This circuit was found in a 73 magazine for radio amateurs from the 90s. The circuit uses high voltage transistors and the relay is high voltage as well.
This second-order filter was found in a 1978 publication and can use common operational amplifiers like...
We found this circuit in a 1988 publication. The power supply for the operator must be symmetrical and...
This circuit is from a 1982 documentation. It can be modernized using LEDs in series with 470 ohms...