Logic GatesHow Boolean logic is implemented

• Transistors used as switches to implement Boolean logic:

AND OR

Logic with Transistors

Logic Gates

• Digital electronics : design of binary systems• Gates are basic building block:– One for each of 6 Boolean logic rules

at tip means NOT

Not

Plus one additional gate• NOT gate reverses a signal

Input Output

0 1

1 0

Combinations

• Output of one gate can serve as the input to the next gate

• Use to produce new logic:– Ex: Two ANDs used to make a 3 input AND

Interactive

Combinations

• Can also create basic gates from other ones• Ex: AND fed into NOT same as NAND

• NOR and NAND gates are universal– Can be used to make any other gate

• Ex: AND made with NORs:

Interactive

Universal Gates

• NOR gates can be single transistor– Electrically simple to design– Apollo’s mission computer was built

exclusively from NOR gates

NOR

Universal Gates

Useful Circuits:

• Simple Memory• Decoder• HalfAdder

Building A Truth Table

• Inputs are always the same (need every combo)– You figure out outputs:

• More inputs = more combinations 3inputs = 8 rows4 inputs = 16 rowsetc…

Input0 Input1 Output00 0 ?0 1 ?1 0 ?1 1 ?

Using the Circuit Simulator

• Always start with DC power• Use Toggles as inputs– Click name below components to rename

• Use LEDs as outputs• Wire by dragging from one

connector dot to another

Off On

Using the Circuit Simulator

• Red connector dots have power, black do not– Top input has power at

output– Bottom input is not letting

power through

• Toggle switches are “On” when light part is at bottom: