I started with How can you measure the caffeine content of a liquid at home? in the cooking stackexchange, but the test strips were only qualitative (yes/no) and don't seem to exist anymore, and the part about rubbing dichloromethane on ones gums frightened me off.
The 2014 NY Daily News article Scientists working on at-home caffeine detection test links to Validation of Caffeine Dehydrogenase from Pseudomonas sp. Strain CBB1 as a Suitable Enzyme for a Rapid Caffeine Detection and Potential Diagnostic Test (Mohanty, S. K., Yu, C. L., Gopishetty, S. and Subramanian, M., J. Agric. Food Chem., 2014, 62 (31), pp 7939–7946) but that's not for home use. I found this nice experiment but that is still serious analytical chemistry.
However the article does mention the use of UV spectrophotometry, and that's something I could imagine playing with at home personally, using UV LEDs or other light sources, a Raspberry Pi camera or some photodiodes, a piece of diffraction grating film, basic physics, duct tape, etc. I'm OK with the optics and electronics, it's the chemistry I need the most help with.
The 1994 paper A Simple Method for Determination of Caffein Content in Tea Samples shows a correlation of caffeine assay from various types of brewed tea with absorption at 440 nm. Strong coffee would contain perhaps 5X more caffein, but being coffee, would probably be 100X or more higher in absorption in blue or UV.
This and this discuss UV absorption of caffeine in a solvent, but I am not sure if there are any solvents that I can find or use at home. Further, while I might be able to find a 275nm LED, it probably limits me to glass optics and expensive detectors, most plastics probably absorb too much there, and most silicon detectors have thin inactive layers that absorb this wavelength of UV; backside-thinned detectors are usually necessary. However, possibly the longer wavelength tail in the direction of 300nm might be useful.
My Chemistry Question: Are there any ways to separate the caffeine in coffee from the organics that make it so dark, using "household chemistry", so that I could try to check blue or UV optical absorption?
I'm only looking for a way to tell if brewed coffee has a big or a small caffeine "kick" to it, so even something as bad as +/-20% repeatability might do in a pinch. I don't need an absolute measurement - just something that roughly tracks the caffeine content.
Possibly helpful reddit, What wavelength should I measure the absorbance of my coffee to get an idea of how strong it is?, which links to an article I can't read right now, as I am drinking coffee on the wrong side of the paywall: Measurement of caffeine in coffee beans with UV/vis spectrometer.
above: An old Certificate of Analysis from a Caffeine Standards Kit, from Agilent.