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"