Encode and decode SPDT switch over single wire

Question

I'm trying to devise a way to connect a device to a SPDT switch using only one wire. Specifically, I want to connect a wall switch to some kind of smart home thingy, and I only have a single switching wire and a neutral lead at the site of the switch.

One obvious way of encoding this would be to have one side of the switch connect via a resistor, and the other one directly. The other side would then see either no resistance, some kind of resistance I get to pick, and infinite resistance.

^{simulate this circuit – Schematic created using CircuitLab}

Basically, what to put in the box?

My limited knowledge suggests that I am going to at least need two transistors to pull the wires down to ground. I could then fiddle around with resistors in such a way that when R1 is connected to the box, one of them turns on, and when it's directly connected to ground, the other transistor turns on. But: how to get the first transistor to switch off in this case?

Answer 1

You could use two 24VDC relays as follows:

^{simulate this circuit – Schematic created using CircuitLab}

The COM is wired through the NC contact on RLY3 to provide "break before make". If you don't need that feature you can avoid using the NC contact.