Found in a Popular Electronics from the 1990s, this circuit is a 1-digit counter that advances under the command of S2. The circuit can be expanded to more digits.
The presented circuit emits signals in the FM band, which can be picked up from a distance of a...
I found this circuit in a documentation for radio amateurs from 1975. The FET could be the BF245....
The traditional inverter amplifier configuration with the operational 741 shown in the figure is...