An Affordable Open-Source Turbidimeter

Turbidity is an internationally recognized criterion for assessing drinking water quality, because the colloidal particles in turbid water may harbor pathogens, chemically reduce oxidizing disinfectants, and hinder attempts to disinfect water with ultraviolet radiation. A turbidimeter is an electronic/optical instrument that assesses turbidity by measuring the scattering of light passing through a water sample containing such colloidal particles. Commercial turbidimeters cost hundreds or thousands of dollars, putting them beyond the reach of low-resource communities around the world. An affordable open-source turbidimeter based on a single light-to-frequency sensor was designed and constructed, and evaluated against a portable commercial turbidimeter. The final product, which builds on extensive published research, is intended to catalyze further developments in affordable water and sanitation monitoring.


S1. Introduction
This Supplementary Material provides additional detail on the construction of the device outlined in the article "An Affordable Open-Source Turbidimeter". The sections below detail the necessary parts and tools, wiring, assembly, and microcontroller programming. Readers are encouraged to contact the corresponding author with any questions about the construction, operation, or expansion of this device.

S2. Component List
All components of the open-source turbidimeter are indicated below, along with potential distributors. Indicated costs are for individual pieces and do not include shipping and handling or taxes. Please note that the parts list and diagrams presented below represent the design for the open-source handheld turbidimeter evaluated in the main article, and differ slightly from the most current model hosted at https://github.com/wash4all/open-turbidimeter-project. Please visit this site for updated parts lists and assembly instructions.

S3. Structural Components
The open-source turbidimeter case consists of four different parts: (1) bottom, which includes ports for button and on/off switch; (2) cuvette holder, which houses the sensor and light source and holds the glass cuvette during device operation; (3) top, which has openings for the seven-segment LED display and the top of the cuvette holder; and (4) battery lid, which slides out from the case assembled case top and bottom to expose the battery holder. Electronic files of these objects (in STL format) are available upon request. We wish to note that any container that provides good shielding from incidental light may suffice (e.g., a nested pair of cardboard boxes).

S4. Tools
A utility knife, soldering iron, and solder are required for assembly of the open-source turbidimeter, and safety goggles are recommended. To replicate the printed four-part case, one must have access to a 3D-printer with black ABS filament. The microprocessor can be programmed via a USB-to-Serial adapter (Mouser carries one [#A000059] for $14.95) or with an Arduino (as described at http://arduino.cc/en/Tutorial/ArduinoToBreadboard). Electrical tape is very helpful (for grouping wires together), as are a pair of wire strippers.

S5. Turbidimeter Breadboard and Schematic
The microprocessor and connected components of the open-source turbidimeter are depicted below in Fritzing ( Figure S2) and EAGLE formats ( Figure S3). Figure S2 depicts what the device looks like internally, while Figure S3 is a more formal schematic.  Table S1 and Figure S3).  Table S1 and Figure S2).

S6. Basic Assembly Instructions
1. Gather the components listed in Section S1, and the tools listed in Section S3. 2. Print the parts described in Section S2 (or devise your own light-shielding case). 3. Wire together the internal components of the open-source turbidimeter according to Figures S2 and S3. Here are some key points: a. It's hard to see some connections of the circuit, such as the wiring of the voltage regulator (VR1), on Figure S2. The Arduino website has a well-illustrated tutorial on wiring the ATMega328P microprocessor (http://arduino.cc/en/Main/Standalone) that the reader may find useful. b. The semi-circles on the microprocessor (IC1) and shift register (IC2) indicate orientation of these components. It is vital that chips are aligned properly using these guides. Wiring them in the opposite orientation will destroy the chips once voltage is applied! Please note that the semi-circle on the TSL230R sensor in Figure S2 is merely a visual aid, and is not present on the actual chip. c. The pins of the seven-segment display are diagrammed by the distributor Electrodragon (http://www.electrodragon.com/w/index.php?title=7-Segment_Display). The linked document maps pin placement to function and representation in Figure S3 (e.g., the bottom left pin on the seven-segment display is pin 12, which maps to segment E of the display, which connects to port QE on the shift register). d. Resistors R2 and R3 combine to form a voltage divider, which is used to measure the voltage provided by the batteries. To achieve useful results, use resistors with tolerance of 1% or less. Tolerance levels of 5% are suitable for the other resistors in the device. e. The internal components list generally should be followed closely, however the power switch (S1) and button (S2) can easily be changed to suit the builder's aesthetic.
f. As always, exercise caution. Do not attempt construction of any electrical device without knowledge of proper technique and safety. 4. Test open-source turbidimeter by placing water sample in glass cuvette, inserting glass cuvette into the device, and analyzing. Note: The experiments described in the article were conducted with quartz cuvettes, which may improve optical clarity but at great expense (~$16 each). If using borosilicate cuvettes, it is advisable to calibrate the turbidimeter manually, rather than using the calibration constants provided in section S7. When using borosilicate glass cuvettes, it is particularly important to visually inspect the cuvette for scratches, and to always measure turbidity with the cuvette inserted into the sample chamber at the same orientation (this is good advice even with quartz cuvettes). The easiest way to do this is to nick the lid of the cuvette with a knife, and align this nick with the centerline of the turbidimeter case.

S7. Programming
The following program, written in Arduino-C, runs the assembled open-source turbidimeter. Connect the device to a computer using a properly wired USB-to-Serial adapter (this requires the use of an additional 100nf capacitor, see http://arduino.cc/en/Main/USBSerial) or an Arduino board (tutorial given at http://arduino.cc/en/Tutorial/ArduinoToBreadboard). Then install the Arduino programming environment (http://arduino.cc/en/main/software). Open a new "sketch" (empty program) and past the code below into it, staying mindful of any formatting quirks that may result from the article formatting process. Compile the new sketch and upload it to the microprocessor.