Slow-scan CRTs were never exactly common compared to their faster cousins, but given the popularity of Slow Scan TV (SSTV) amongst hams and NASA broadcasts, many of you are probably familiar with them. The slow scan rate of SSTV meant it required much less bandwidth, but in the early days you needed a CRT with a long-persistence phosphor to hold onto the image. [AJRussell]’s Glow Engine works much the same, with one key difference — instead of cathode rays, he’s using a frikkin laser beam.
In this case, the phosphor is Strontium Aluminate, the same stuff that gives most glow-in-the-dark toys and filament its kick. Energized by a 405 nm laser of questionable wattage, the phosphor will glow for several seconds, allowing the creation of an image. So while this is a laser projector, it works more like a CRT than most galvo projectors, which rely on Persistence of Vision to create an image. Here it’s persistence of fluorescence.
Because the phosphor is so slow, you don’t need the rapid scan rate you would with a laser projector, so [AJRussel] can skip the mirror drum and just mount the mirror on a gimbal motor. Field Oriented Control makes the precise sweeping of the gimbal possible, via a hall-effect sensor and the SimpleFOC library that we featured last year. The other axis just moves the laser and gimbal assembly on a big stepper. The whole thing is driven via an ESP32. The biggest downside is that the short focus range of the repurposed engraving laser means it’s smack dab in front of the screen.
This is a work in progress and still changing, so it’s not clear which — if any — of the various SSTV modes the Glow Engine can handle. Given the number of scanlines in the photos it looks like a good use case, and without trying it the timing might work, too: [AJ] reports scanning left-to-right to generate a frame takes about eight seconds, depending on the resolution, and depending on the PWM power setting on the laser the image can last up to a minute.
Given the one-minute decay time with this particular phosphor, perhaps he can make a clock. If you have a longer-lasting glow powder, we’ve seen uses for such a persistent display as well.
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This articles is written by : Nermeen Nabil Khear Abdelmalak
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