Logic probe

While most logic probes are powered by the circuit under test, some devices use batteries.

This is required as these families have different thresholds for the logic-high (VIH) and logic-low (VIL) circuit voltages.

Some logic probes have an audible tone, of which vary across models.

An oscillating signal causes the probe to alternate between high-state and low-state tones.

The logic probe was invented by Gary Gordon in 1968 while he was employed by Hewlett-Packard.

Elenco LP-560 logic probe. It has a red LED for high state, green LED for low state, amber LED for pulsing state, and a changing audible tone. [ 1 ]