## 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.

### Comments

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

i don’t understand

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

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

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

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

what about full adder?

• written by: Mihai on June 29, 2021 at 2:58 pm
5. written by: mark on December 9, 2023 at 4:42 am

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

Yes. It’s a mistake in the schematic.

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

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

Here you can find a block diagram showing how to connect multiple adders.
https://www.antimath.info/electro/4-bit-relay-computer/

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.

This site uses Akismet to reduce spam. Learn how your comment data is processed.