"Robert Nichols" wrote
in message ...
On 02/06/2016 07:41 PM, DoN. Nichols wrote:
On 2016-02-06, Jim Wilkins wrote:
Tbe one that impresses me most is the old analog telephone which
does
everything over two wires without active electronics, only one
very
clever transformer, speaker and carbon mike. I couldn't quickly
find a
circuit description and should return to fixing my fallen TV
antenna
that had me abseiling down the snow-covered roof.

I could tell you (in too much detail) how automated connections
were made by at least one set of equipment -- the "Strowger
switch",
invented by a somewhat paranoid undertaker who believed that is
competitor's wife (who was a telephone operator) was directing
potential
customers to her husband). That was initially picked up by a
company
called "Automatic Electric".

AT&T (Ma Bell) used that system for small-down setups later,
calling them "10x10s" in contrast to the crossbar switches used in
the
more complex exchanges.

Mechanically, the switch is an amazing bit of engineering, and
relay logic was my first experience with logic circuits (long
before the
ICs which continued the principles), with special relays set up to
be
slow to pick up, or slow to release along with the normal speed
ones.
The connection to the phone came though a two-coil relay, which
both
acted to pick up the dialing pulses, and to isolate the audio from
the
battery (48 VDC, FWIW) and serve as a balanced line, to be more
immune
to induced noise.

It's the Panel Switch that takes the engineering prize. It's been
called a "Telephone office designed by a mechanical engineer." I
visited one of the last panel offices just before it was taken out
of service. Impressive to watch it in action. Glad I didn't have
to maintain it.

https://en.wikipedia.org/wiki/Panel_switch

--
Bob Nichols AT comcast.net I am "RNichols42"

I theoretically had to repair stepping-relay switches that still
existed in some of our more remote Army sites near the Border in
Germany. Luckily the computer-type gear I maintained had dedicated and
usually modem-quality lines, but a phone call made the relays clatter,
and sometimes was routed through Berlin and undoubtedly monitored in
the DDR. The Germans would warn me when that happened.

Communicating with German engineers about line distortion problems
sorely strained my limited technical vocabulary which was oriented
more toward math and chemistry, and buying VW parts. OTOH knowing at
least a little of it may have kept me from Vietnam.

-jsw