I made a ring oscillator circuit with transistors, where the LEDs light up in sequence.
If you connect an odd number of NOT gates in a looping manner, the signal will propagate in sequence and oscillate as it loops. by using transistors instead of NOT gates and adding a capacitor between them and the next stage, you can see the signal slowly propagating.
The circuit is quite simple. It is a combination of a transistor emitter-grounded inverting switching circuit and an RC circuit, connected in triplicate. More than three will work as long as the number is odd.
This circuit does not start oscillation just by turning on the power, so you have to push the start switch on the right side of the circuit to force it to start oscillation.
Here is the operation when the supply voltage is 5V. When the switch is pressed to start the circuit, you can see the LED light shifts from left to right in sequence.
This is how it works at 2.7V. You can see how the capacitor is charged and discharged, and how the light is transmitted in turn.
The analog-like behavior, as if a light bulb is glowing, gives me a pleasant feeling.
If the voltage is lowered any further, the signal may disappear in the middle of the transmission, or may not be transmitted at all. It's a strange circuit that feels like it's alive.