Perhaps you can find an application for these funky little
displays and have as much fun building with them as I did.
My requirements were simple. Each display had to be
a stand-alone unit with its own driver. I needed them to be
physically located at different positions in the telephone
project, which didn’t lend itself to grouping them in most
cases. So, I made
each one an
with three stacked
boards as shown
in Figure 3.
They also had
to be front-mountable in
in a panel — much like mounting LCD displays.
Lastly, they had to accept two types of inputs: a
totalizing pulse input (1-10 pulses) from a relay contact or
telephone dial; and a daisy-chained SPI data/clock input
that was addressable from a host microcontroller.
The plan was to build a total of nine daisy-chained
displays for the telephone project. Figure 4 shows the
completed three-board assembly, with six daisy-chained
wires going in and out.
Let’s Get Started
This article will describe how I designed and built the
drivers and some of the problems I encountered.
The first task was locating a suitable bi-directional
driver for each segment, like an H-bridge. I found
references to a 40-pin FP2800A IC that could drive
multiple digits, but I never could find a source. Besides, I
really wanted something smaller since I wanted the digits
to be independent and compact.
If the displays were to be used for something else like
a clock, they could all be mounted on a longer PCB and
there would be plenty of room for an FP2800A (if you can
After more research, I finally settled on the Toshiba
TB6612 which contains two motor drivers and is normally
used to control small DC motors. However, I blanched
when I saw it only came in a tiny SSOP package. I had
used many SMD parts in the past but never any with a
36 July/August 2018
■ FIGURE 4. Multiple displays can be daisy-chained
together to respond to SPI generated addresses and
■ FIGURE 3. The
stack of boards
and four TB6612
drivers to decode
and flip segments
The full link to eBay for the individual seven-segment
electromagnetic displays for $9 from Hungary is: