I absolutely CANNOT take credit for this. But I searched and searched the community for a solution to no avail.
A very standard way to wire a 3-way switch in the US is: Switch 1 supplies the power and connects to Switch 2 with a 3 conductor cable. The Switch 2 side has the Load (so the cable has a neutral and 2 travelers). So, there is no full-time power on the Load side (so no shelly 1 there…). The Switch 1 side has full time Power, but no way to control the load (so no shelly 1 there…).
A fellow named David Eads came up with a very elegant solution. I HOPE IT’s OK to post an outside link https://medium.com/@dceads1972/how-i-configured-a-shelly-smart-relay-to-control-a-3-way-switch-4b21a20b6c85 https://medium.com/@dceads1972/how-i-configured-a-shelly-smart-relay-to-control-a-3-way-switch-4b21a20b6c85 .
Basically, he solves the problem with a 2 channel Shelly placed at the Switch 2 load side. Channel 1 is a normal edge triggered switch with the switch connection going to the center terminal of the 3-way. Channel 2 is a FLIP switch with an inverted input. The Ch 2 input goes to the Traveler 1 switch terminal (along with the traveler coming from the Switch 1). The Ch 2 output connects to the other input on Switch 2 (where Traveler #2 was originally connected). Remove Traveler 2 from Switch 1 (load side) and connect Traveler 2 to full time power. Again, on the Switch 2 side, Remove Traveler 2 from the Switch and connect it to the Shelly’s power input.
IT WORKS GREAT. No new wiring, uses existing wiring.
My take on the explanation. Shelly Channel 1 is just like a normal 3-way install as if you had Line, Load and Neutral in the same box. Now the trick, Channel 2 is a copy Ch 1 except Inverter. Just like the old fashioned basic 3-way switch (Traveler 1 is ON when Traveler 2 is OFF and vise-versa.) so the Shelly Channel 2 switch logic replaces Traveler 2. Leaving Traveler 2 available to become full-time power
