Flip-dots are small electromechanical discs with one matt black face and one brightly coloured face. The discs are permanently magnetised and pivoted so they can freely rotate to show one face or the other. Beneath each disc is a coil wound on an iron former. The iron core is also magnetised and, depending on its polarity, causes the disc above to be repelled or attracted with either the black or coloured face uppermost.
Passing a current pulse through the coil flips the magnetic polarity of its iron core and consequently flips the disc above. Passing a current in the other direction flips the polarity and the disc back.
Each disc needs quite a large current pulse of a few hundred milliamps for several milliseconds to change state. Discs retain their state when power is removed, current is only consumed when they change state.
Flip-dots are visually attractive but perhaps the most compelling thing about them is the sound they make - a rustling, fluttering sound. It’s quite hypnotic.
Flip-dots are supplied in strips of 7 units. I used a total of 4 strips or 28 flip-dots in this design and needed to be able to control the current in both directions for each coil individually. H-bridge drivers seemed the most obvious solution - the classic L293D dual H-bridge.
Each L293D has 4 outputs which can independently sink or source current. I wired the flip dots with one common connection per strip of 7 dots and individual connections to the other terminal of each coil, making 8 connections per strip neatly matching a pair of H-bridge chips. Four strips of flip-dots equated to 8 x L239D in total.
On reflection there are probably ways of wiring them using less H-bridges but the current design does work reliably.
In each strip of 7 dots, 6 were used for telling the time. That gave me 24 dots in total to display the minute in the hour therefore each flip-dot would represent 2½ minutes. That seemed a reasonable time resolution. The minutes within the hour can be displayed (to the nearest 150 seconds) by a single flip-dot showing its coloured face. But how to display the hour as well ?
The answer was to flip a contiguous string of dots so that the first dot with coloured face upward in the string (moving clockwise) represented the hour and its length represented the minute. For example, quarter past the hour is indicated by 6 dots and half past by 12 dots.
The four corner flip-dots were unused. I made use of them as optional 'tick tock' quarter-minute indicators.
I also included a mode that produced a short animation on each hour for additional visual interest.
A PIC microcontroller with a crystal oscillator keeps track of the time with a couple of pushbuttons to set the initial time. The crystal is a higher specification 10ppm part for better accuracy and there is a feature in the firmware that allows fine tuning by setting a number of seconds per day calibration offset, stored in non-volatile memory.
Power is via a barrel jack connector, 9V dc.
The quarter-minute 'tick tock' feature:
Some of the 'turn of the hour' visual effects: