Es ist Mittwoch.

Sometimes blog posts just have to sit in the drafts for a while. This one had the opportunity to mature for about a year. The actual project lies back about two and half years by now. The pictures I have of it haven’t got any better in the meantime, so I might just as well hit ‘post’ on this split-flap calendar I built for a friend.

Most of the time, it doesn’t do much. Once a day, it advances by one flap and ⅐ of the time, it announces the day of the week, Wednesday.

For dramatic effect, this video has the calendar go through all seven flaps. In reality, it would just advance by one to limit battery consumption (and because it’d be annoyingly noisy at midnight).

The flaps are made from ordinary copier paper, folded in half with a steel wire through the crease. A few coats of acrylic lacquer give them plenty rigidity and, through spotty spray-painting, a delicately rough surface texture, which blends well with the imperfectly laser-printed black backgrounds.

The electronics are straightforward: An ATTiny24A runs straight from three AAA batteries. Four NPNs drive the stepper motor, which, with its built-in gearbox, drives the display directly at a pleasant speed. A reflective light sensor together with a white color dot on one of the hubs is used to home the display on power-on.

The wide split-flap display, disassembled: the flaps are taken out and lie in front of it. The controller PCB can be seen mounted in the back (it is normally hidden behind the flaps).
The case is built from PCB material, as are the seven-holed hubs holding the flaps, which are simply bent a bit to insert them.

Some of you might be asking: No buttons anywhere? How do you set the date? Easy: there’s a note in the battery compartment telling you to only insert fresh batteries on Wednesdays.

Published 2025-05-07 in #weekend project, #gift idea, and #display. Last updated 2025-05-02.