This circuit was found in a 1967 Japanese documentation. It consists of a touch control system with two transistors from the time. The circuit uses a power source and sensitive relays.
Today we have more compact solutions. This is from a Radio Electronics magazine from October 1990....
This filter was found in Texas Instruments documentation from the 1980s. The circuit uses a common...
16 commercial test circuit for tubes and transistors appeared in Radio Electronics magazine in...