Digital multimeter using a mechanical ADC

[joeqsmith]

and no nixie tubes!   

https://www.youtube.com/watch?v=OIATKbSkYxw&feature=youtu.be

http://www.stevenjohnson.com/nls/

https://en.wikipedia.org/wiki/Non-Linear_Systems

https://en.wikipedia.org/wiki/Kelvin%E2%80%93Varley_divider

[karoru]

Now imagine working in a lab with few of these working simultaneously - joy comparable only with having electromechanical telephone exchange (Strowger or something like that) nearby:)

[chickenHeadKnob]

Now imagine working in a lab with few of these working simultaneously - joy comparable only with having electromechanical telephone exchange (Strowger or something like that) nearby:)

Back in the early 1980's I was at a work-site in a  large telephone exchange servicing a mixed business/residential area of a city. I didn't work for the telephone company  I was there installing some experimental gear which performed digital traffic analysis, a kind of deep packet inspection, but that is unimportant now. The exchange had a mix of new digital switches  and the old legacy relays in tall aisles you could walk past. Most of the time while I was working there was a more or less constant random clacking. That changed around mid-morning, when in Canada it is customary for office workers to take a break and make personal phone calls. Slowly the noise level would build and close to the peak traffic began to exhibit patterns, like fast waves  running up and down the aisles. It was a bit like the behavior of crowds  in sports stadiums doing "the wave" only faster and with an element of chaos. I found it mesmerizing so I would just sit in the middle of the exchange and listen to it sing and breathe like it was some kind of organic beast.

[Red Squirrel]

Woah that's really cool.  It does kinda remind me of a telco exchange, the old stepper switch ones. 



What's interesting is the newer switches still have relays (the typical ones you'd see on circuit boards) and when you walk through the aisles you can hear them.  But the actual sound is mostly just fans as it's mostly all digital.  Each phone line has it's own digital card and thats where the relays on, I think they're used for physically connecting your line to the next part of the switch, sending ring tones etc.

[kandrey89]

I wonder why the newer ones like you said use relays as well, because the line has ~30VDC, but there should be FETs that handle this stuff, reliability?

[Red Squirrel]

My guess is that the line voltage may have a different ground reference than the logic ground of the circuitry would have so any kind of fet or transistor would not really work or be easy to implement.  The ring generator is also in the order of 170vac as well, the phone line is -54vdc (-48v nominal).   I try to avoid brushing skin against the terminal blocks when working on the distribution frame.  If that line gets a call you get a little surprise.