Found in Popular Electronics from the 90s, this circuit is based on a PLL567 and employs high-end or ultrasonic speakers depending on the frequency of oscillations.
We found this circuit in a 1998 publication. The circuit has controls for both frequency and...
This circuit for conventional lines was obtained in a 1996 documentation. The circuit uses common...
This is the conventional version of capacitance multiplier with operational amplifier that appears in...