Simple two bulb circuit

Question

Im trying to draw up a circuit with two bulbs and two spdt switches. In this circuit one of the two bulbs must always be on, and both switches must be able to swap which bulb is on.

So to avoid any confusion: Both lights must change state (on to off or off to on) when either switch is used, One light must be on the other off, I only have two single pole double throw switches and must use both.

This is seems like a simple circuit like but i am having a very hard time working this out. Thanks for your help.

Answer 1

The simplest (but non-qualifying) solution

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. By changing the switch type the problem becomes simple.

AC solution

^{simulate this circuit}

Figure 2. By not changing the switch type the problem is more difficult!

Gray code sequence

A trick to looking at this solution is to write the truth table in Gray code rather than a normal binary sequence. This is important because only one switch will be changed at a time and it will be easy to see the lamp status change.

Table 1. Switch combinations in Gray code sequence.

SW1  SW2  L1   L2
-------------------
 0    0   ON   --
 1    0   --   ON
 1    1   ON   --
 0    1   --   ON
 0    0   ON   --   <-- First step repeated

With the Gray code it is easy to see that every one-switch change results in a reversal of the lights' status.

DC solution

^{simulate this circuit}

Figure 3. The DC version.

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Binary sequence

Here is the pattern in binary code sequence.

Table 2. Switch combinations in binary code sequence.

SW1  SW2  L1   L2
-------------------
 0    0   ON   --
 1    0   --   ON
 0    1   --   ON
 1    1   ON   --
 0    0   ON   --   <-- First step repeated

Note how much more difficult it is to visually verify that the requirements have been met.

Answer 2

This might be the answer.

^{simulate this circuit – Schematic created using CircuitLab}