hope of not botching the job. On the video, I also
learned about using flux pens which made the
solder flow perfectly.
As I soldered the driver ICs, I got better with
each chip but still had a long way to go to match
the finesse in the video. Figure 7 shows me
slaving over a hot iron, happy as a clam.
Additional closeup photographs of the fronts
and backs of the final populated PCBs are shown
in Figures 8 and 9.
I chose to program the ATtiny2313 in
assembly using Atmel Studio because I had used
the same chip in another recent project. The only
tricky part was the “missed pulse” section that
locked in the dial count when the rotary dial
finished turning. The rest was easy.
Actually, if you just want to use SPI to talk to
the displays, a smaller chip could probably be
employed, including PICs. The assembly program
is available with the article downloads.
To program the µC, two different Atmel In-System Programmers (ISP) are available. The
AVRISP mkII is obsolete but still available on eBay
for about $50. The current AVR
programmer/debugger from the manufacturer is
the ATATMEL-ICE for $91. In both cases, when the
chip is being programmed, it has to be powered-up by a separate 5V supply.
That concludes my description of the design,
construction, programming, and operation of the
EMDs. However, I’d like to say a few more words
about my “ambitious” telephone project, in case
anyone is interested.
So, What’s this
In days gone by, telephone operators manually routed
the calls by using switchboards with plugs, jacks, lever
switches, and lots of wires. It was labor-intensive, but nice
to hear the operator say “Number Please?”
In 1891, a man named Strowger envisioned an
electromechanical system to replace the operators. The
end result was the rotary dial which selected the number
to call, and electromechanical step-by-step (SXS) switches
to automatically route the calls.
Soon, the Central Offices were filled with thousands
and thousands of clicking step-by-step switches, and
operators were only needed for long distance calls. Over
the years, improvements were made to the switches, but
electronic switching gradually took over. By the 1990s, the
SXS switches were completely phased out.
This brings us to my vintage step-by-step switching
system. I embarked on the project — called a Private
Branch Exchange (PBX) — because when I was a kid I
went on a field trip to the telephone company and the
rows and rows of clicking SXS switches left a lasting
impression on me. With the advent of eBay, it’s much
easier to find old telephone equipment.
I bought the five SXS switches from a guy on the east
coast. I built a sturdy frame for them and added a ton of
electronics, displays, relays, switches, lights, power
supplies, and an Arduino Mega to make them do some
A photo of the backside of the PBX with its numerous
July/August 2018 39
■ FIGURE 9. Rear view: The five-pin single inverter chip (U8) is
located on the back of the left driver board, in the lower right.
■ FIGURE 8. Front view: The six-pin ISP programming
connector is in the upper right of the right MicroC board.