Full subtractor circuit

full subtractor circuit diagram 26

Full subtractor circuit,full subtractor circuit diagramFull Subtracrtor

Today’s Tutorial(full subtractor circuit diagram) we are going to discuss combinational logic circuit named Full Subtractor .  This circuit helps us in performing subtraction of 3binary inputs.

As you see in the block diagram Subsector have 2 input A & B and third one borrow in and 2 output Difference (D) and borrow out. The disadvantage of half subtractor is that it cannot cater the Borrow in.

As you see in the truth table 3 input mean 8 possible combination (0-7).

Now considering a combination from  the truth.

A                      B                     Bin                              D                           Bout

1                      0                      1                                 0                              0

For example B is subtracted from A,  A-B=1, Bin is subtracted from A-B , 1-Bin=0, mean D is 0 and we does not take any borrow so the Bout is also 0.

After completing the procedure of subtraction between 3 bits (input variables) we get 2 outputs variable Difference (D) and Borrow out (Bout).

full subtractor circuit diagram

Full subtractor circuit

The output variables are simplified with K-maps & we make logic expression.  After obtaining 2 output variables and their expression you need a circuit/ logic diagram. You can see the logic diagram in your screen. It consist 2 XOR, 2 AND, 2 NOT  and 1OR GATES. TOTALL 7Nos gates are used in construction of Full Subtractor . Interesting thing is that it also consists of NOT gates.



  • 74Ls86 1Nos
  • 74Ls08 1Nos
  • 74Ls04 1Nos
  • 74Ls32 1Nos
  • Bread Board
  • Toggle Switch 3Nos (for Input logic 0/1)
  • LED 2Nos    (for checking output status)
  • Resister 470 Ohm 2 Nos
  • 5volt DC Power supply


  • DMM
  • Side Cutter
  • Tweezers
  • Connecting Wires
  • Wire Striper

Assemble the circuit as you see in your screen. At the end connect the Power supply.

Now we come to circuit testing process full subtractor circuit diagram

When all inputs (A,B, Bin)  on Logic 0 than D and Bout is also on Logic 0.

If we set inputs A on logic 1 A=1,B on Logic 0 B=0 and Bin on Logic 1 Bin=1 with help of Toggle Switches.

we see the outputs D and Bout both outputs are on Logic 0 and no one LED is turn ON.

At the same way you can satisfy your truth table. So viewers following thesw designing step you can construct an operational Full subtractor.










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