering of that signal using a Sallen-Key second order active filter. The properties of the filter can be adjusted by selecting different passive (capacitor/resistor) components. There is also a passive filter that can be activated by a tactile switch. Both of the filters are tied to analog inputs to the microcontroller in order to explore the strengths and weaknesses of these two approaches.

The four-terminal wire block provides power to an LED source. To minimize potential damage to the microcontroller, the source, which can be switched on or off by a digital pin on the microcontroller, is triggered through a transistor. The end user can alter the brightness of the source either through pulse width modulation of the signal or adjusting the current limiting resistor. The other two terminals on the wire block accept an LED as the detector. The idea here is that LEDs can serve as wavelength-selective detectors, so matching the source LED with the detector LED can greatly diminish interference from other wavelengths.

Lastly, there are two terminals that allow the user to utilize the filters with other detectors. This feature isn’t necessary to operate the turbidity meter, but may find application in future projects.

In addition to learning about signal amplification and filtering, students will get experience with 3D printing the sample compartment.

But the electronics are only half of the story. In this module, I anticipate that students will explore digital fabrication strategies, namely the design of a sample holder. Turbidity measurements are made with the source and detector at a 180 angle (assuming the vertex is the center of the sample). Nephelometry measurements are made at 90 or 135 degree angles. The current cell design allows the user to rapidly change the angle and switch to different techniques. Students will learn how to create the current design and have the opportunity to explore modifications.

Example data – the blue line shows the amplified signal of LED (930 nm) light transmitted through a slurry of 0.05 um alumina particles. Green trace is the filtered signal. Instrument programming and data visualization is done with the Mu editor here.

There’s still plenty of work to do on this project, and in fact, if you look closely, you’ll notice that there’s something funky going on with the op amp. It turns out that I made some pretty silly mistakes when transcribing the circuit from my breadboard to the PCB layout. Fortunately, a workaround involves just a bit of chip hacking (literally) and creative soldering. Once I’m satisfied with the board, I’ll get to software design and course content.

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