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1 Construction of a Photometer As an Instructional Tool for Electronics Construction of a Photometer as an Instructional Tool for Electronics and Instrumentation Robert L. McClain Supporting Information I. Curricular Context We use this experiment in our “Chemical Instrumentation” course, which is considered an advanced analytical chemistry course. Most students in the course are junior and senior chemistry majors, but we do get students, both undergraduate and graduate, from other departments. The course pre-requisites are first semester analytical, two semesters of organic, and one year of calculus based physics. The topics covered in the course are chemical separations, mass spectrometry, spectroscopy, electrochemistry and electronics. The laboratory experiments we use are timed to coincide with the lecture coverage of the topics. The students have one three hour lab session per week with all the students doing the same experiment each week. We do not use formal laboratory reports, but students complete a laboratory report sheet as they work on their experiments. The reports sheets contain the student’s data, calculations, plots, tables, observations, and answers to questions. We feel the report sheets help students think about what they are doing while they are doing it, as opposed to working through the procedural part of the experiment during the laboratory, and doing the analysis and thinking at home. Each laboratory section has eight students and the students work in pairs. This requires our lab to have four identical experimental stations. We do not use any high-end, user friendly, commercial instrumentation in the laboratory, but instead use a “modular instrument” approach. Each station is equipped with a monochromator, various light sources, a photomultiplier tube detector, a charge coupled device detector, a sample compartments, a potentiostat, a home built lock-in amplifier, oscilloscope, function generator, digital multimeter, various power supplies, and a computer with LabView software. We also have hot plates, stirrers, glassware, etc., which we use for separations and sample preparation. The experiments are all “hands-on” with a goal of getting students to think about chemical instrumentation at a fundamental level. The construction of the photometer fits well with our 1 overall approach, and provides a great introduction to practical electronics in the context of chemical instrumentation. II. Experimental Procedures including detailed circuit diagrams Day 1: Making a photometer with an LED light source and a CdS detector in a voltage divider Students begin by making a voltage divider and confirming Ohms law for their circuit according to the circuit diagram shown in Figure 1. The voltage divider is a fundamentally important circuit that is often used to introduce basic direct current (DC) electronics. In this activity, the students learn to use a digital multimeter to make resistance, voltage and current measurements and how to lay out circuits on a breadboard. R1 1k Vs=12V Vm R2 1.5k R2 Figure 1. The voltage divider circuit used to introduce students to Ohms Law. Students replace the 1.5 k Ω resistor with a cadmium sulfide (CdS) photoresistor to make the detector circuit for the first version of the photometer. They construct the light source with a green LED and a 470 Ω resistor as shown in Figure 2. The resistor limits the LED current to about 25 mA. They complete the photometer construction by placing a sample cell between the source and detector as shown in Figure 3. We make the cell holders with a small Keck clamp epoxied to a piece of printed circuit board that is also epoxied to a 16 pin integrated circuit socket. 2 470 1k Vs LED Vs=12V V CdS m Figure 2. The LED light source and CdS photoresistor circuits. Figure 3. The completed CdS detection based photometer After getting the photometers working, the students measure the output voltages for a series of standard chromium (VI) solutions complexed with the diphenylcarbazide and a water blank. The output voltage is used to calculate the resistance of the photoresistor from the voltage divider equation, and since there is an inverse relationship between resistance and light power, a Beer’s law plot can be generated. R − log D(DI ) ∝ Absorbance . )1( R D(sample ) 3 Days 2 and 3: Making the photometer more sophisticated with AC filters and operational amplifiers Over the next two laboratory periods the students construct a more sophisticated version of the photometer as shown in Figure 4. The design contains 6 different sub-circuits providing students with experience working with resistors, capacitors, diodes, a transistor, and operational amplifiers. Each sub- circuit performs a basic function for the device. The students use an oscilloscope to measure the voltages at important points in the circuit in order to confirm the circuit is working, to help see and understand how the sub-circuit functions, and to troubleshoot problems in their circuitry. 