# Simple two bulb circuit

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.

• OMG I knew what the question was going to be before I read the whole thing. It's how lights in the house are wired when you got 2 fliky switches. Aug 24 '16 at 9:12
• Wait a second, I didn't predict correctly. I'm not sure if this is possible. Aug 24 '16 at 9:20
• I think the XOR gate is what you are looking for... Aug 24 '16 at 9:44
• "Must use both." Is this a homework assignment?
– JRE
Aug 24 '16 at 9:58
• Smells like a HW assignment. But I believe more components should be involved. Aug 24 '16 at 9:59

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.

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.

• He said to only use two SPDT switches, not a SPDT and a DPDT. Aug 26 '16 at 4:06
• I know but I don't think there's a solution without using more components - either some electronics or a relay to double up the available contacts. I don't think the OP has given us all the constraints. (Nobody else has solved it either.) Aug 26 '16 at 6:08
• Well my answer got trashed on. And I was within the restrictions. D; Aug 26 '16 at 8:05
• It's a tough world in here! ;^) OP needs to state AC, DC, wattage, LED/filament, what other components are allowed ... Aug 26 '16 at 8:10
• Well honestly the OP did not specify how the circuit could be powered, so I assumed it could be in any way. But he did say bulbs which most likely means incandescent bulbs, not LED. Also since he did not specify a power supply, I could get a power supply that has an abnormally high resistance like 1k ohm. Aug 26 '16 at 8:13