This circuit is a relaxation oscillator (specifically a triangle/square-wave generator) that controls a transistor to gradually fade four parallel LEDs in and out. The reference voltage (Voltage Divider) The two 47 kΩ Resistors: Configured as a symmetrical voltage divider. They divide the 9V supply exactly in half, this creates a stable 4.5V reference voltage at the center node. The top op-amp's (+) input and bottom op-amp's (-) input.
The top Op-Amp: Functions as a charge-controlling integrator. (Virtual Ground) which keeps its (-) input held steady at the 4.5V reference level. The 22 µF Capacitor functions as the primary timing component that determines the flashing speed. The potentiometer (80 kΩ / 20 kΩ) & 25 kΩ Resistor: Control how fast current flows into or out of the capacitor. When the bottom op-amp is HIGH, current pushes into the timing network, causing the capacitor to charge and the top op-amp output to ramp down. When the bottom op-amp switches to LOW, current pulls out, causing the capacitor to discharge and the top op-amp output to ramp up.This continuous cycle generates a smooth, repeating triangle wave at the top op-amp's output.
The bottom Op-Amp & 75 kΩ Resistor: Functions as a non-inverting Schmitt trigger (comparator with hysteresis). The 75 kΩ resistor provides positive feedback to the (+)input. Its output quickly snaps between two extreme states: HIGH (9V) and LOW (0V). It acts as the master switch that reverses the charging direction of the capacitor.
The LED Driver & Output Stage has an NPN Transistor configured as an emitter-follower current amplifier. Driven directly by the smooth triangle-wave voltage from the top op-amp. As the voltage rises slowly, the transistor gradually conducts more current (fading the LEDs in). As the voltage sinks, it conducts less current (fading them out). The 47 Ω Resistor (Emitter): Limits the maximum current passing through the LEDs to prevent burnout. The 4 LEDs (Red, Yellow, Green, Blue) are wired in parallel to illuminate and extinguish simultaneously in response to the transistor's current.
