We found this circuit in a 1964 Electronic Design. It provides 0.5 V signals from 4 kHz subcarriers. Transistors are from the era.
This circuit, which can be used as a shield, was found in American documentation from the 1970s....
This circuit for low frequency data transmission was found in the May 1977 issue of Radio...
This circuit activates a relay that has a lamp as a load. The circuit is from a Radio Electronics from...