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Basic Logic Gates
12 questions By Tony R. Kuphaldt
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Question 4 of 12
Crude logic gates circuits may be constructed out of nothing but diodes and resistors. Take for example this logic gate circuit:
Identify what type of logic function is represented by this gate circuit (AND, OR, inverter, etc.).
Reveal answerThis is an OR gate circuit.
Notes:Diode-resistor gate circuits such as this are rarely used in real digital circuitry because they have no capacity to amplify weak input signals. They also have no capacity for signal inversion, rendering them incapable of representing a great many logic function types. Gates made from transistors are much preferred over diode-resistor technology.
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Question 5 of 12
Crude logic gates circuits may be constructed out of nothing but diodes and resistors. Take for example this logic gate circuit:
Identify what type of logic function is represented by this gate circuit (AND, OR, inverter, etc.). Also, trace the directions of all currents in this circuit.
Reveal answerThis is an AND gate circuit.
Follow-up question: show how the two SPDT switches could be replaced by SPST switches (without the need for pullup or pulldown resistors).
Challenge question: explain why it may be impractical to design this diode-resistor gate circuit to directly drive an LED so that it lights up when the output state is “high.”
Notes:Diode-resistor gate circuits such as this are rarely used in real digital circuitry because they have no capacity to amplify weak input signals. They also have no capacity for signal inversion, rendering them incapable of representing a great many logic function types. Gates made from transistors are much preferred over diode-resistor technology.
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Question 6 of 12
Identify each of these logic gates by name, and complete their respective truth tables:
Reveal answer
Notes:In order to familiarize students with the standard logic gate types, I like to given them practice with identification and truth tables each day. Students need to be able to recognize these logic gate types at a glance, or else they will have difficulty analyzing circuits that use them.
