In response to a recent comp.home.automation question on how to retrofit wiring to provide dimmed lighting and (preferably) provide automation to a house that uses 1950's vintage relay-controlled lighting (Subject: "What can I replace this latching relay system with?"), I bread-boarded a concept using INSTEON power line control (PLC) devices that I had previously tested with X-10 but didn't end up using.
The original poster's challenge is that the existing wiring to the wall switches is suitable only for low voltage and installing new wiring is not acceptable. The low voltage switches on the wall control latching relays in the ceiling that switch the AC power to the ceiling-mounted lights. The AC wiring to the ceiling lights is not individually "home-runned" to the entrance panel, so centralized dimmers would require re-wiring the AC wiring which is also not acceptable.
The circuit below solves these issues and provides for both dimming and home automation control of the lighting using the existing wire infrastructure. It transmits powerline control signals and sufficient AC power to power an INSTEON device for input and signaling purposes only over the existing low voltage wiring from the AC power and ceiling lamp to the wall switch.
The wall-mounted INSTEON dimmer(SW1) sends dimming and other signals to INSTEON dimmer SW2 which in turn controls the light. INSTEON SW2 also repeats the signal received ensuring that it is available in the AC wiring throughout the house for conventional home automation control.
I tested this arrangement 'on the bench' using both ~200 feet of CAT5 wire and ~200 feet of 24-gauge non-twisted telephone wire as the interconnect. The gauge of the wire needs to be sufficient for the load. Current through the nominal 24AC control line was about 200mA with an INSTEON ICON switch (relay) connected for SW1.
One could also control a small load on the isolated INSTEON device by using sufficiently large transformers and interconnecting wires of appropriate gauge and installation.
120VAC C1 C3 Hot ------+----------+-||-+ +-||-+ (black ) | | | | | (black) INSTEON(red) + +-->>-----------