🚀 go-pugleaf

A circuit for a twin-T network useful for viewing
signals on the power line is described in the book
"Handbook of Remote Control & Automation Techniques" by
John E. Cunningham & Delton T. Horn (TAB Books # 1777).
The circuit is a 60 cycle notch filter. One leg has DC
continuity - so you must be sure that that leg is connected
in the neutral line - else sparks will fly when you connect
to your scope!  It might be prudent to measure between that line
and your scope ground before making the connection just to be
sure.

Well, here is the circuit given in the book:


                          |-----------------------------|
                          |                             |
            |----r2-------|---r2-----|                  |
            |                        |                  |
Line in     |                        |                  |
o-----------|                        |--------o  out    |
            |                        |                  |
            |                        |                  |
            |----c1---|-------c1-----|                  |
                      |                                 |
                      |        |------------------------| 
                      r1       c2                       
                      |        |
o---------------------|--------|---------------o Neutral out


Component values:

c1 - 9 uf at 600v
c2 - 18 uf at 600v
r1 - 500 ohm pot set to 147 
r2 - 500 ohm pot set to 295

Unfortunately, with these values the device is going to be bulky and
expensive.  The author justified these values because he had some
9 uf surplus caps available.

It is my opinion that at the frequencies you want to see,
much smaller capacitors could be used.  Perhaps the following
values would be ok:

c1 - 0.05 uf at 600v     or    c1 - 1 uf at 600v 
c2 - 0.1 uf at 600v            c2 - 2 uf at 600v
r1 - 26500 ohm                 r1 - 1325 ohm
r2 - 53100 ohm                 r2 - 2650 ohm

Some adjustment of the pots will be necessary to null out the
60 hertz signal due the the inaccuracy of the values of the caps.

Disclaimer:  I have not tried this circuit.  It looks like it should
   work, but there is danger any time you are dealing with the
   power line.  Be careful.

BTW, Jensen Tools sells a Line Noise Viewer that provides line isolation
for the tiny sum of $450 or so.  I don't know if the bandwidth is good
enough to see X10 signals, though.

Roy Frederick
Dallas County Data Services
504 Records Bldg
Dallas, TX 75202