Logic probe

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A logic probe is a hand-held pen-like probe used for analyzing and troubleshooting the logical states (Boolean 0 or 1) of a digital circuit. It is powered by the circuit under test (some devices use batteries). It can be used on either TTL (Transistor-Transistor Logic) or CMOS (Complementary Metallic Oxide Semiconductor) integrated circuit devices.

There are usually three different colored LEDs on the probe's chassis; - A RED and GREEN LED indicate High and LOW states respectivily; - An AMBER LED is indicates that a Pulse (a single occurring event). The pulse detecting electronics usually has a "pulse stretcher" circuit so that even very short pulses become visible on the AMBER LED. A control on the logic probe allows the capture & memory of a single event or to run continuously.

When the logic probe is either not connected to a valid logic level (IE. a fault condition) or not connected at all, no LED's will be on.

Another control on the logic probe allow selection of either TTL or CMOS family. This is required as these families have different thresholds for V_IH & V_IL

Some logic probes have a separate audible tone for each of the logical states. An oscillating signal causes the probe to alternate between "High-state and Low-state tones".

A Logic probe is a cheap, versatile and convenient digital test instrument.

When many of logic levels need to be observed simultaneous or recorded a Logic Analayzer is used.