How to Simulate Full Adder Circuit

A full adder takes three single-bit inputs, two operand bits A and B and a carry-in bit Cin, and produces two outputs: Sum and Carry-out (Cout). This tutorial walks you through using the simulator to verify each of the eight input combinations and confirm that the outputs match the standard full adder truth table.

• How a full adder extends a half adder by incorporating the carry-in bit
• How to toggle three independent inputs in the simulator
• How to read and record Sum and Cout for all eight input combinations
• How to verify your results against the full adder Boolean expressions: Sum = A XOR B XOR Cin and Cout = AB + BCin + ACin

• Understanding of binary addition and what carry means
• Familiarity with XOR, AND, and OR gate behaviour
• Completion of the half adder lab or equivalent knowledge

1. Step 1: Open the simulator
   Navigate to the Full Adder simulator page. You will see three toggle switches on the left panel labeled A, B, and Cin, and two LED indicators on the right panel labeled Sum and Cout. All three inputs start in the OFF (0) state.
   Tip: Confirm both LEDs are dark before you begin. If any LED is lit, click the corresponding toggle to reset it to 0.
2. Step 2: Test the 0,0,0 case
   Leave A, B, and Cin all at 0. Observe that both Sum and Cout LEDs remain unlit. This confirms that 0 + 0 + 0 = 0 with no carry.
   Tip: Write down the result immediately. Keeping a running table prevents confusion when you compare rows later.
3. Step 3: Set Cin to 1 only
   Click the Cin toggle to turn it ON. A and B remain 0. Sum should light up and Cout should stay dark. This confirms 0 + 0 + 1 = 1 with no carry.
   Tip: Sum lighting without Cout means the total is 1, which fits in one bit.
4. Step 4: Test A=1, B=1, Cin=0
   Click Cin back to 0, then click A and B to ON. Sum should now be dark and Cout should be lit. This represents 1 + 1 + 0 = binary 10, where the 0 is Sum and the carry is Cout.
   Tip: This is the first case where a carry is generated without a carry-in. Note that Sum is 0, not 1.
5. Step 5: Test A=1, B=1, Cin=1
   With A and B still ON, click Cin to ON as well. Both Sum and Cout should now be lit. This represents 1 + 1 + 1 = binary 11, where Sum is 1 and Cout is 1.
   Tip: This is the maximum input case. If Cout is dark here, check that your B toggle is actually in the ON state.
6. Step 6: Complete the remaining four rows
   Work through A=1,B=0,Cin=0 then A=1,B=0,Cin=1 then A=0,B=1,Cin=0 then A=0,B=1,Cin=1. For each, toggle the inputs one at a time and record Sum and Cout before moving to the next combination.
   Tip: Change only one input per step where possible. This makes it easier to isolate which toggle is having an effect.
7. Step 7: Compare against truth table
   Once all eight rows are recorded, compare your Sum and Cout values against the standard full adder truth table. Every row should match. If any row differs, re-run that combination by toggling all three inputs to 0 first and then setting only the required inputs.
   Tip: A mismatch on multiple rows that all share the same Cin value often means Cin was accidentally left in the wrong state.

For all eight input combinations, the Sum and Cout LEDs should match the full adder truth table exactly. When A=1, B=1, Cin=1, both LEDs are lit. When A=0, B=0, Cin=0, both LEDs are dark. The Sum LED follows A XOR B XOR Cin and the Cout LED follows AB + BCin + ACin for every row.

• If Sum is always lit regardless of inputs, the simulator may not have loaded correctly. Refresh the page and allow all assets to load before toggling any input.
• If Cout is dark when A=1, B=1, Cin=1, check that all three toggles are visually in the ON position. A toggle that looks ON but was clicked twice quickly may have returned to OFF.
• If Sum and Cout are both lit when only one input is ON, the circuit logic in the simulator is showing a stale state. Reset all inputs to 0 by clicking each toggle twice, then proceed again.
• If the LED colours are identical for ON and OFF states, your display brightness may be masking the difference. Look for a glow or ring around the LED circle rather than colour alone.
• If results match for all rows except those where Cin is involved, verify that you are toggling the correct switch. Cin is the rightmost toggle in the input panel, not the middle one.