1bit half adder with one relay

With just one double relay, you can make an adder capable of performing 1bit additions:

0 + 0 = 0
0 + 1 = 1
1 + 0 = 1
1 + 1 = 0 ( with Carry = 1 )


The circuit design isn’t mine, I found it with a quick google search.



This is the complete schematic of the above circuit.


  1. written by: model k on April 22, 2014 at 1:29 am - Reply

    i don’t understand

  2. written by: model k on April 22, 2014 at 1:38 am - Reply

    that schematic is not clear to me i have a short of seven years of studying electronics as an hobby and i can’t get what is drawn

    • written by: Mihai on April 22, 2014 at 9:39 pm - Reply

      That is a double relay, A and B are inputs ( and as it is shown in the truth table, A and B can only have two values, “1” (V+) or “0” (GND), this is done by switches ), SUM and CARRY are outputs, therefore connected to some indicators ( light bulbs, LED, voltmeters, etc).
      V+ is the operating voltage of the relay used.

  3. written by: Bilal on May 28, 2014 at 10:07 pm - Reply

    I wonder if you could make a detailed schematics for your design , I have been trying to make my own copy but I could not manage to work B input , except that all functions are working.

  4. written by: akismet on June 28, 2021 at 6:17 am - Reply

    what about full adder?

  5. written by: mark on December 9, 2023 at 4:42 am - Reply

    Thanks, good work. Just a question:

    1) surely the resistor needs to be before the LED?

    • written by: Mihai on December 15, 2023 at 3:18 pm - Reply

      Yes. It’s a mistake in the schematic.

  6. written by: mark on December 9, 2023 at 5:19 am - Reply

    My second question is: Can you use two of these (with the carry from the first adder being the input to the second) to make a full adder? I don’t have the where-with-all to draw it out, as my head is hurting enough trying to figure out your drawing as it is… Any chance you can draw it out to make a full adder with just two SY4008 relays as you have in the original half adder… Thanks, M

    PS: in your drawing, it is a little confusing if wires cross it is not clear if they are ‘jumping over’ or ‘joined’… Thanks, M

    • written by: Mihai on December 15, 2023 at 3:24 pm - Reply

      Here you can find a block diagram showing how to connect multiple adders.

      Sorry, indeed the schematic is not the prettiest.
      It was drawn a very long time ago, and somewhat in a hurry if I remember correctly. If there is a dot where the lines meet, it means there is an electrical connection, if not there is no connection.

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