This circuit was found in a 1971s English magazine. We can still assemble it using BC548 transistor. The other components are common. The supply voltage of 6 or 12 V depends only on the relay.
This circuit uses an LED (called SSL) to regulate current in a configuration with complementary...
A dual function using common transistors and diodes can be implemented as shown in the figure...
Found on a 1964 CQ this circuit is intended for tone frequency change modulation (FSK) telegraphic...