Logic probe diagnosis: A logic probe is placed on the input of a digital circuit and the probe's lamp blinks slowly. What condition does this slow blinking indicate about the signal at that test point?

Introduction / Context:
A logic probe is a handy troubleshooting tool for quick checks on digital nodes. Most probes use simple visual indicators (LEDs) or lamps to show whether a node is at a logic HIGH, a logic LOW, or is changing over time. Many also include a pulse indicator that blinks when the signal transitions. Understanding what the different indications mean is essential for quickly diagnosing timing and activity in a circuit.

Given Data / Assumptions:

• The probe is connected to a valid powered digital system ground and Vcc.
• The probe has indicators for HIGH, LOW, and pulse/activity.
• The observation is a slowly blinking lamp rather than a steady ON/OFF indication.

Concept / Approach:
Logic probes typically show steady HIGH or steady LOW with fixed indicators. When the node is switching, a pulse or activity indicator blinks. The perceived rate of blinking correlates with the repetition rate: slow blinking suggests a low-frequency pulse train or infrequent transitions. For very high frequencies, some probes latch or show a special pulse-only LED because human eyes cannot follow rapid blinking. Therefore, slow blinking corresponds to low-frequency toggling, not a steady state and not necessarily a fault.

Step-by-Step Solution:

Verification / Alternative check:
Confirm with an oscilloscope: a low-frequency square wave or intermittent pulse will produce a clear waveform matching the probe’s slow blink rate. If the signal is steady, the scope will show a DC level; if very fast, the scope will show a high-frequency waveform the probe might flag as “pulse” rather than visibly blinking.

Why Other Options Are Wrong:

• A steady HIGH logic level would show a solid HIGH indicator, not a blink.
• A high-frequency pulse train typically causes a pulse LED to flicker rapidly or latch, not slow, distinct blinks.
• An open/undefined level or supply problem often yields erratic or no indication, not a consistent slow blink.
• A steady LOW would show a solid LOW indicator.

Common Pitfalls:
Assuming any blinking means a fault; misinterpreting very fast activity as steady due to probe limitations; forgetting to attach the probe ground clip; probing tri-stated or high-impedance nodes that can float unpredictably.

Final Answer:
A low-frequency pulse or toggling activity is present at the input