Applied Electronics: Construction of a Simple Spectrophotometer

Abstract

In this paper we describe the construction of a simple spectrophotometer intended for use as a capstone circuit for an electronics laboratory exercise. Students in the undergraduate laboratory have learned basic circuitry and are now able to read a schematic and construct an instrument that has practical applications in chemical analysis. A simple LED is used as the source and is easily changed to correspond to different species being studied. Detection is accomplished through the use of one of three signal transducers (a photoresistor, a photodiode, or a photodarlington) and a current-to-voltage converter. A box lined with black felt is used to shield the spectrophotometer from ambient light. This spectrophotometer shows excellent Beer-Lambert law linearity over a large concentration range and has been used in the laboratory to determine the concentration of a metal cation in solution on the basis of a calibration curve. Of the transducers used, the photoresistor showed the best response at multiple wavelengths with error in concentration determination not greater than 6%. Through the building and testing of this circuit, the students can visualize applications of electronic circuitry.

Review of Student-Built Spectroscopy Instrumentation Projects

“…Many low-cost devices are constructed from cardstock or cardboard boxes, − ,,,− but other options include plastic ,,− or aluminum housings, ,, wood, − PVC pipe, or professional optical mounts. , A unique device using a graduated cylinder permitted careful investigation of the effect of path length on absorbance . More recently, LEGO blocks (Figure b) ,,,− and 3D printing (Figure c) ,,− have become popular.…”

“…In general, LEDs have lower cost than photodiodes do but are also less precise and less sensitive. , LDRs are generally even more sensitive in the visible range than photodiodes, and phototransistors are more sensitive still . Student comparisons of a photodiode, LDR, and photodarlington suggest that the LDR produces the most accurate results . If possible, students should evaluate multiple detector options (experimentally or through reading and research) and justify their choice based on the need for sensitivity, precision, and accuracy.…”

An accurate, precise, and affordable light emitting diode spectrophotometer for drinking water and other testing with limited resources

“…Educators and their students at the high school, college, and university levels often build simple LED spectrophotometers as part of the chemistry, physics, or electronics curriculum [8][9][10][11][12][13][14][15][16]. These simple LED spectrophotometers are very useful for education; however, they usually have simple power, source, and signal conditioning circuits, which limit their performance.…”

Measuring device for optical properties of plant leaves

“…There are commercially manufactured devices available on the market. Yet many researchers prefer to design and manufacture them on their own, for example, a low-cost, low-tech colorimeter with an interchangeable LED [13]; a more advanced LED-based spectrometer with a total of seven wavelength selections [14]; spectrophotometer [15] and photocolorimeters of various design [16; 17] for educational purposes. The available measuring devices for the optical properties of solid objects, which include the plant leaves, have certain weak points.…”