This circuit is from an ancient American documentation. The integrated circuit is 741 and the power supply must be symmetrical. The passing band is centered at 1 kHz.
This circuit is from a 90's Radio Electronics. The circuit operates accurately up to about 15 kHz...
This circuit produces low power flashes in a LED. Frequency is adjusted by P1 and depends also on...
Found in an old Popular Electronics, this circuit warns when a remote lighting system is not...