Michal Zalewski Tracks Down Things That Glow in the Dark — with a Clever Phosphorescence Detector
When the glow of charged powered milk disappears in a few milliseconds, you're going to need something clever to capture it.
Michal Zalewski has been on a hunt for things that glow in the dark — aided by a homemade phosphorescence detector, built from a tin with a photodiode and a quartet of ultraviolet LEDs controlled by a Microchip AVR Dx microcontroller.
"Some time ago, my eldest son decided to make glow-in-the-dark pigments by doping strontium aluminate with rare earth elements," Zalewski explains of the project. "After a bit of trial and error, he succeeded — and the relative simplicity of the process made me wonder if there are any naturally-phosphorescent materials in our homes."
The most obvious way to check is, of course, to turn the lights off and see if anything glows. As anyone who has been disappointed by commercially-available glow-in-the-dark decorations will know, though, that doesn't really work: phosphorescent materials need to be "charged" by exposure to strong light, ideally with an ultraviolet component, before they'll glow — and unless they're specifically built for the task, said glow is likely to be both hard to see and short-lived.
To solve that, Zalewski designed a phosphorescence detector: a light-proof metal tin with a single Martech MT03-023 photodiode and four 385nm LEDs. The photodiode is then connected to a Texas Instruments TLV3541 op-amp to provide a 220,000-times amplification — run through a low-pass filter and the amplified twentyfold once more before being connected to a Microchip MCP33151-10 analog to digital converter (ADC) and a AVR Dx microcontroller.
The idea behind the detector is simple: place the material under test into the chamber and seal the lid, then turn the UV LEDs on for a few seconds. Extinguish the LEDs, activate the photodiode and chart its response — seeing if there's any residual glow from the materials that would indicate phosphorescence.
"I first tested the circuit with a plastic glow-in-the-dark trinket dyed with zinc sulfide," Zalewski writes. "Although the object glows in the dark for a good while, the intensity of the effect already decays exponentially on a sub-millisecond scale.
"One of the finds that exhibited pretty strong phosphorescence was powdered milk. Another unexpected glow-in-the-dark foodstuff? Gelatin! It initially registers about 30% higher than powdered milk, but decays more quickly."
Zalewski's full write-up, including schematics, is available on Substack.