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I have a selector switch with four wires: one common (C) and three position wires (P1, P2, P3). Normally, the output follows the selector so that C is connected to whichever P is active. I want to override this behaviour with two controllable relays (or two-way switches if you like) that are placed between the existing selector switch and its output terminals. These relays should provide three additional states to force a connection between C and one of P1, P2, or P3 regardless of the selector’s position. In other words, I need these four output states:

  • State 0 (Normal): Output follows the selector (C connected to whichever P the switch selects).
  • State 1: Force connection between C and P1 only, regardless of the selector state.
  • State 2: Force connection between C and P2 only, regardless of the selector state.
  • State 3: Force connection between C and P3 only, regardless of the selector state.

I’d like to achieve this with no extra components (e.g., diodes or transistors). I’m aware that a solution exists using two DPDT relays or three SPDT relays, but is it possible to design the circuit with just two SPDT relays? If so, how should the relay contacts be interconnected?

If there is no solution possible, please provide an explanation or, even better, proof. I've spent quite some time on this and I do not think it is likely a solution exists. But I would like to be sure.

Edit: As an example, below is the solution I found using two double-pole switches / DPDT relays. But I would like to know whether it's possible with two SINGLE-pole switches / SPDT relays.

Solution

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    \$\begingroup\$ Your solution has nothing to prevent, for example, P1 and P2 or P3 being turned on at the same time. \$\endgroup\$ Commented Apr 8 at 20:57
  • \$\begingroup\$ @thebusybee the actual question is: is it possible with two SINGLE-pole switches. The image I posted is an example of how I would do it with DOUBLE-pole switches. The image is not a solution to the actual question. I'll clarify that in the description. \$\endgroup\$ Commented Apr 9 at 7:15
  • \$\begingroup\$ @Transistor It does. Follow the signal lines for any switch state and you'll see that in any selector & relay state, exactly one P line is connected to C. \$\endgroup\$ Commented Apr 9 at 7:26
  • \$\begingroup\$ OK. I think I see what the schematic is representing. \$\endgroup\$ Commented Apr 9 at 9:47

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schematic

simulate this circuit – Schematic created using CircuitLab

I don't think it's possible with only two switches. One would be dedicated to the override function and two SPDT switches would be required to duplicate the three-way action.

With relays you have the option of working with additional contacts.

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  • \$\begingroup\$ Thanks! Yes, I already found this solution (and it works for me) but I would like to have proof it is not possible with just two. \$\endgroup\$ Commented Apr 8 at 19:28

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