Digital Circuits

Basic Logic Gate Troubleshooting

22 questions By Tony R. Kuphaldt

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Question 4 of 22

Predict how the operation of this logic gate circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Output of NAND gate U₂ fails low:

Output of buffer gate U₃ fails low:

Output of NOR gate U₁ fails high:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Output of NAND gate U₂ fails low: Gate U₄ output stuck in the low state.

Output of buffer gate U₃ fails low: Gate U₄ output stuck in the low state.

Output of NOR gate U₁ fails high: Gate U₄ output simply equal to [C]D, no other inputs have any effect on U₄‘s output.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 5 of 22

This circuit is supposed to energize a lamp when the input voltage (V_in) falls between the two reference voltages set by R_pot1 and R_pot2. Predict how the operation of this circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Comparator U₁ output fails low:

Comparator U₁ output fails high:

Comparator U₂ output fails low:

Comparator U₂ output fails high:

Wire connecting V_DD to R_pot1 fails open:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Comparator U₁ output fails low: Lamp energizes when V_in > V_ref(low), even if V_in > V_ref(high).

Comparator U₁ output fails high: Lamp energizes only when V_in < V_ref(low).

Comparator U₂ output fails low: Lamp energizes only when V_in > V_ref(high).

Comparator U₂ output fails high: Lamp energizes when V_in < V_ref(high), even if V_in < V_ref(low).

Wire connecting V_DD to R_pot1 fails open: Lamp refuses to energize.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 6 of 22

This circuit is supposed to energize the green lamp when the input voltage (V_in) falls between the two reference voltages set by R_pot1 and R_pot2, and energize the red lamp when the input voltage exceeds both reference voltages. However, something is wrong with this circuit: the green lamp operates just as it should, but the red lamp never turns on even when it is supposed to.

A technician decides to replace the red lamp, thinking it is burned out. This, unfortunately, does not fix the problem. Identify two possible component faults that could account for this problem, and describe what further diagnostic steps you would take to determine the precise nature of the fault.

Reveal answer

U₃ and Q₁ are the most suspect components, given the behavior of the circuit. I’ll let you figure out what to measure next!

Notes:
Discuss your students’ answers to this question and their troubleshooting strategies. The latter part of the question, where students are asked to explain what they would do next, is the most important part!