Audible Logic Probe

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:

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]