Electromagnetic display

Electromagnetic displays are nice. They have a very high contrast, even in bright sunlight. Not to forget the beautiful clicking sound when they change state. If the display changes not often the power consumption is even very low compared to bight LED displays, which makes battery operation possible. Unfortunately they are not easy to obtain and usually expensive.

Because such a display contains a lot of equally shaped parts I thought it would be a perfect application for 3D-printing. And in fact a single digit prototype turned out quite nice.

Electromagnetic 7-segment display
Electromagnetic 7-segment display

The bores for the segment axles are finished with a 0.5 mm drill. The coils are driven by software PWM at about 1.6 A peek current and a duty cycle of about 50%. With that current it takes less than 10 ms to flip the segment reliably. The segments are flipped after each other to limit the peak power consumption of the whole circuit.

In the first place the driver was designed to drive a passive matrix of 4 x 7 segments. But setting a segment without changing the neighbor segment reliably didn’t quite work. (In a passive matrix the neighbor segments gets supplied with half of the nominal current) For the single digit prototype it works fine anyway.

Circuit diagram of the controller/driver
Circuit diagram of the controller/driver

The segments contain a small neodymium disk magnet (1 x 4 mm) and the coils a 1 mm iron core to concentrate the magnetic flux. The iron cores hold the segments in the last position if no current flows through the coil due to the remanence of iron.

Dismantled segment
Dismantled segment

Download stl-files, driver circuit and source code: 7segment-files.zip