With great pleasure I accepted Elektor's challenge to design an analog version of the well known digital PWM one.
My goal was to develop a discrete analog circuit with a minimum of transistors and with just one capacitor for the timing.
As shown the finished circuit consists of an analog Schmitt trigger using complementing transistors T1 and T2, a single transistor integrator around T3 to generate the triangle wave and a buffer (emitter follower T4) to drive the LED.
The amplitude of the triangle is set by R3 for continuous LED fading without full on or off time.
The results of this circuit are attached with Vout2 being proportional to the current and intensity of the LED.