Energy from dead batteries using a Joule Thief circuit

Major Challenge entry to the Robogals Science Challenge 2016
Name of participant: Anoushka Gupta
Age category: Junior

Project title: Energy from dead batteries using a Joule Thief circuit

1. Why did you choose this project?
Usually, I do not ride my bike at night. One day I got late at the library, and it was dark by the time I left. My bike light would not turn on as the batteries had run out. I wished there was a way to get some reserve power from batteries.
I did some research and found there is a way to boost the voltage of run out batteries enough to do something useful like lighting up LED lights. We can do this by connecting a circuit called a voltage boosting oscillator popularly known as a Joule thief.
2. What did you enjoy most about the project?
I enjoyed using my project as a nightlight to keep monsters away. I could use a spent 1.5-volt battery light up a LED as a nightlight for a whole week. I have done several science projects, but this was the first one that I could use.
It was incredible to see what seemed to be impossible – to light a 3V blue led from flat 1.5V battery become possible by using just three extra parts.

3. What have you learned from this project?

I learnt a huge lot from this project. I have understood that electricity and magnetism are very closely linked. I did the “Oersted's Experiment” and “What Happens to a Current-Carrying Wire in a Magnetic Field” in minor challenges and “Making an Electromagnet” last year, though I did not send in all of these.
I learnt the properties of inductors and basic concepts of Faraday's law of induction and Lenz’s law.
I also learnt basics of electronics like how to read the value of a resistor, and identify the pins of a transistor.
I learnt how a transistor works and how it is used a fast switch.The inventors of transistor won the Nobel Prize.
This project combines physics, electrical engineering and a concept from biology called persistence of vision which means several separate images blend together in our brain, like in flipbooks and animated films. This circuit produces a very fast flashing light, but it appears as if is not flashing.

4. How did your project change your understanding of science/engineering?

By cleverly combining properties of different parts we can make what seems to be impossible, achievable quite easily.

5. How did your parent/guardian or mentor help you?

My dad helped me understand the working of the parts, helped with the soldering iron and editing the video.
Understanding how a transistor works was the hardest part for me. I was glad to get very patient explanation of this topic
I also got a lot of help to understand Faradays laws of electromagnetic induction.

6. What do you think are the engineering applications of the knowledge found in your science project? In other words, if you were an engineer, what would or could you create out of your science project?

If had I designed bike lights or torches, I could make an “emergency power” switch built in that would turn on a joule thief circuit to get the remaining power from the batteries if needed.

7a. Briefly, explain the underlying scientific theories behind the project; why you chose this method and equipment; and whether the experiment is repeatable and why?

The basic theory behind this project is that energy is stored in a magnetic field in an inductor, and there is no minimum voltage for this to happen. The stored energy can be released at a high enough voltage to turn on the LED if the magnetic field collapses suddenly – which happens when the switch is turned off, or when the transistor stops conducting, the current stops flowing immediately, so the magnetic field collapses.
Faraday's laws of electromagnetic induction and Lenz’s law give the voltage that is generated by a changing magnetic field.
Lenz’s law is
Induced Electromotive Force = - dφ/dt
φ is the magnetic flux, and dφ/dt is how fast the flux is changing.
The negative sign shows the direction of Induced Electromotive Force is opposite to the direction of changing magnetic flux.
The circuit is arranged so that the transistor is a very fast automatic switch. A silicon NPN transistor starts to conduct when the voltage at the base is about 0.6 volts more than emitter. When a transistor is conducting the collector current is β times the base current, where β is the current gain of the transistor when conducting. For BC 547, β is between 100 & 800. If the voltage at the base falls below 0.6 volts respect to emitter the transistor turns off.
This project is very easily repeatable. I made several copies! It's important to connect the first coil in such a way that the magnetic field produced by it is in opposite direction to the magnetic field produced by the second coil. One easy way to do this is just wind one coil and put a centre tap, or if the circuit does not work just swap the connections of one coil around.
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