You can turn on a 3 Volt LED just using 1.5V Battery. It is the Joule thief circuit doing the trick. Joule thief circuit is a self oscillating voltage booster used to drive light loads like LED. The name Joule thief is given to the circuit because; it is stealing the Joule or energy from the battery. That means, it is extracting energy even from an almost dead battery to power a load. Try it. This is one basic circuit you should know as a starter in electronics.
We cannot light an LED using 1.5 V battery, since the forward voltage of all LEDs is above 1.8V. So by using the Joule Thief circuit, we can boost the 1.5V of a battery to 3V or more. The Joule Thief circuit is very efficient so that it will light the LED even if the battery voltage is as low as 0.8 V.
See the Forward voltage of the common LEDs
Something about Joule thief circuit
Joule thief circuit works by rapidly switching a transistor connected to a Toroid transformer having both primary and secondary windings. The Toroid can be easily made by winding plastic wires in a Ring core. When the circuit is connected to the 1.5V battery, the transistor rapidly switches on/off. Initially, current flows through the secondary winding, resistor and the base-emitter of transistor. This triggers the transistor and its collector current starts to flow through the Primary winding.
Since both the two windings are connected in the opposite directions, this induces a voltage in the secondary winding which is positive due to the polarity of winding. This turns on the transistor with higher bias. This Self stroking or Positive feedback instantly turns on the transistor and the Collector – Emitter path works like a closed switch. Since the Primary winding is across the battery, the current flow increases at a rate proportional to the supply voltage divided by the inductance. In short, once the current in the coil stops increasing, transistor turns off and opens the Collector-Emitter switch. At the same time, the Magnetic field collapses inducing much voltage to drive the load. When the field returns to zero, the whole sequence repeats.
You needs the following components to make the Joule thief circuit
1. NPN Transistor BC 547 – 1
2. 1K Resistor – 1
3. Toroid core – 1
4. Two coloured thin wires – 30 cm each
5. LED – 1
6. 1.5V Pen cell
7. Pen cell holder
Toroid winding is an important job for making the joule thief circuit. Procure a Core from an old SMPS or electronic chock. You can also purchase from electronic shops. Take 30 cm length two coloured thin wires preferably Toroid winding wire. Wind closely through the Ring core to make 10-15 windings. If the size of the Core is large, and windings are numerous, efficiency of the Toroid increases. After completing the winding, the Toroid will have four wire ends. One wire is primary and the other is secondary.
So now there will be three leads. Connect one lead to the collector of transistor and the other to IK resistor that going to the base of transistor. Finally, connect the jointed wires to the +ve of battery holder. The negative of the battery holder should go to the emitter of transistor. Now connect the Anode of LED to the collector and Cathode to the emitter of transistor. Joule thief is ready. Connect the 1.5V Battery in the holder. LED will light instantly. Actually, the LED is blinking very fast based on the oscillation of the circuit. But we will not feel this blinking because the oscillation is very fast.