A white LED delivers a wide color spectrum and better visibility than do monochromatic LEDs. However, a white LED presents a higher forward-voltage drop than do its colorful counterparts and thus poses problems for operation from a single 1.5V cell. The self-oscillating step-up converter in Figure 1 features a minimal component count and an easily assembled transformer, T₁.

During the time it takes to charge T₁'s primary inductance, resistor R₁ and T₁'s added secondary winding provide sufficient base current to turn on Q₂. Q₂'s collector current increases until its base current can no longer hold the transistor in saturation. When Q₂ comes out of saturation, T₁'s magnetic flux and secondary-voltage polarity reverse. During T₁'s primary-discharge interval, the combination of T₁'s secondary voltage in series with Q₁'s base-emitter voltage applies reverse bias to Q₂'s base and turns off the transistor. When Q₂ turns off, the voltage across T₁'s primary inductance adds to the battery voltage and applies a forward bias to the LED, D₁. The current through R₁ determines the power applied to the LED and applies forward bias to Q₁'s base-emitter junction to provide temperature-compensated bias voltage for Q₂.

The breadboarded circuit's transformer, T₁, comprises eight turns of AWG #30 insulated wire wound around the body of an unshielded 100-µH axial-lead inductor, producing approximately 400 mV p-p across the secondary winding. (Editor's note: Observe the winding's polarity dots. If the circuit fails to oscillate, reverse the connections to either the primary or the secondary winding.) The circuit operates over an input voltage range from just above Q₁'s base-emitter voltage drop of approximately 0.6V to the LED's forward-voltage drop of approximately 3V. The circuit's switching frequency exceeds 340 kHz at 1.5V input.