This project is a simple 2-bit binary adder built entirely with transistors and discrete components.
Four pushbuttons are used to enter two binary numbers, and three LEDs display the result.
Each input number can have a value from 0 to 3, so the circuit can add two 2-bit numbers and produce a 3-bit result from 0 to 6.
How does it work ?
The circuit uses four pushbuttons as binary inputs.
SW1 represents the right-hand bit of the first number.
SW2 represents the right-hand bit of the second number.
SW3 represents the left-hand bit of the first number.
SW4 represents the left-hand bit of the second number.
A pushbutton that is not pressed represents 0, while a pressed pushbutton represents 1.
The result is displayed using three LEDs:
LED 1 represents the right-hand bit of the result.
LED 2 represents the middle bit.
LED 3 represents the left-hand bit.
The circuit is made from several transistor-based logic gates that perform the binary addition step by step.
The first bit
The first stage adds the two right-hand input bits, SW1 and SW2.
A two-transistor XOR gate controls LED 1.
An XOR gate produces a 1 when exactly one of its inputs is 1. Therefore, LED 1 lights when either SW1 or SW2 is pressed, but not when both are pressed.
The first pair of transistors therefore produces the right-hand result bit.
Another pair of transistors forms an AND gate. It detects when both SW1 and SW2 are pressed.
When both inputs are 1, the AND gate produces the carry that must be added to the next stage.
The second bit
The second stage adds three values:
the carry from the first stage,
SW3,
SW4.
This part of the circuit uses a three-input XOR gate.
An XOR with three inputs produces a 1 when an odd number of inputs are 1. Therefore, LED 2 lights when either one of the three inputs is 1 or when all three inputs are 1.
The three possible input values are:
carry + SW3 + SW4
The transistor network performs this XOR operation and drives LED 2.
The carry information from the first stage is therefore taken into account when calculating the middle bit of the result.
The final bit
LED 3 represents the final carry of the addition.
This part of the circuit uses three OR functions connected in parallel.
The final carry is produced when at least two of the three values involved in the second stage are 1:
SW3 and SW4 are both 1.
SW3 is 1 and the carry from the first stage is 1.
SW4 is 1 and the carry from the first stage is 1.
These three conditions are combined to control LED 3.
The circuit therefore performs the complete addition using transistor-based logic gates instead of integrated circuits.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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