Energy-saving pocket-sized white light LED flashlight

This example describes a white LED flashlight that fits in an empty glue stick tube and has a long battery charge life. The circuit consists of only a few common parts. This flashlight has proven to be very durable, with the author's prototype having been in use for nearly five years and still in good working order.

The flashlight is powered by a 1.2V/2500mAhr NiCad battery (Figure 1). A simple transistor-based booster based on a tapped inductor effectively steps up the voltage (by about 80%) to the level required for a typical white LED (in this case, about 3V). Q1 and Q2 form an astable multivibrator, producing square waves at the collectors that are 180° out of phase.

Assume that at power-up, Q2 is off and Q1 is on. Under this condition, the collector of Q2 is high and Q3 is turned on through the collector resistance of Q2. When Q3 is on, current flows through the first half of the inductor (from terminal 1 to terminal 2).

At the end of the first half cycle of operation, the multivibrator flips to the other state: Q2 is on, Q1 is off, and Q1's collector is high. Q3 is off, and Q4 and Q5 are turned on through Q1's collector resistance. The decaying inductor current flows between terminals 1 and 3. Since L1-2 is identical to L2-3 and they share a common core, L1-3 has four times the inductance of L1-2 and L2-3. This increased inductance (and the corresponding increase in the number of turns on the core) results in a decrease in the current amplitude but an increase in the voltage across the LED. During this phase, current flows through the LED while the 10μF capacitor is charged. The duration of this phase is determined by the RC value of the astable circuit.

Once the RC time constant has been exceeded, the process repeats: Q1 turns on, Q2 turns off, and the other transistors switch as before. The current through coil terminals 1 and 2 increases again, storing energy from the battery in the inductor. During this phase, the 10μF capacitor powers the LED.

Figure 2 shows how the circuit components are assembled to both sides of the circular universal board. Figure 3 shows how the flashlight fits into a tube of glue stick. Once the flashlight is assembled and powered up, the 100kΩ potentiometer in the astable circuit can be adjusted to achieve maximum brightness. Note: If desired, an additional transistor can be added in parallel with Q3 to increase the energy stored in the L1-2 inductor. Whether or not to add an additional transistor depends on how quickly and deeply Q3 goes into saturation.