This project is a simple 3-LED ring oscillator built with only three transistors and discrete components.
The most interesting feature of this circuit is that only one LED is illuminated at a time. The active LED moves continuously from one stage to the next, creating a simple three-LED running light.
How does it work ?
In each stage, the LED and the NPN transistor are connected in parallel. This means that the LED is on when the transistor is switched off.
When one transistor is conducting, the corresponding LED is switched off. When the transistor is switched off, the LED is illuminated.
As the voltage at the base of one transistor changes through the coupling capacitor, the transistor switches state. This causes its LED to turn off, while the next stage switches to the state that illuminates its LED.
The sequence therefore continues:
LED 1 → LED 2 → LED 3 → LED 1 → ...
A perfectly symmetrical ring oscillator can sometimes have difficulty deciding which state to start in. For this reason, the circuit deliberately uses a different base resistor value on the last stage.
This small imbalance gives the circuit a preferred starting condition when power is applied. Once the oscillation has started, the three stages continuously influence one another, and the circuit keeps running.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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