The 1.5 volts from the flashlight battery are boosted to the 3.5 volts required by the LEDs

The power supply described here boosts the voltage from 1.5 volts of a flashlight battery to the 3.5 volts required by the LED, while the power supply connects the LED in series with the flashlight battery. This circuit is designed to improve flashlights with LEDs. The boost circuit replaces one battery in a two-battery flashlight, and the LED device replaces the incandescent lamp (see Figure 1).

The result is that the LED flashlight can last a long time using only one flashlight battery. The biggest difficulty in this design is: increasing the voltage when only one end of the power supply is connected to the battery and the other end is connected to the incandescent lamp.

The power supply circuit shown in Figure 2 relies on a trick to work. In the LED device, there is a diode connected to the LED so that when the power supply is sinking current, the current also passes through the diode. During this time, the LED is reverse biased. After a short period of time, the power supply stops sinking current and switches to sourcing, so that the LED is forward biased.

The "working part" is an intermittent oscillator that uses two transistors instead of the usual one. When the transistors conduct, the PNP transistor connects one end of the inductor to the positive power supply, and the NPN transistor connects the other end of the inductor to the negative power supply. Current accumulates in the inductor until one transistor is saturated. The reversal of the base voltage causes the two transistors to turn off quickly.

When both transistors are turned off and the voltage in the inductor coil is reversed as current flows through the inductor, the inductor releases current through diodes D1 and D2. As a result, the end that was originally the positive pole is now connected to the negative pole of the battery through D2, causing the inductor to flip. In addition, the end that was originally the negative pole is connected to the positive pole of the battery through D1. When the circuit oscillates, the inductor constantly changes between these two connection conditions (current accumulation through the transistors and release through D1 and D2).

The 4.7 kilohm resistor can be placed in parallel with the LED to slightly reduce the turn-on voltage. In the original design, the turn-on voltage was reduced from about 1.3 volts to less than 1.1 volts. This makes sense because a new alkaline flashlight battery will probably take twice as long to discharge to 1.1 volts as it does to 1.3 volts.

The 100 kΩ resistor connected to the 2N4401 is used to ensure that there is always enough drain current to turn on the transistor and start the oscillator. The inductor is made of 45 turns of continuous #32 single-strand magnetic metal wire on a ferrite soft magnetic material TC8.2/3.7/4-3E7 toroidal core. There are taps at 15 and 30 turns, so that three parts of 15 turns each are formed. The oscillation frequency is 17kHz at a battery voltage of 1.6 volts.