Where’s The Logic In That?

The above is a modified image from page 193 of our best-selling book Make: Electronics by Charles Platt. It illustrates the 7432-series Integrated Circuit, a Quad 2-input OR gate IC, which is the centerpiece (along with a pair of 555 Timers) for Game Show Buttons, a quiz show circuit for two players. The OR gate is one of seven common logic gates, the complete list being: AND, NAND, OR, NOR, XOR, XNOR, and NOT (or as most call it, an Inverter). The OR’s output, as the name implies, is true if either gate A “or” gate B are true (or if both are true).

There are several ways to visualize – to aid in understanding – logic gates. These can include truth tables (the OR truth table is shown here), Venn diagrams (see Wikipedia’s entry on logic gates), or my favorite the “mechanical comparison” which Platt makes on pages 187-189 of his book, visualizing the notion of logic gates with physical sliding plates on a bubblegum machine. There are also several ways to visualize the pin order of gates. Sometimes these are shown as lines from left to right through the IC, with inputs on the left and outputs on the right. But the most common, and by far the most helpful, is the technique shown above, which imposes the gates on a diagram of the IC, with inputs and outputs shown as line traces, and symbols corresponding to the type of gate.

We’ll explore one other type of logic gate in a future installment of Weekend Projects, but be sure to pick up the Make: Electronics books for further reading on logic gates, as well as several breadboard experiments that put these theories to use!

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