This simple circuit can work with either 4.5 V or 6 or 9 V if a 1k resistor is added in series with the set of LEDs. The circuit is from an old French publication.
This TTL circuit uses LED indicators and must be powered by a voltage of 5 V. The capacitor...
Low power RF transistors such as the BF494 or BF495 and even those in the BC series can oscillate well...
This switched power supply was found in a 70s documentation. The basic components are from that...