This bidirectional circuit was found in an American documentation from the 80's. The LEDs are common red, and the resistor is calculated according to the formulas next to the diagram.
This amplifier was found in a 1957 Rayrtheon documentation. The circuit can be fitted with a BC558...
This circuit was obtained from the May 1971 issue of Radio Electronics magazine and is intended for...
Although it is a common configuration today, this circuit was taken from an old publication. The...