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Somebody taught me this circuit in my childhood and I haven't forgotten it ever since.

enter image description here

If you close SW1 first, Lamp1 will light at full brightness, then if you turn on SW2, both Lamp1 and Lamp2 will light at full brightness.

If you close SW2 first, still Lamp1 will light up first at full brightness, then if you turn on SW1, both Lamp1 and Lamp2 will light at full brightness.

I shall be amazed again and again. :-)

I find it smart but once upon a time somebody told me that he can do this priority switching using fewer contacts.

Do you think it is possible? I could never find a simpler solution with switches and wires.

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    \$\begingroup\$ Instead of thinking of it as a "priority" function, think in terms of LAMP1 = SW1 "or" SW2, and LAMP2 = SW1 "and" SW2. Then an answer like Jonathan's becomes fairly obvious (and could be simplified even further). \$\endgroup\$
    – Dave Tweed
    Dec 18, 2022 at 13:32
  • \$\begingroup\$ Nice and clear description, thank you. But even further? I will be thinking on it. \$\endgroup\$
    – ha3flt
    Dec 18, 2022 at 14:08
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    \$\begingroup\$ It's an OR gate and an AND gate situation. \$\endgroup\$
    – Andy aka
    Dec 18, 2022 at 14:19
  • \$\begingroup\$ Thanks, I would upvote comments and answers but I have no permission for that yet. \$\endgroup\$
    – ha3flt
    Dec 18, 2022 at 14:27

1 Answer 1

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I have made a design using 4 switches. It functions as you said.

enter image description here

Case 1: When you turn ON SW1 first, Lamp 1 will glow. Thereafter, if you turn ON SW2, both Lamp 1 and Lamp 2 will glow.

Case 2: When you turn ON SW2 first, Lamp 1 glows. If SW1 is switched ON after that, both lamps would glow.

The switches in this case work together to function like an OR gate in the case of Lamp 1 and like an AND gate in the case of Lamp 2.

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  • \$\begingroup\$ Oh, yes, so easy. Thank you. :-) \$\endgroup\$
    – ha3flt
    Dec 18, 2022 at 14:03
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    \$\begingroup\$ Note that while you do need 4 sets of contacts, there's no reason that the circuits for LAMP1 and LAMP2 need to be interconnected in any way. \$\endgroup\$
    – Dave Tweed
    Dec 18, 2022 at 14:37
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    \$\begingroup\$ The interconnection can be removed if SW2 in series with lamp 2 is removed and placed in parallel with SW1 in the branch containing lamp 1. \$\endgroup\$
    – Jonathan_the_seagull
    Dec 18, 2022 at 15:05
  • \$\begingroup\$ I think you should remove the first schematic. The second one is simpler and makes the AND/OR situation much more clear. Having less clear matter in front that is unnecessary can only add confusion, not remove it. \$\endgroup\$
    – cjs
    Dec 19, 2022 at 5:10
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    \$\begingroup\$ @Jonathan_the_seagull Keep in mind that the point of the site is to find the best answer, not to document conversations and ideas eventually leading to the best answer. (Nonetheless, the older version will still be available in the edit history if anybody wants to look at it.) Treat answers here as if you were including them as a chapter in a book or a section of a magazine article: would your editor say that it's better to show both versions, including the more confusing one first, or better just to show the best and clearest one? \$\endgroup\$
    – cjs
    Dec 19, 2022 at 13:22

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