Found in a 90's EDN this circuit can be used as a shield. The transistor is a general-purpose type and the frequency of the emitted sound is determined by R1 / C1.
This circuit, found in 2002 documentation, triggers LEDs when a phone call is received. The LEDs...
This simple version of a function generator with the XR2206 was found in the May 1977 issue of...
A pulse triggers this 800 Hz horn for a time determined by capacitor C1. Powered by 5 V the...