Eight LEDs to Replace the Word Purple

Found the opamp first. TL072, dual op-amp, in the parts drawer under the coil of magnet wire. Beside it: three photodiodes still in their static bag, BPW34 silicon detectors, ordered months ago for something I can’t remember. That’s how today started—not with a plan, but with a transimpedance amplifier circuit I’d been avoiding building because I hadn’t known what to measure.

The refractometer calibration ritual last week set the precedent: ±0.001 specific gravity per 5°C temperature deviation, zero reference at exactly 20°C, measurement precision tied to environmental control. If I’m going to analyze mushroom dye extracts and reef tank trace elements past “looks purple” or “smells like iron,” I need wavelength-specific absorption data. That means building an LED spectrophotometer from discrete components.

Searched “DIY absorption spectroscopy” and landed on a Hackaday project using the AMS AS7265x sensor