When we say we have a "digital" circuit, it means that we create an entire new domain that is purely represented by an analog signal. What that means is, referring back to our basic circuit of a voltage source and a resistor, we can say that the different voltage levels can represent a "low" or "high". Using electrical and computer engineering conventions, we classify a logic "high" as anywhere from 4.5 volts to 5.0 volts and a logic "low" as anywhere from 0.1 to 0.5 volts.
So basically, we say that a 0 is inputted when the voltage is between 0.1-0.5 volts and a 1 if the voltage is 4.5-5 volts. Ideally however, we want 0.0 and 5.0 volts to represent 0 and 1 respectively and pictorially is represented below.
high 1 or on
low 0 or off
X 100M
X 100K
X 100
X 100m
X 100μ
Ω
A
DC
AC
C L
X 100M
X 100K
X 100
X 100m
X 100μ
Ω
A
DC
AC
C L
With this in mind, we can now move on and discuss logic gates which form the foundation of combinational circuits and analysis.
In electronics a NOT gate is more commonly called an inverter. The circle on the symbol is called a bubble, and is used in logic diagrams to indicate a logical inversion between the external logic state and the internal logic state (1 to 0 or vice versa). On a circuit diagram it must be accompanied by a statement asserting that the positive logic convention or negative logic convention is being used (high voltage level = 1 or high voltage level = 0), respectively). The wedge is used in circuit diagrams to directly indicate an active-low (high voltage level = 0) input or output without requiring a uniform convention throughout the circuit diagram. This is called Direct Polarity Indication. See IEEE Std 91/91A and IEC 60617-12. Both the bubble and the wedge can be used on distinctive-shape and rectangular-shape symbols on circuit diagrams, depending on the logic convention used. On pure logic diagrams, only the bubble is meaningful.
