NE556 timer can be used as an indicator of static of digital logic audible terminal. Audible logic probe is useful for inspecting a part visually while we also need to check the logic state on other point far from the part. NE556 acts as an oscillator controlled by (SN74132N) and IC2 (SN7486N). Any logic transition from 0 to 1 and vice versa will be followed with audio beep, lasting at least 50 nanoseconds. LED provide visual indication. When the input logic is 1, the LED will glow.
Source : seekic.com
