DIY Low-Budget Smartphone Photometer

From LED Photosensor via Smartphone to a Photometer

  • Smartphone photometer in the hands of the authors: f.l.t.r: Scheffler, Elsholz and Rodrigues Elsholz © Ulrich Scheffler and Heiko ThämlitzSmartphone photometer in the hands of the authors: f.l.t.r: Scheffler, Elsholz and Rodrigues Elsholz © Ulrich Scheffler and Heiko Thämlitz
  • Smartphone photometer in the hands of the authors: f.l.t.r: Scheffler, Elsholz and Rodrigues Elsholz © Ulrich Scheffler and Heiko Thämlitz
  • Fig. 1: main components (A), intermateable components parts (B) and complete photosensor (C) © Ulrich Scheffler
  • Fig. 2: photosensor in the housing © Ulrich Scheffler
  • Fib. 3: Display of the smartphone photometer; after blank measurement (A) and after first sample measurement (B) © Ulrich Scheffler

A LED photo sensor, designed as a kit and assembled more than 100 times by school groups, has been fundamentally revised. In the new version, the light detector is now a component of digital electronics and is connected to a Wifi- (WLAN-) enabled microprocessor, which transmits its data to a smartphone. At approx. 30 €, the price of the required components is quite moderate, and instead of the previously required multimeter, now any commercially available smartphone can be used.

This article describes a photometer that can be produced in “DIY” (do-it-yourself) fashion at a low budget, even by people with little previous knowledge in the fields of electronics, computers and software. This photometer is used in student experiments in a project called “Schullabor mobile Analytik” (mobile analytics laboratory for schools). The combination of a self-built sensor with a smartphone has sparked enthusiasm at the previous events, and has stimulated the motivation of students to deal with photometry.

The basic components of a photometer are a light source, a monochromator, a cuvette and a light sensor as well as a system, which processes the measurement data and, if necessary, visualizes it. Monochromatic light is sent onto a liquid sample, and its absorption, transmission or absorbance is measured. A description of the frequently used variants can be found at “photometry-compendium” [1].

The Photosensor

The photometer presented here is a further development of a previously described LED photo sensor [2]. The light source is a LED which already emits monochromatic light, thereby rendering the expensive monochromator unnecessary. The previously used analogue photosensor has been replaced by a light intensity sensor with a digital interface. This new sensor TSL2561 (manufactured by TAOS) is characterized by its highly dynamic sensitivity range, its integrated 50/60 Hz suppression to eliminate intensity fluctuations of the ambient electrical light, and its very low energy requirements. The TSL2561 has also proven its worth for photometry in a reference project by Oliver Happel [3]. In conjunction with a Wifi-enabled microprocessor, a simple data transfer to computers is possible.

To avoid complex soldering work, only components available in modular design have been selected.

Therefore, only a small number of soldering operations (mainly on the adapter board) are necessary to connect the main components, shown in Fig. 1A (LED, light sensor and microprocessor) to intermateable component parts as shown in Fig. 1B. The LED is then plugged into the main module with its face to the light sensor. Depending on the application (wave length), a corresponding LED is used (Fig. 1 C).

The photosensor can be used without housing. However, as ambient light might disturb the measurement, it is better to surround the device with a housing. For this purpose, there are inexpensive options, such as no longer required plastic packaging. However, for the described setup, a housing design is also available as an STL file for printing on a 3D printer. Fig. 2 shows the photosensor fitted into such a housing.

A detailed description of the soldering work and further information on the housing production is available from the authors upon request.

From Photosensor to Photometer

For data processing and transmission, it is necessary to install software on the microprocessor module WeMos D1 mini. The module is connected to a PC via the USB interface, and a program in the Arduino IDE code of about 400 lines is compiled on the microcontroller. This software (available on request) accomplishes the reading of the light sensor data, the conversion of the data into a LUX value, and the calculation of an absorbance value from two LUX values (e.g., blank value and sample value). After the software installation, the photosensor can be separated from the PC and can be used in conjunction with a smartphone, tablet or laptop as a mobile photometer.

