# Help with circuit to alternate a light switch with solid state

I have designed a PCB that allows me to turn a light on with a mechanical relay. The difference between your average implementation here is that I also wired the board up to a wall light switch which allows me to alternate the state of the light switch.

I managed to alternate the behavior of the light by using an SPDT relay and a 3-way switch (which is wired in U$3 below). The theory of operation is pretty simple. The HOT wire is connected on the relay's center pin and the current is either brought out of the NO or NC pin. The light switch is connected similar to the SPDT relay but in opposite (you can think of the switch as an SPDT relay). ie: The center pin P$2 is the output and the other 2 states bring the current from NC or NO out to TRIGGER. Thus flipping the light will alternate the current from NC to NO and the relay essentially can do the same to alternate the current.

The issue with the current design is that I'd like to get away from using a mechanical relay and move to solid state for all of the typical reasons (audibly noisy, low life, sparks, ...). How can I achieve the same goal using solid states?

SPDT Triacs virtually don't exist. I found a few designs for SPDT triacs using a few SPST triacs, but I wasn't exactly thrilled to see the large number of components and I'm a bit worried that I may lose any significant power gains I may have gotten from moving to solid state.

Are there any other clever ideas that anyone can think to achieve my high-level goals of XOR'ing the state of the light switch with a relay and a light switch?

• You may get a lot more attention to your question if you took the time to provide a schematic of what you have done or at least intend to try. – Michael Karas Jun 2 '18 at 10:20
• Use an SSR (solid state relay). – Andy aka Jun 2 '18 at 10:42
• @MichaelKaras Good idea. I've added one. I also clarified what I think was a pretty confusing intro. – ti_chris Jun 2 '18 at 21:27
• @Andyaka SPDT relays are virtually impossible to find. They don't tend to like to make those. It's all SPST. – ti_chris Jun 2 '18 at 21:28
• You mean "SPDT solid-state relays are virtually impossible to find". – Transistor Jun 2 '18 at 21:37