Powering a logic probe: Before a handheld logic probe can indicate HIGH/LOW/pulse states, its red and black leads must be connected to what?

Introduction / Context:
A logic probe is a simple but essential tool that reports logic levels and activity on digital nodes. Like most active test instruments, it requires power to operate its internal comparators, indicators, and pulse-stretch circuitry. This question clarifies the necessary initial connection before probing any signal line.

Given Data / Assumptions:

• Standard logic probe with red (V+) and black (ground) leads.
• Target circuit provides suitable supply rails (e.g., +5 V and ground for TTL, or +3.3 V for modern CMOS).
• The probe thresholds are referenced to the connected supply.
• The probe is not a passive device like a simple LED tester.

Concept / Approach:
The logic probe’s detection thresholds and pulse indicators must be referenced to a known supply to correctly interpret what constitutes a HIGH or LOW. Connecting the red lead to the system’s V+ and the black lead to ground powers the probe and aligns its thresholds to the system’s logic family. Without a power source, the probe cannot operate or may misreport levels due to floating references.

Step-by-Step Solution:

Verification / Alternative check:
Most probes include a power LED that lights when properly powered; some provide a self-test or reference pad to confirm proper operation before use.

Why Other Options Are Wrong:

Common Pitfalls:
Powering the probe from a different voltage than the circuit under test (threshold mismatch); forgetting ground reference; probing high-voltage or non-logic lines and damaging the probe.

Final Answer:
power source