Initiating the Photometer

For an initial test, a powerbank or a regular power supply is connected to the USB port of the photo sensor. The built-in LED should light up for approx. 5 seconds and then turn off again. The photosensor sets up its own Wifi network. A smartphone, tablet or laptop can now be connected to the photosensor network and access the web page of the photometer via a web browser. A web page with two control buttons and a table is displayed (Fig.3).
Inserting a cuvette with water or the blank solution, and pressing “Leerprobe” (this means “blank”) performs a measurement to determine the luminous flux I0 and enters the data into the table. Next, insert a cuvette with standard or sample solution and press “Probe X” (German for sample X); X represents the sample number, which is given consecutively. By pressing the button, another measurement is carried out to determine the luminous flux I and the data is entered into the table once more. The software calculates the absorbance using A = log (I0/I) and presents the value in the table.

Determining the Level of Silicate

An example for an application is the measurement of silicate in drinking water. Sotriffer [4] describes further reactions for easily transferable parameters such as nitrate, nitrite, ammonium, phosphate and carbonate, among others.

For the silicate measurements, three reagents were prepared, each freshly.

Molybdate solution: Fill approx. 150 mL of pure water into a 250 mL volumetric flask, add 4 mL conc. sulphuric acid and 7.5 g of hexa-ammonium hepta-molybdate, and fill the flask up to the mark with ultrapure water and shake.

Oxalic acid solution: Transfer 11 g of oxalic acid to a 250 mL volumetric flask and fill up to the mark with ultra-pure water and shake.

Ascorbic acid solution: Transfer 4 g of ascorbic acid to a 100 mL volumetric flask and fill up to the mark with ultra-pure water and shake.

Four calibration standards with 1, 2, 4 and 10 mg / L silicate were prepared from a 1 g / L stock solution. One mL of each of these solutions and 0.5 mL of the reagents listed above were placed in a single use cuvette, which was then covered with parafilm and shaken upside down. After a reaction time of 10 minutes, the blue-colored solutions were measured, using a classical spectrophotometer at 880 nm and by using a smartphone photometer using a red LED. The absorbance values were 125 (100), 217 (171), 495 (397) and 1202 (950) mAU (milli Absorbance Units). The values in brackets are those measured with the smartphone photometer. They are, as expected, slightly lower than those measured at the optimum wavelength, but are nonetheless well-suited for the measurement of silicate in the concentration range indicated, as the measurement of a water sample shows. The result of measuring silicate levels in the tap water in Hamburg-Bergedorf was 8.70 mg / L with the spectral photometer and 8.78 mg / L with the smartphone photometer.

Acknowledgement
The authors like to thank York Zöllner for the support in the translation.

Authors
Olaf Elsholz1, Tereza Cristina Rodrigues Elsholz2, Ulrich Scheffler3

Affiliations
1Laboratory of Instrumental Analytical Chemistry, Hamburg University of Applied Sciences, Faculty of Life Sciences, Department Environmental Engineering, Hamburg-Bergedorf, Germany
2 UEMG (Universidade Estadual de Minas Gerais), Campus Ituiutaba, Ituiutaba, Brazil
3Laboratory for Bioprocess Automation, Hamburg University of Applied Sciences, Department Biotechnology, Hamburg-Bergedorf, Germany

Contact
Prof. Dr. Olaf Elsholz

Head of Laboratory of
Instrumental Analytical Chemistry
Hamburg University of Applied Sciences
Faculty of Life Sciences
Department Environmental Engineering
Hamburg-Bergedorf, Germany
olaf.elsholz@haw-hamburg.de
 

A detailed description how to build this device you find by clicking here or on the "Read Topstory-PDF" button on top of this page

 

References:

[1] Kusserow A.: GIT Labor-Fachzeitschrift 7 /2015, 27 (2015); Photometry-Compendium

[2] Elsholz O. und Rodrigues T.C.: GIT Labor-Fachzeitschrift 6 /2005, 519-520 (2005)

[3] Happel O.: Analytische Methoden mit dem LED Photometer. AATIS-Rundschreiben Sommer-Herbst 2015, 20-23 (2015)
www.aktuelle-wochenschau.de/main-navi/archiv/chemie-und-licht-2015/kw40-bau-eines-led-photometers-und-seine-einbindung-in-den-schulunterricht.html

[4] Sotriffer A.: Umweltchemie im Schüler/innenexperiment unter Einsatz kostensparender Mikromethoden. IMST-Fonds, Wien (2009),

The Project Photometry-Compendium

Description DIY Photometer (German spoken): http://bit.ly/Ph-DL

Contact

Hamburg University of Applied Sciences

Hamburg-Bergedorf

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