The Opposite Switch (aka the NOT Logic Gate)

In digital electronics, a special circuit symbol is used for the NOT Logic Gate consisting of a circle attached to the point of a triangle on its side. Figure 5-3 shows the digital electronics circuit symbol for the NOT Logic Gate.

Figure 5-3. The NOT Logic Gate circuit symbol

In the Fritzing wiring diagram shown in Figure 5-2, the +3V battery provides the input binary data for the NOT Logic Gate and the electrical circuit output using an LED. With the switch open, the LED turns on. When the switch is closed, the LED turns off. Therefore, the purpose of the NOT Logic Gate circuit is to make a decision that is the opposite of the norm.

The NOT Logic Gate is traditionally used to invert a control signal used by smart machines like robots. To show the NOT Logic Gate’s decision operation graphically, a truth table (TT) is used; this is shown in Figure 5-4. A truth table is used in digital electronics to show the operation of computer logic circuits in a simple data table. The input column is the digital data or information applied to the logic gate. The output column shows the logic gate’s decision.

TECH NOTE

Another term for a NOT Logic Gate in digital electronics is “Inverter.”

Figure 5-4. The NOT Logic Gate truth table