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A logic probe is a hand-held pen-like test probe used for analyzing and
troubleshooting the logical states of a digital circuit. While most are powered
by the circuit under test, some devices use batteries. They can be used on
either TTL (transistor-transistor logic) or CMOS (complementary metallic oxide
semiconductor) integrated circuit devices.
There are usually three differently-colored LEDs on the probe's body:
- Red and green LEDs indicate high and low states respectively
- An amber LED indicates a pulse (as used in a NOID Light to test for pulses
to fuel injectors on a electronically controlled fuel injection vehicle)
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 either the capture and storage of a single event or
A low-cost logic probe When the logic probe is either connected to an invalid
logic level (a fault condition or a tri-stated output) or not connected at all,
none of the LEDs lights up.
Another control on the logic probe allow selection of either TTL or CMOS
family logic. This is required as these families have different thresholds for
VIH and VIL.
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,
but can test only a single signal at a time. When many logic levels need to be
observed or recorded simultaneously, a logic analyzer is used.
It is recommended when you make any digital circuit to check the high & low
status (1 or 0)