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I had been starting to work on this idea in 2018, being an extension of a previous project, a colorimeter. My intension was to use a e-paper display, so the colorimeter could be used as a stand-alone solution without the requirements for an external monitor, e.g. for class room or field applications.
I had some time to play on the project over Christmas vacations 2018/2019, but, while even a draft of the instructable had already been written, a few things I intended to do were still missing. Then I had to concentrate again on the job, had to finish my projects there and started in a new position in April. So I had not much time for silly projects for a while, and finally the project below became one of several ideas and concepts hibernating in my small “Bastelecke” (“tinker corner”?), being untouched since January 2019.
If it would not be for the “Finish it already” contest, this instructable might be still unpublished for years.
So as Pentecost 2020 is nearing now, I decided to make a just few changes to the draft instructable’s text and layout, and publish it.
And maybe I will find the time to build a housing for the device and perform these enzyme kinetics measurements I wanted to present someday. Or you will do that before me.
Happy Tinkering
In this instructable I would like to describe a small, inexpensive and mobile six channel photometer composed of a Raspberry Pi Zero with an Inky pHAT e-ink display, an AS7262 six color sensor breakout, a cuvette holder and some push buttons, LEDs and cables.

To assemble the device does not require much specialized skills or tools above the soldering of header strips. The device might be of interest for educational, hobby or citizen science applications and could be a nice STEM project.

In the configuration described here, instructions and measurement results are displayed on the e-ink display and on an optional computer display. The results of the measurement are also stored in CSV-files on the SD card of the RasPi, allowing a subsequent data analysis.

Instead of the Inky pHAT you could use other displays as well. But the e-ink display has a number of benefits, including very low power consumption and very good legibility even in bright daylight, allowing to build devices for in-field applications that can run for hours being powered by a power pack or batteries.

I am using the AS7262 six channel color sensor. This sensor measures the intensity of light at relatively narrow ranges (~40 nm) throughout the visible spectrum, covering violet (450 nm), blue (500 nm), green (550 nm), yellow (570 nm), orange (600 nm) and red (650 nm). This allows much more precise measurements compared to RGB-sensors as the TCS34725. A minor limitation is that a few areas of the visible spectrum, e.g. cyan, are not well covered. But as most dyes will have a wide absorption spectrum, this issue should not be too relevant for most applications.

The program is written in Python3 and uses the Adafruit Blinka and AS7262 libraries as well as the Pimoroni Inky pHAT and the GPIOzero libraries. It therefore should be easy to modify and optimize the script for your special application.

As several parts and concepts already have been described in previous instructables, I like to refer to these for some details or layout options.”

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