Chemistry Everyday for Everyone edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Low-Cost Science Teaching Equipment for Visually Impaired Children H. O. Gupta* and Rakshpal Singh Department of Computer Education and Technological Aids, National Council of Educational Research and Training (NCERT), New Delhi 110 016, India
Science education is increasingly becoming accessible to visually impaired children (VIC) in the developed countries (1–8). Recent developments (9) in this direction have given VIC access to various software and talking computer terminals enabling them to do a variety of experiments in science. But the situation in the developing countries has not improved much. One of the main reasons is lack of suitable low-cost science teaching equipment and aids (10). In our department, we are developing very simple and relatively inexpensive science teaching aids and equipment for VIC (11). This article describes some of this equipment that uses simple electronic circuits consisting of commonly available operational amplifiers, resistors, and piezoelectric beepers for generating audio signals for output indication. Earlier *Corresponding author.
equipment for VIC has been described, in which the read-out system is based on the digital-multimeter Optacon approach (3) and analog bridge-type meters (6 ). For the equipment described here, we have made a simple low-cost read-out system consisting of an embossed dial over which a pointer is rotated. The pointer is attached to a knob, which in turn is attached to a variable resistor acting as one of the arms of a bridge circuit encompassing the sensor for determining the experimental variable. The readout is indicated by an audible null detector and the reading is noted tactually on the dial. Null Detector The utility of a null detector is well known in a number of experimental setups using bridge circuits. For sighted children, the null detection is normally achieved through a galvanometer, which requires meter reading. For VIC, it has
Figure 1. Circuit diagram of a null detector.
Figure 2. Circuit diagram of an electronic thermometer.
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Journal of Chemical Education • Vol. 75 No. 5 May 1998 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
to be either audible or tactile. We developed a null detector with audio output. It consists of an operational amplifier, piezoelectric beeper, and a dc power supply, as shown in Figure 1. When the output from a bridge is applied to this instrument, the audio signal from the beeper continues as long as the bridge is unbalanced, owing to differential output of the operational amplifier. As soon as balance is achieved in the bridge, the output drops and there is no sound generated in the beeper, enabling the VIC to locate the balance condition. This audio null detection circuit has been utilized in the electronic thermometer and colorimeter described in the following sections. Electronic Thermometer Tombaugh (2) mentions thermometers with voice output or audible null detection using stainless steel probes. We chose a negative temperature coefficient thermistor for sensing temperature, utilizing the bridge circuit design by Droms (12). This thermistor (R2) forms one of the bridge arms. A fixed resistor (R1) forms the adjacent arm. A linear potentiometer is introduced as part of the variable arm of the bridge as shown in Figure 2. Since the resistance of a thermistor varies exponentially with its temperature, the actual values of the bridge components have been chosen so that the temperature varies linearly with the scale of a direct reading system at balance for a limited range of temperature from 0 to 75 °C. The direct reading system consists of a knob with a pointer connected to the linear potentiometer, which forms the variable arm of the bridge. The knob is rotated over an embossed dial, which was calibrated using temperatures of 0, 25, 50, and 75 °C. The output signal from the bridge is applied to the audio null detection circuit as described for the null detector. A slight change in temperature manifests as a change in the value of the resistance of the thermistor, indicated by an audio signal generated by the piezoelectric beeper. The actual temperature of a liquid in which the thermistor is immersed can be read by manipulating the knob until no beat is heard. The temperature is then read tactually on the embossed dial. The smallest increment of the temperature scale is 1 °C. The electronic thermometer is shown in Figure 3.
Figure 3. Electronic thermometer with embossed dial connected to a probe (0 – 75 °C).
Colorimeter This equipment can be used either as a color change indicator or as a colorimeter for qualitative or quantitative experiments, respectively. As a color change indicator, the device may be used in a number of experiments in chemistry. Some of them are listed below: Volumetric titrations Study of chemical equilibrium Detection of acids and bases Detection of chemical reactions involving change of color
Here, we have introduced a light-dependent resistor (LDR) in one arm of a bridge circuit, as shown in Figure 4. The actual values of the bridge components have been chosen so that the balance conditions for various colors are obtained on an embossed measuring dial fixed on the top of the equipment case. Light from a light source is passed through the solution under examination and is allowed to fall on the LDR. The bridge is balanced by rotating a knob (attached to the linear variable resistor (R4) forming the variable arm of the bridge) with a pointer over the embossed dial before the color change. When a color change occurs due to chemical reaction, the value of resistance of the LDR changes, affecting the balance point. The balance is again obtained by varying the position of the knob until no tone is audible from the beeper. The readings can then be noted tactually on the embossed dial. The equipment was tested for quantitative use for different colors. It was found to be quite sensitive to various colors. A linear graph was obtained by plotting dial positions
Figure 4. Circuit diagram of a colorimeter.
JChemEd.chem.wisc.edu • Vol. 75 No. 5 May 1998 • Journal of Chemical Education
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Chemistry Everyday for Everyone Figure 5. Colorimeter with embossed dial for indicating a color change and estimating the strength of a colored solution.
rating appropriate types of sensors. We are now developing a pH meter and conductimeter, which will be reported in a future publication. Acknowledgments Financial assistance from UNICEF is gratefully acknowledged. Thanks is due to the Blind Men’s Association, Ahmedabad; Faith India, Trivandrum; Electronic Research and Development Centre; Trivandrum; and a number of experts belonging to these institutions, for their roles in this developmental program.
versus concentration of the pink, green, blue, and red solutions in the range from 0.1 to 0.5 M. However, the equipment was not sensitive for different concentrations of the yellow solution. The equipment is shown in Figure 5. Conclusions The preceding discussion shows that the equipment described can be fabricated using locally available, inexpensive electronic components. The output is read directly from a calibrated dial fixed to the top of the equipment case. There is no need to attach a costly voice analog multimeter or an Optacon digital multimeter for reading the output. The approach described in this article would be generally applicable to a wide variety of science equipment for VIC by incorpo-
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Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Lunney, D.; Morrison, R. C. J. Chem. Educ. 1981, 58, 228. Tombaugh, D. J. Chem. Educ. 1981, 58, 222. Anderson, J. L. J. Chem. Educ. 1982, 59, 871. Smith, D. J. Chem. Educ. 1981, 58, 226. Hiemenz, P. C.; Pfeiffer, E. J. Chem. Educ. 1972, 49, 263. Tallman, D. E. J. Chem. Educ. 1978, 55, 605. SAVI/SELPH project. Center for Multisensory Learning: University of California, Berkeley, CA 94720. Ayres, D. G.; Hinton, R. A. L. SSR. 1985, 66 (238), 18. Lunney, D. J. Chem. Educ. 1994, 71, 308. Gupta, H. O.; Singh, A. School Sci. 1994, 32, 21. Gupta, H. O. Blind Welfare 1993, 34, 23. Droms, R. C. In Temperature, Vol. III; Herzfeld, C. M.; Dahl, A. I., Eds.; Reinhold: New York, 1962; Part 2, pp 339–346.
Journal of Chemical Education • Vol. 75 No. 5 May 1998 • JChemEd.chem.wisc.edu
