Debugging a 1-Bit Adder

Posted on: Feb 17 2025

I'm working on building an 8-bit adder using transistors. While soldering together this 1 bit adder I made a mistake and had to debug it to figure out where the error was. Here's how I did it.

Soldered 1-bit adder

Testing

When testing the board I immediately realized I had made a mistake, because it did not produce the expected output.

1-bit adder truth table:

A	B	Cin	Sum	Cout
0	0	0	0	0
0	0	1	1	0
0	1	0	1	0
0	1	1	0	1
1	0	0	1	0
1	0	1	0	1
1	1	0	0	1
1	1	1	1	1

1-bit adder A-input

Expected:

A	B	Cin	Sum	Cout
1	0	0	1	0

Actual:

A	B	Cin	Sum	Cout
1	0	0	1	1

1-bit adder B-input

Inputs: A=0, B=1, C_in=0 Expected output: Sum=1, C_out=0 Actual output: Sum=1, C_out=1

Expected:

A	B	Cin	Sum	Cout
0	1	0	1	0

Actual:

A	B	Cin	Sum	Cout
0	1	0	1	1

1-bit adder C_in-input

The Carry-in input (C_in) resulted in the correct output.

A	B	Cin	Sum	Cout
0	0	1	1	0

Comparing to the simulator The next step was to compare these results with the circuit I had build in Logisim. Here's a screenshot of the same circuit (at the logic gate level) in Logisim:

logisim 1-bit-adder

Here's the circuit with the gates labelled to match the simulator:

labelled_gates_1_bit_adder_debugging

By comparing the state of each gate, we can see that only the I (Sum) gate is incorrect, so it was likely that one of the inputs to that gate was soldering incorrectly. Looking at the inputs to the I (labelled III in Logisim) and considering the truth table for a NOR gate, the only way for that gate to be ON is when both inputs are OFF.

NOR gate truth table:

A	B	OUT
0	0	1
0	1	0
1	0	0
1	1	0

We expect the top input of that gate to be connected to the output of the E gate, which is correctly ON in this state, so it is a safe guess that the connection between the output of the E gate and that input to the I gate to be soldering incorrectly.

That turned out to be correct and here is the underside of the completed circuit: 1-bit adder underside

Now I just need to make 7 more...

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