Circuit states

Let's talk in terms of "on" or "off". In an electrical circuit, when a voltage is present we say that the circuit is "on". When there is no voltage present, we say that the circuit is "off".

We represent these states with the digits 0 and 1:

• the "zero" state means the circuit is off -> 0
• the "one" state means the circuit is on -> 1

Consider the three diagrams shown below, the binary logic ciruit, the AND gate and the OR gate. Each of these example circuits have the following components:

• a power source (such as a battery)
• a switch (or switches)
• a light bulb

These circuit components are represented by the following symbols, and in the case of the switch, the logic state that it can represent.

Component	Component Description	Logic State
	Wire, an electrical conductor that transmits signals	0
	Switch is shown open (the switch is off)	0
	Switch is shown closed (the switch is on)	1
	Switch is shown closed (the switch is on) and is labeled "S1". Labeling is common in circuits where there is more than one of a type of component in a circuit.	1
	Lightbulb	n/a
	Battery (the power soure)	n/a

 

In the circuits below the switches are initially open, in the off position, and the circuit is known to be in the 0 state. When the switch(es) are turned on in the proper configuration, the circuit is in the 1 state and the light bulb illuminates.

And now, a description of each of the circuits.

Binary logic (left most circuit in the below diagram)

We can represent the position of the switch in a circuit, along with the corresponding states, in a table:

Switch Position	Light Status
0	0
1	1

AND logic (center circuit in the below diagram)

Now consider two switches connected in series (i.e., one right after another in the circuit). We now have two switch positions that influence the light status:

Switch 1	Switch 2	Light
0	0	0
0	1	0
1	0	0
1	1	1

We call this combination of switch positions AND logic:

• 0 AND 0 yield a 0 result
• 0 AND 1 yield a 0 result
• 1 AND 0 yield a 0 result
• 1 AND 1 yield a 1 result

OR logic (right most circuit in the below diagram)

Now consider two switches again, but this time in parallel (i.e., connected on separate branches from the "main" circuit). We still have two switch positions to consider in determining the light status, but the outcomes behave in a different way:

Switch 1	Switch 2	Light
0	0	0
0	1	1
1	0	1
1	1	1

We call this combination of switch positions OR logic:

• 0 AND 0 yield a 0 result
• 0 AND 1 yield a 1 result
• 1 AND 0 yield a 1 result
• 1 AND 1 yield a 1 result

Note: In the following diagram, click on the switches to see how they affect the circuit.

Try the diagram below to familiarize yourself with the different combinations of 0 and 1 to see the result:

Or view the interactive diagram in a new window.

We call these diagrams truth tables, and they form the basis for what is known as Boolean logic. Boolean logic contains the basic principles of digital operation that allow the CPU to operate in a coordinated manner to achieve the desired result.