Full Adder (1-bit)
Full Adder (1-bit) is a runnable microcontroller simulation example on SensBench — open the wired circuit, inspect the code, and compile real firmware in your browser, no install needed. Last updated Aug 14, 2026.
FREE SIMULATOR EXAMPLE · INTERMEDIATE
Full Adder (1-bit)
This example computes a 1-bit full adder on the Arduino Uno: three buttons are A, B and the carry-in, and the sketch prints the sum and carry-out for every combination. It is the exact arithmetic cell inside every CPU’s adder, reduced to buttons and LEDs.
| Board | Arduino Uno (ATmega328P) + logic / power circuit |
|---|---|
| Difficulty | intermediate |
| Category | circuits |
| Libraries | None — just the Arduino core |
| Runs in | Sensbench browser simulator — free, no account |
The exact circuit you get when you open this example — already wired, ready to run.
The sketch
// 1-bit full adder in software
void setup() {
Serial.begin(9600);
pinMode(2,INPUT_PULLUP); pinMode(3,INPUT_PULLUP); pinMode(4,INPUT_PULLUP);
pinMode(5,OUTPUT); pinMode(6,OUTPUT);
}
void loop() {
bool a=!digitalRead(2), b=!digitalRead(3), cin=!digitalRead(4);
bool sum = a ^ b ^ cin;
bool cout = (a&b) | (cin&(a^b));
digitalWrite(5, sum);
digitalWrite(6, cout);
Serial.print("A="); Serial.print(a); Serial.print(" B="); Serial.print(b);
Serial.print(" Cin="); Serial.print(cin); Serial.print(" Sum="); Serial.print(sum);
Serial.print(" Cout="); Serial.println(cout);
delay(300);
}How it works
Three buttons on pins 2-4 supply A, B and Cin (inverted from the pull-up wiring), and the loop computes the sum with a ^ b ^ cin — XOR counts the odd number of HIGH inputs — and the carry with (a&b) | (cin&(a^b)). Pins 5 and 6 drive the sum and carry LEDs, and the serial line prints the full truth-table row.
The 300 ms polling turns the three buttons into a slow-motion truth table: press combinations and read the printed row. A=1, B=1, Cin=0 gives Sum=0, Cout=1 — the 1+1=10 moment where binary addition produces a carry, exactly like decimal 5+5=10 produces a tens digit.
Before you build it for real
The software XOR solution hides what hardware gates must handle: a physical full adder is built from many gates with propagation delays, and the carry output arrives later than the sum — the classic carry-propagation delay that limits every ripple-carry adder’s speed (the reason real CPUs use lookahead carry). The pull-up inversion applies to all three inputs: forget the ! on any one and that bit reads backwards in the truth table. And one full adder is just one bit position — chaining to a real byte-wide adder multiplies the delay, which is the step from this example to actual computer arithmetic.
Get the parts
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FAQ
Why three inputs for adding two bits?
Adding two bits can produce a carry — the third input (Cin) is the carry coming in from the previous bit position. The full adder sums A + B + Cin, producing a 1-bit sum and a carry out. Chain these cells together and you have a multi-bit binary adder — the exact structure inside every CPU’s arithmetic unit.
What does the Cout formula mean?
Cout = (A AND B) OR (Cin AND (A XOR B)) — a carry results if both A and B are set, or if one of them is set while a carry is coming in. The formula looks intimidating but is just the English sentence “at least two of the three inputs are on” written in boolean algebra.