1. Electronics: Principles and Applications (Instrumentation Labs)
Eighth Edition
(Instrumentation Labs)
Charles A. Schuler
Lab 3
Introduction to the
Logic Probe
McGraw-Hill
©2013 by The McGraw-Hill Companies, Inc. All rights reserved

2. Lamp OFF … LOGIC LOW
Lamp ON … LOGIC HIGH
Lamp DIM … OPEN CIRCUIT (FLOATING) OR BAD LEVEL
(Other brands of probes may differ)

3. Digital logic levels as % of VSUPPLY
TTL
CMOS
(5 V supply)
(3 to 18 V supply)
100 90
HIGH
= 1 = ON 80
HIGH 70 60 50
UNDEFINED
= ? = FLOATING 40 30
UNDEFINED 20
LOW
= 0 = OFF 10
LOW
% of VSUPPLY

4. Common faults that can be detected with logic probes
Internal short
(stuck high)
Open bond
(floating output)
Solder bridge
(stuck low)
Defective input

5. Logic probe with pulse memory (often used to catch “glitches”)
1. Set TTL/CMOS switch
to family under test.
2. Place tip on circuit
under test.
3. Press MEM/CLR
(light goes out).
4. Light comes on when
a single pulse (“glitch”)
occurs.

6. Pulse trains cause the probe to flash at less than a 10 Hz rate
even if the pulse frequency is much higher. (up to 80 MHz)
This probe will “stretch” pulses as
short as 10 ns and the lamp will flash.

7. Logic Probe Quiz When the lamp is off, the logic level is LOW
When the lamp is on, the logic level is
HIGH
When the lamp is dim, the logic level is
bad or floating
When the lamp is flashing, the logic level is
a pulse train
A probe with pulse memory is useful when
looking for
glitches