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I'm trying to design a simple circuit that is fail-safe. I have an infinite number of SPDT switches at my disposal and the situation I'm trying to avoid it having all lamps lit simultaneously.

With two lamps, the solution is quite trivial, and I've illustrated it below. In the circuit shown, it is physically impossible to have both lamps lit at the same time.

I now want to extend this to three lamps: any combination of zero, one or two lamps can be lit, but having three lamps lit at the same time should be impossible. Is this even possible?

enter image description here

aidan
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    A 3 way push button type switch with only one selectable at any one time. Have two of those and you get what you describe... – Solar Mike Mar 18 '20 at 06:21
  • There are some implicit assumptions about failure modes that you are making. If the lower switch fails short between NO and NC you can have both lamps on. – Spehro Pefhany Mar 18 '20 at 07:20
  • Think about What is a PARITY bit using XOR? with as many data bits as you want Choose ODD to mean the Lamp is always odd – Tony Stewart EE75 Mar 18 '20 at 07:46
  • @Tony Stewart Sunnyskyguy EE75 Here I understand he wants to keep using only hardware switches as logic gate. That's like 19th century electronic. :) – Fredled Mar 18 '20 at 09:12
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    SPDT is EQUIV to XOR @Fredled Design by Logic Table...implement any way you want. – Tony Stewart EE75 Mar 18 '20 at 09:13

2 Answers2

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Here's one way to do it.

schematic

simulate this circuit – Schematic created using CircuitLab

I've used one more switch than is strictly necessary, but seeing as you have an infinite number that shouldn't be a problem :)

Jasen Слава Україні
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"Buddy, can you spare a diode?"

With only SP switches I can't see a solution. You could "gang" them as shown on the left two switches below in which case Jasen's solution would work.

enter image description here

Figure 1. Left two switches are ganged. Right two switches are mechanically interlocked so they can only switch up if the left two are up. Source: EE.SE unattributed.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. With diodes the configuration becomes simple.

Transistor
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  • Diodes would be a great solution, but unfortunately, although I've drawn a DC circuit above, I'm actually working with AC. I love the hardware fail-safe! – aidan Mar 18 '20 at 13:57
  • Well why did you do that! I can only answer the question you asked, not the one that's in your head. Please fix your question. – Transistor Mar 18 '20 at 13:58
  • My mistake - sorry about that. I've updated the question. – aidan Mar 18 '20 at 14:15