This circuit is from an old Japanese publication of 1962. The circuit uses two batteries, one for the filaments of the tubes and the other one of high voltage for the plate circuits.
This circuit was obtained from an Electronics Now from the 90's. It generates the dial signal and...
In the figure we have a way to adapt a phone obtained to the output of a common amplifier. The...
This simple shield for microcontrollers and TTL digital circuits was found in documentation from...