I found a number of vendors on eBay who were
selling surplus 1” high seven-segment
electromechanical displays (EMDs) that go clickity
click (see Figure 1). The segments appear and
disappear in the blink of an eye when the tiny coils
are pulsed. They aren’t quite as fast as LCDs, but
they can add a whole new dimension to your
project: sound! (Check out my You Tube video at
Many months ago, I embarked on an ambitious
project that featured five 1941 vintage rotary
telephone step-by-step (SXS) switches. I thought
these little EMDs would be the perfect match to
display the dialed digits; refer to Figure 2. So, I
ordered a dozen from a guy in Hungary and the rest
is history. You can see nine of them mounted in a
line under the SXS switches.
Reinventing the Wheel
The displays didn’t come with drivers and I couldn’t
find a commercial source. I saw a reference to a 40-pin
FP2800A decoder/driver IC but it was no longer available,
so I decided to build my own. Maybe I missed something,
but it wouldn’t be the first time I set about reinventing the
I learned a few things along the way, and it was a
rewarding experience to design and build the PCB
(printed circuit board) drivers using tiny SMD components.
July/August 2018 35
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■ FIGURE 1. The seven electromagnetic coils are driven by
25 ms bipolar pulses that flip the segments on or off.
■ FIGURE 2. My step-by-step vintage 1941 telephone
switch demonstrator uses nine displays to show the