We found this circuit in an English publication from the 60's. The transistors are of germanium of the time and the headphones must be high impedance magnetic.
This mono circuit for 10 LEDs was found in the January 1991 Radio Electronics magazine. It is...
In the figure we have a line receiver circuit (twisted pair) with the possibility of...
This circuit was originally designed for the practice of Morse code, something common in 1966, the year...