This circuit was obtained from an 80's EDN. The components used are still common. The circuit can be used as a shield and the frequency is approximately 775 Hz.
This circuit is from a 1991 PE Hobbyst Handbook. The circuit uses two trigger transformers for a xenon...
A JFET is used in this circuit to maintain constant gain. The circuit is from an Electronics Now...
Found in a Popular Electronics from the 90s, this circuit uses a double 555 to emit beeps when the...