This is the schematic I traced for an answering machine stopper 
gadget : it cuts off the answering machine in case any line is picked
up. Some answering machines have this feature built-in; the older or
inexpensive ones don't. The gadget is called Message Stopper (R), by
Design Tech International Inc., Springfield, VA. Several other mfrs
make this kind of gadget, so this is just one I happened to buy. 
Cost, roughly $10. 

The gadget is in the same form factor as a wall-plug-in Y-splitter.
That is, it plugs into the wall RJ11 outlet, and has two RJ11 outlets
on it, one marked TEL, the other marked ANS. There are two LED's, the 
green one on top of the ANS outlet, the red one on top of the TEL outlet. 

The two outer wires of the three RJ11's are wired in parallel. The
schematic for the center two wires is : 


         .--------------  phone
      .-----------------
      |  |
C1   ----|
C2   -|  |
      |  |
      |  |
      |  '-----------------  answering machine
      |  
      +--|<--+--|<--->|----
      |      |    two zeners                      
      +-->|--+
         two leds

     Top led is green, bottom is red. 

It turns out the leds are basically idiot lights. The outside design 
makes it look like the green should light up when the ANS is offhook,
and the red when TEL is offhook, but actually, depending on the line
polarity, only one will glow, and only when the ANS outlet is offhook.
If a load is connected on the ANS outlet, both leds will glow when
ring voltage comes through. 

I could not read the zener voltage off the diodes, but I measured 11V
across one of them when operating. 

The theory of operation is straightforward. All telephone extensions
in the house are in parallel with each other, and with any device connected
on the TEL outlet. If any of these devices is offhook, the phone line
voltage is expected to drop below 12V or so, at which point the zeners
isolate the ANS outlet from the line. 

It's a fairly simplistic design, and will not work in all situations,
but it mostly works. 

----

Regards,

/ Jon Sreekanth

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