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

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

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

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

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.

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.

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