We recently featured an entertaining project here, a digital clock with a variety of different retro display technologies forming its numerals. Among those was an extremely unusual device, a rear-projection display with an array of bulbs each able to shine through a different letter or numeral slide. There was such interest in this device that its owner [Suedbunker] subjected one to a teardown for all to see.
The displays came from an organ which he suggests may have been manufactured around 1900. We suspect that may be a rather early estimate due to its use of a printed circuit board, but it is no less a fascinating device for it. A rectangular enclosure secured by twist-tabs opens to reveal a matrix of small filament bulbs on a PCB and supported by a stack of resin boards, in front of which was placed a slide with a letter or number for each one. Before that lies a sheet of glass, and then a molded plastic lens assembly which provides an individual lens for each of the 12 bulbs. When a bulb is illuminated with these in place, the letter or number is projected on the screen at the front of the unit.
It has the advantage of simplicity, no need for a high voltage, and high-quality characters and flexibility in displaying alternatives through different slides, though at the expense of quite a bulky package. The bulbs are quite energy-sapping, so for his clock he replaced them with LEDs. We like it as one of the more practical retro numeric displays, but its size means we probably won’t see a comeback.