Plate readers are instruments which are used to detect properties of samples in microtiter plates, via absorbance, fluorescence, and luminescence.
Karol Szymula, Michael Magaraci, Michael Patterson, Andrew Clark, Sevile Mannickarottu, and Brian Chow published a paper in September 2018 detailing the construction and performance of an open-source plate reader. The extensive supplementary materials provide everything you will need, from software to parts list to a zip file with all the 3D printed and laser cut designs! The parts list pegs the device coming in at just under $3,400.
The project is also listed on metafluidics.org, where the same detailed tutorials, lists and files can be accessed in a slightly more user-friendly manner. The website does require registration, but the parts list seems to be more refined including PCB files for the various electronics involved in the device.
This has to be one of the most thorough jobs making a complicated device possible to understand and build we’ve ever seen.