This joule thief led or high efficiency LED with 1.5 volt driver is a tried and tested circuit built around some discrete electronics components. A common 5 mm high-efficiency white LED (D1) is the key component in this design.
Technical specification of the high-efficiency LED used is shown below
- White color, water clear lens
- 5mm Round type
- Material: GaN/SiC
- Viewing angle: 15 degrees
- Forward current: 20mA
- Forward voltage: 3.0-3.6V
- Luminous intensity: 15000-20000 mcd
With the help of an ordinary low power npn transistor (T1-C547B-Philips) we can drive this 3.6 Volt LED using a single 1.5 Volt AA cell.
Here, the transistor functions as a high-frequency oscillator to generate the required voltage (DC-DC Boost Conversion) for the white LED. Next important component is a home-made toroid core inductor (L1). Total current consumption of the circuit is near 50mA.
If you can’t find a toroid core for L1, note that toroid (ferrite) cores are largely used in Compact Fluorescent Lamps. Just take the core from the PCB of a discarded CFL and wind 10 turns of 30SWG enamelled copper wire. Pull out a centre tap and then wind another 10 turns.
Finally connect the inductor leads to the circuit as shown in circuit (ie Start by winding 10 turns, the start is the collector winding.
Pull out the tap for the battery +ve connection, and then wind an additional 10 turns for the base winding).