This example describes a white LED flashlight that can be placed in an empty glue stick tube and has a long battery life. Its circuit consists of only a few common parts. The flashlight has proven to be extremely durable, and the prototypes that the author has built have been in use for nearly five years, but still maintain good working conditions. 
The flashlight is powered by a 1.2V/2500mAhr nickel-cadmium battery (Figure 1). Using a simple transistor booster based on a tapped inductor, the voltage can be effectively increased (about 80%) until the voltage level required for a typical white LED is reached (in this case, the voltage is about 3V). Q1 and Q2 form an unstable multivibrator, and the collector generates a square wave with a phase difference of 180°.
Assume that at power-on, Q2 is turned off and Q1 is turned on. Under this condition, the collector of Q2 is high and Q3 is turned on by the collector resistance of Q2. When Q3 is turned on, current flows through the first half of the inductor (flow from terminal 1 to terminal 2).
At the end of the first half of the operation, the multivibrator flips to another state: Q2 turns on, Q1 turns off, and Q1 is high. Q3 is turned off, and Q4 and Q5 are turned on by the collector resistance of Q1. The inductor current in the attenuation flows between the terminal 1 and the terminal 3. Since L1-2 is the same as L2-3 and they share one core, the inductance of L1-3 is four times that of L1-2 and L2-3. This increased inductance (and the corresponding increase in the number of turns on the core) causes the current amplitude to decrease, but increases the voltage across the LED. At this stage, current flows through the LED while the 10μF capacitor is charged. The duration of this phase is determined by the RC value of the unsteady circuit.
Once the RC time constant is exceeded, this process repeats: Q1 turns on, Q2 turns off, and other transistors switch as previously described. The current through the coil terminals 1 and 2 increases again, storing the energy in the battery in the inductor. At this stage, a 10μF capacitor powers the LED.
Figure 2 shows how the circuit components are assembled to both sides of a circular universal board. Figure 3 shows how the flashlight is loaded into a rubber tube. Once the flashlight is assembled and energized, the maximum brightness can be achieved by adjusting the 100kΩ potentiometer in the unsteady circuit. Note: If necessary, add a transistor in parallel with Q3 to increase the energy stored in the L1-2 inductor. Whether or not to add additional transistors depends on the speed and depth at which Q3 enters saturation.
Keystone Jack,Ethernet Wall Jack,Ethernet Wall Plate,Rj45 Keystone
ShenZhen Antenk Electronics Co,Ltd , https://www.antenk.com