Photo. This is the test circuit -the basic driver is only two transistors, two resistors, the circuit was evaluated using a white LED, but when it was time to button it up and archive it, I replaced the expensive white LED with a cheap green one.
Othere LED Driver Circuits on this site:
White LED drive from 1.5V
White LED inductorless drive from 1.5V and 3V.
Fast precision LED driver
White LED Stroboscope With Constant Duty Cycle and Constant Current Drive
When Dave and I were kicking around ideas for the best arrangement of components for white led flashlight, Dave suggested putting the voltage converter in place of the second cell in a two cell flashlight, and substitute an LED assembly for the incandescent lamp. “Yeah, I that would be ideal…” was my hesitant response, though I didn’t see how this would be possible. It took someone like Dave, who didn’t’ know it was impossible, to suggest doing it. Eventually a topology emerged that can actual run a 3.5 volt LED when it and the driver in series with a 1.5 volt flashlight cell. Well kind of, anyway. The trick was to put one additional component, a diode that is actually part of the driver, in parallel with the LED. Read on.
I was already using energy stored in an inductor that is part of a blocking oscillator to boost the voltage available for an LED (see the schematic below).
While this provided the necessary voltage boost, it required that the blocking oscillator be powered from both terminals of the battery. It seemed that some re-arrangement of the parts might result in a practical topology that would allow current built up in the inductor to be dumped through the LED. The circuit below emerged.
This circuit above is that of a series connected voltage booster. The schematic represents an LED assembly (D3 and D4 with an optional 4.7k resistor), a 1.5 flashlight cell, and the series boost circuit. If you compare the circuit in Figure 3 with the basic blocking oscillator circuit in Figure 2, it should be apparent that the series connected voltage booster is designed around two blocking oscillators in parallel with a single inductor core in common. The oscillators are locked together by virtue of their using the same core. The PNP blocking oscillator connects and disconnect the positive power supply to the inductor and the NPN blocking oscillator connects and disconnects the negative supply to the inductor.
In order for the oscillators to run, a diode, D3, needs to be placed across the LED (D4) to power the circuit while current builds up in the inductor. When the transistors turn off, the inductor current flows through steering diodes D1 and D2, and LED D4. Diode D3 that conducted during the build up of current in the inductor is reverse biased while the inductor discharges. The 4.7k resistor across D3 and D4 is optional; it lowers startup voltage to below 1.2 volts, about 100 millivolts lower than the normal 1.3 volt startup voltage.
Read more: Series Connected Voltage Boost Circuit for a Battery Operated LED Lantern using microcontroller