Converting a power transistor to an optical transducer as an alternative to a commercial photo-detector for the construction of a simple and low-cost photometer

Abstract In this article, design and construction of a low-cost and simple homemade photometer for the determination of iron is described. Affordability and availability of required electronic components are two important factors that have been considered in the design of this photometer. An inexpensive low-wattage light-emitting diode was used as the light source, so there was no need to add a filter for wavelength selection. Instead of using a commercial phototransducer, a readily available NPN power transistor (2N3055) was modified as a proper substitute. To do this, the metallic cover of the transistor was carefully cut and removed so that the semiconductor junction of the transistor was revealed. It should be noted that this part of the transistor is photosensitive and can be used as a photo-sensing device. When the electromagnetic radiation strikes this photosensitive part, a small electric current is developed in the corresponding circuit. As this current is proportional to the radiation intensity, thus it is possible to use the uncovered transistor instead of a commercial phototransducer. The output electrical signal of this homemade phototransducer was amplified by a simple current-to-voltage converter and then measured with a digital voltmeter. As an application, the constructed photometer in this work was applied successfully to the assay of the iron content of vitamin tablets.