This filter, with a cut-off frequency of approximately 1 kHz, has a high Q and employs two operational ones. The power supply must be symmetrical.
We found this circuit in an old Popular Electronics. It is possible to light a common neon lamp...
In the figure we have the way to control a triac of the TIC series with a low voltage through a...
Found in an Elektor from the 90s, this circuit converts signals from the 280 kHz band to 516 kHz,...