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.
Here we show an oscillator that uses an LC network. The upper limit depends on the power-supply...
This circuit from a Popular Electronics from the 90s allows you to use an old PC as a digital...
of the National Semiconductor of 1994. The same technique can be used with other operators and the...