Here I present to you a single AA battery led circuit. This circuit is a variant of the Joule thief in that it contains 2 transistors and a simple standard 2 lead inductor instead of a hand wound 2 wire inductor.
You can use it as a white led torch. You can use a fresh AA battery, but the circuit is special in that it accepts empty batteries. Down to 0.7 volts. Do you have a box of empty batteries? They are still useful to power this led circuit.
In the schematic of the led circuit you can see the power source (V1) that represents an empty battery. It has only 1 volts remaining and an internal resistance of 50 ohms. High internal resistance is a characteristic of an empty battery. Capacitor C2 stabilizes the voltage so the switching parts of the led circuit can draw enough current to function correctly.
The circuit functions in a negative feedback loop. Let me explain. Q2 is the switching transistor that charges the inductor L1. After a while Q1 starts conducting and short circuits the base of Q2. That means that Q2 no longer conducts and let inductor L1 release its energy into the white led. The white led needs more than 1V to turn on, L1 will provide it.
Observant readers will notice that typically C1 is of a larger value and is accompanied with a series resistor. I found out that a smaller capacitor and no series resistor works just as well.
This graph show you the voltage measured at the white led. You can see at the left the charging of C2. It charges relatively slow because of the empty battery. When the input voltage of the led circuit reaches about 0.6 volt the circuit start oscillating and the white led turns on. The led circuit oscillates at a frequency between 250KHz and 400KHz.
Want to simulate the circuit yourself? Download LTspice IV and then load white led circuit joule thief schematic.asc (right-click save as). You can play with the values of the components and discover what happens with the output voltage.
I soldered the led circuit using SMD components:
From left to right, top to bottom you see all the components used in the led circuit. The (empty) battery, C1: 1uF, L1: 470uH, C1: 47pF, R2: 10K, Q1: MMBT2222, R1: 1K, Q2: MMBT2222 and the white led.
Want to build the led circuit yourself? You can use standard through hole components in a breadboard. Not all components are critical. Look into your component junk box and just search for similar components. Do you have a 330uH or 680uH? Simulate if it works, it probably will.
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