Capacitive Relay (T1527E)

Found in Popular Electronics from August 1956 this sensitive two-tube relay is powered directly from the mains power supply.

Complementary Oscillator (CB24720E)
This is a circuit that we use very often in our projects, and it also appears in many technical...
High pass variable filter (CB7571E)
Found in an American paper from the 90's, this circuit can be adjusted to pass frequencies above 30 to 300...
Preamplifier Using a PNP Transistor (CB239E)
This preamplifier is indicated for low and high-impedance sources as microphones, transducers,...