100k rectified signal 10u +12V sample cuvette 10k + 10p A oscillator out 1N4148 10k 100k photodiode -12V 470 4 100nF 10k 10k -12V LED - 6 -12V 4700 pF -12V 2 OP27 - - + 2 + OP27 OP27 DC out to ADC - 100k 7 3 47k + + 6 +12V + + 741 2N3904 3 + +12V + -12V +12V 7 4 +12V I to V converter high pass filter 10k 22k active half wave rectifier active low pass filter -12V relaxation oscillator transistor current amplifier Figure 4 . A photograph and circuit diagram for the completed photometer. 4 Students begin the circuit construction of the more sophisticated design by replacing the CdS detector from Day 1 with a silicon photodiode. Light striking the photodiode detector induces a current that is proportional to the incident light power. The current is converted to a measurable voltage with a standard operational amplifier I to V converter as shown in Figure 5. The I to V converter is one of the most widely used circuits in chemical instrumentation and is the basis for the pre-amplifier circuitry in most optical and mass spectrometers and potentiostats. The photodiode in our detector circuit is reversed biased, which improves the linearity of the detector circuit. The students adjust Rf, to get an output voltage near 1.0 V when the LED light hits the detector. This usually requires a resistor between 100k and 1 M. I f Rf V-- 4 cathode - Vout 2 6 PD I op27 sh + + anode 3 7 V++ -12V Figure 5. The detection circuit made with a silicon photodiode and current to voltage converter The detector responds to both the LED light and the background room light. To discriminate against the background light, the students modulate the LED light source near 1.5 kHz and add a high pass filter to the detection circuit. Students construct a relaxation oscillator using an open loop operational amplifier, as shown in Figure 6, to modulate their LED. The output current of the operational amplifier is insufficient to drive the LED, so a transistor relay is added to provide the current required to drive the LED. Students begin with a 1 µF capacitor for C to get the oscillator to work at about 2 Hz, which is visually seen in the flickering of the LED. After replacing the 1 µF capacitor with a 4700 pF to change the frequency to about 1.5 kHz. Oscilloscope traces at various points in the oscillator circuit are shown in Figure 6. To understand the oscillator, students must understand the voltage divider and the charging of a capacitor, two fundamental concepts in electronics. 5 +12V oscillator out 470 C 100k LED - 100k 741 transistor + + 10k 22k -12V Figure 6. Oscillator circuit and associated oscilloscope images. The orange trace is the output of the oscillator which oscillates between the positive power supply voltage and the negative supply voltage. The pink trace shows the voltage at the non-inverting input of the operational amplifier and cyan trace shows the voltage at the inverting input. After the oscillator is complete, students add a high pass (R=47 k; C= 0.1µF) filter to the detection circuit. See Figure 7. A 10 pF capacitor is also added to the operational amplifier feedback loop to minimize any “ringing” in the detector circuit. The final stage of circuitry converts the AC signal of the LED light source to a DC signal. An “ideal” rectifier constructed from an op amp and two signal diodes followed by an active low pass filter are used for the AC to DC conversion. Figure 7 contains the oscilloscope images after the high pass filter, the rectifier and the low pass filter. The final output is a DC voltage directly proportional to the intensity of the LED light striking the detector. 6 I f A 10p 4 C - 6 2 op27 Vout + + 3 7 R -12V Figure 7. Current to voltage converter for the photodiode detector and associated oscilloscope images. The orange trace shows the signal at pt A in the circuit. This signal is due to both the oscillating LED and room light striking the photodiode. The cyan trace shows the signal after the high pass filter which eliminates the DC component. A slight distortion of the square wave can also be seen due to the attenuation of the lower Fourier components of the square wave. The pink trace shows the signal after the half-wave rectifier, the signal is inverted due to the operational amplifier, and the green trace shows the final average DC output after the low pass filter. The active filter also inverts the signal so the final output increases in the positive direction as more LED light strikes the photodiode. 7 rectified signal 10u 10k 10k 10k 10k AC in - - op27 op27 DC out + + + + active half wave rectifier active low pass filter Figure 8. An AC to DC converter constructed from an “ideal” half wave rectifier and active low pass filter.
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