Logic systems that yield two or more signal outputs in the presence of the input are scarce. A universal logic system consisting of plasmonic MoO.sub.3-x nanodots and 3,3'-diaminobenzidine (DAB) for fabrication of visual contrary logic pairs and circuits are presented here. They do not require the use of expensive instrumentation but can be visually read. It is based on the facts that the blue dispersion of MoO.sub.3-x nanodots turns to colorless after oxidation, while the colorless reagent DAB is oxidized by various oxidants to generate a brown color. On this basis, the complete contrary logic pairs and circuits such as YES-NOT, AND-NAND, OR-NOR, XOR-XNOR, INH-IMH, and MAJ-MIN can be fabricated. Various oxidants serve as inputs, and absorbances as outputs. A smart logic voting system with "one-vote deny" function is also described that is based on the cascade of MAJ logic circuit and INH logic gate using ascorbic acid (AA) as the superior denier. All the logic operations can visually read due to the appearance of distinct color changes.