Logic probe
A logic probe is a cheap handheld tool used to see whether a line in a digital circuit is at 0 or 1. If you need to watch many signals at once, you’d use a logic analyzer. Probes are usually powered by the circuit they’re testing, though some run on batteries. They work with TTL or CMOS logic families (like 7400-series and 4000-series) that use similar voltages.
Most probes have LEDs to show the current state. They can either capture a single event or monitor continuously. If a wire isn’t connected or is at an invalid/tri-stated level, the LEDs stay off. A switch lets you choose TTL or CMOS because these families have different high and low voltage thresholds. Some models also make sounds: a tone for a high state, or different tones for high, low, or no connection. An oscillating signal makes the tone switch between high and low.
The logic probe was invented by Gary Gordon in 1968 at Hewlett-Packard.
This page was last edited on 2 February 2026, at 22:10 (CET).