To lightÂ blue and white light-emitting diodes (LEDs) the voltage of 3V is required. By the way, whiteÂ LED is the same as blue LED with additional luminophore inside. This luminophore is activated by blue light and lights up. Finally itÂ lights as white.
What to do if you wishÂ toÂ connect a white or blue LED to a simple AA or AAA battery orÂ even worseÂ to the dead battery? It’s very simple. It’s enough to assemble a self generator onÂ one transistor and the LED will light. That’s a real sample:
On the Internet this scheme is often called “Joule Thief”. Though the idea of self generation is well known and used in various strange devices.
What do you need for repeating the experiment? A simple transistor firstly. I used the transistor NPN 2N3904. Â Secondly, you needÂ 1K resistorÂ and Â a small transformerÂ wound on a ferrite ring. The ring can be very small,Â as device is not very powerful. My ring has 2 x 30 winds of thin copper wire.
WindÂ the transformer as follows: take 1m ofÂ thin wire and fold it in half.Â So you have two wires. Wind them on the ring simultaneously. When the ring is wound, cropÂ and tin each endÂ of the wires.Â Connect the beginning of one windÂ to the end of the other wind and to the positive post of a battery. (Check with a tester or in any other way if the connection was made correctly. You can use coloured wires, it will be easier to recognize them).Â Â The secondÂ wire of the transformer should be connected to the resistor, the third – toÂ the collector. Connect Led’s anode (+) to the same collector and free end of the resistor to transistor base (toÂ the central leg). Transistor emitter and LED’s cathode (-) should be connected to the negative post of the battery.
Â Scheme is here:
Theoretical remarks (untested).
You can use PNP transistor, but it will be necessary to reverse emitter with collector, anode with cathode and to connect the power supply in opposite way. You can also use old Russian germanium transistors. Then the device will work more stable at low voltage. Such device could be powered from “primary power sources” as lemons or apples with two different wires inside…