This project uses three NPN transistors to build a 3-stage ring oscillator that smoothly cycles through the colors of an RGB LED. Powered by a 3.7 V supply, the circuit switches the red, green, and blue channels in a continuous loop, creating a shifting rainbow glow without using a microcontroller or an integrated circuit.
How does it work ?
The circuit consists of three identical transistor stages, with each stage controlling one specific color channel of a common-anode RGB LED.
The RGB LED contains three individual LED chips inside a single package: red, green, and blue. By cycling through these channels sequentially, the circuit constantly alters the overall color emitted. Because the transitions are gradual rather than instant, the channels briefly illuminate together, blending their primary colors to produce intermediate shades like yellow, cyan, and purple.
Three 100 µF capacitors connect the transistor stages in a closed loop. They transfer sudden voltage changes from the collector of one transistor directly to the base of the next stage. When one transistor turns on, it drops the voltage on its collector, sending a negative pulse through the capacitor to temporarily turn off the next transistor in line.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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