Logic Probe With Sound


This logic probe can be selected to operate on TTL or CMOS
logic levels, depending on switch S1. A string of resistors associated
with switch S1 sets the threshold levels for a window comparator
comprising IC1a and IC1b. Depending on whether the level applied to the
probe is high or low, the window comparator turns on LED1 (high) or LED2
(low). The 1.2M and 680k resistors set the probe signal to a midrange
value when the probe is open-circuit, thereby preventing either LED from being lit.

Circuit diagram:

Logic Probe With Sound

Logic Probe With Sound

If a pulse signal is present, the output of IC1a will toggle the clock
input of flipflop IC2a. This drives LED3 which either lights for each
pulse or continuously, depending on the setting of switch S2. Finally,
the outputs of IC1a & IC1b are connected by diodes D5 & D6 to
the base of transistor Q1 which is connected to the Reset input of
flipflop IC2b. This has a piezo sounder (not buzzer) connected between
its Q and Q-bar outputs so that it produces a sound which echoes the
input pulse signal.

Author: Tom Hughes
Copyright: Silicon Chip Electronics

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