Half Adder, Full Adder and 4-bit Ripple-Carry Adder in Logisim-evolution
Adders are the most common first lab assignment in digital logic courses: design a half adder, extend it to a full adder, then chain four full adders into a 4-bit ripple-carry adder. Here is the whole path in Logisim-evolution.
Step 1: The half adder
A half adder adds two single bits, A and B, and produces a Sum and a Carry:
- Sum = A XOR B
- Carry = A AND B
Build it: place two Input Pins (Wiring library), one XOR gate and one AND gate (Gates library), and two Output Pins. Wire A and B into both gates, connect the XOR output to the Sum pin and the AND output to the Carry pin. Poke the inputs with the Poke tool (the hand icon) and check all four combinations against the truth table.
Step 2: The full adder
A full adder adds three bits: A, B, and a carry-in. The standard construction uses two half adders and one OR gate:
- Feed A and B into the first half adder → gives Sum1 and Carry1.
- Feed Sum1 and Carry-in into the second half adder → gives the final Sum and Carry2.
- OR Carry1 and Carry2 together → the final Carry-out.
Instead of redrawing the half adder twice, turn it into a subcircuit: select Project → Add Circuit…, name it HalfAdder, build the half adder there, then place it twice like any other component. See our subcircuits tutorial for the full workflow.
Step 3: The 4-bit ripple-carry adder
Chain four full adders: the carry-out of bit 0 feeds the carry-in of bit 1, and so on. Use Splitters (Wiring library) to break your 4-bit input buses into single wires, or keep it simple with eight individual input pins and five output pins (4 sum bits + final carry). The “ripple” name comes from the carry rippling from the least significant bit to the most — you can watch it happen with Simulate → Single Step Propagation (Ctrl-I).
Quick check
Test 0101 + 0011 = 1000 (5 + 3 = 8). If any wire turns red, two outputs are driving the same wire — see troubleshooting.
Next: half and full subtractors, which mirror this exact structure.