Audible Logic Probe

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We can use NE556 timer IC to make indication of static of terminal in digital logic audible.  Audible logic probe can be very useful when we have to visually inspect a part while need to check the logic state on other point apart from the part.  Here’s the circuit diagram:

Audible Logic Probe

NE556 will act as oscillator controlled by IC1 (SN74132N) and IC2 (SN7486N). Audio beep will indicate and detect any logic transition from1 to 0 or 0 to 1, lasting at least 50 ns. LED provide visual indication . LED will glow when input logic is 1.  [Source: seekic.com]