LED-flashlight circuit works at voltages as low as 0.5V

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Most commercial LED flashlights use three AAA or AA batteries in series that produce 4.5V. The batteries then drive four white LEDs that connect in parallel. These LEDs can work at voltages as low as 2.7V and, in some cases, 2.4V. At those voltages, the LEDs become dim, and you must frequently change the batteries. Thus, the lowest working voltage in this case is approximately 0.8 to 0.9V per battery.
LED-flashlight circuit
When a 1.5V alkaline battery discharges to 0.9V, it still has more than 10% of its original energy left. If you replace or discard the battery, you waste that energy. You can, however, use this small amount of battery energy with the circuit in Figure 1. The Linear Technology LT1932 LED driver is a step-up voltage-booster chip with constant-current capability for LED lighting. It works with input voltages of 1 to 10V, and it can drive several serial LEDs.
The trick is to choose the supply voltage. Because LT1932 can work at voltages as low as 1V, using a two-cell, 3V supply results in the lowest working voltage: 0.5V per cell. Choosing a three-cell, 4.5V supply results in a lower voltage of 0.33V per cell. A 4.5V supply can power as many as eight LEDs. Tests show that this circuit works from 4.5V to 0.94V, which is lower than the data-sheet-specified 1V. The LED driver uses a 4.7-μH inductor.
flashlight circuit
Setting the value of resistor R1 regulates the constant current through the LEDs. Setting a higher resistance results in lower brightness. In this case, the current is 18 mA. The LT1932 is available only in a surface-mount, tiny, six-pin SOT package, with fine pitch as small as 0.037 in. So, using it requires a PCB (printed-circuit board).
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About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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