Nuts and Volts - July/August 2018

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 https://www.youtube.com/watch?v=inIxU27QgN0.)

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 wheel.

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.