A guided-inquiry project designed to teach students the basics of spectrophotometric instrumentation at the second year level is presented. Students design, build, program, and test their own single-wavelength, submersible photometer using low-cost light-emitting diodes (LEDs) and inexpensive household items. A series of structured prelaboratory assignments guide students through the processes of researching background information, designing a photometer device, and developing their own procedure to test the performance of the device. Students also learn basic skills of data acquisition by programming an easy-to-use LabVIEW interface for their device. Using a colorimetric indicator dye, students use their photometers and LabVIEW interfaces to determine the endpoint of an acid-base titration and compare the linear response of their device against that of a commercially available spectrophotometer. Students who completed the experiment indicated that the experience improved their understanding of spectroscopy, as well as their critical thinking skills and research ability.