Hi, my name is Eric. I’m a rising sophmore and I go to the Trinity School in New York. For my starter project, I made a MintyBoost, a boost converter that charges a phone with two AA batteries. I picked this project because it seemed like the most practical and useful project, as was my main project. My main project is a Personal PowerPlant. It converts solar and kinetic energy to electric energy that can be harvested for more practical applications. I found this project on Instructables, and made my own version of it, with help from instructors at BlueStamp Engineering.
FINAL PROJECT:
The Personal Powerplant is a small scale generator which converts solar and kinetic energy to electricity. A hand cranked motor and solar panel charge a 7.2 V NiMH battery which leads to an output terminal that can be used to power electronics. It also uses a “visual” multimeter that gives approximate voltages going into and being stored in the battery. The original creator of the project has posted instructions here: http://www.instructables.com/id/personal-powerPlant/.
DOCUMENTS:
BOM (Bill of Materials):
https://docs.google.com/spreadsheet/ccc?key=0An8IC3yEMPVKdDhsUFdEY3N6dWFvLXJISnNjSUtFMHc#gid=0
Schematic:
http://www.instructables.com/files/orig/F4W/3FRZ/F3LMO09N/F4W3FRZF3LMO09N.pdf
Mechanical Drawings:
(for Gears): http://www.instructables.com/files/orig/F1F/LAKX/F3LMO0A7/F1FLAKXF3LMO0A7.pdf
Please note that the large and small gears were scaled to be 1” and 4” respectively.
Instructions to Build Project:
https://docs.google.com/document/d/1V2R7tJCGlptFE8avw82SYv5gY4Q9JZFQlq3ILPzu3Z8/edit
Disclaimer: I built this project as something to do as part of an educational program and not as a product.
Here is a video explanation of my final, completed project.
Reflections:
Completing this project taught me much about mechanical engineering. On the first day of the program, in an attempt to learn more about the parts I was using, I typed “capacitator” into Google, and shamefacedly clicked on the Google spelling suggestion. Now, at the end of this project, I can confidently explain all parts of my project (and spell them correctly). Throughout this project, I gained finesse in mechanical engineering, and learned about the values of good mechanical and electrical engineering. Through completing this project, I am not only more confident in my math and physics skills, but also sure of myself in knowing what I truly enjoy doing. Making this personal Powerplant has become my inspiration to become more engaged in electronics, electrical engineering, and circuitry. I am grateful to my instructors for giving me this amazing and awesome engineering experience.
THIRD MILESTONE POST:
The last and most interesting part of my project was assembling the “visual multimeter,” which gauges how much the voltage is in and/or being sent to the battery. Rails (open wires) connect the parts before and after the rectifiers. A momentary switch connects one lead to the rail, and the other end to the a joint with three zener diodes. Zener diodes differ from the ideal diode in that they limit a current flow forward, but are also designed to allow current to flow in the reverse direction if it is above a certain voltage, known as an “avalanche point.” I installed three zener diodes in parallel, each with a different avalanche point: 3.9V, 5.1V, 7.5V. I purposely installed the diodes “backwards” in the direction against the current. This was so that what current did pass through the diodes had to be at least a certain voltage. After each diode, I added either a resistor or some other jumper, to further limit the voltage. I used a 100 ohm resistor, and 150 ohm resistor and a jumper for the 3.9V, 5.1V and 7.5V respectively. After these, I placed LEDs, green, yellow and red respectively. I connected the ends of the LEDs with a solder jumper and a wire jumper leading back to the terminal connecting it to the battery.
Now, when the battery discharges, it sends current running through the rectifiers again and when the momentary switch is pressed, it goes through the zener diodes, and continues past them based on the voltage of the current. If the voltage is high enough to surpass the avalanche point, it will continue through the resistor/jumper and light up the LED.
This is the reason I used a parallel circuit. A parallel circuit is designed to conserve the same voltage for every component of the circuit, while a series circuit involving these diodes placed end to end would reduce the voltage after every component. Since I used a parallel circuit, I could be sure that the same amount of voltage was getting through to every diode, and it was not being reduced continuously. With a series circuit, the reading would have been unreliable, as a 7.2 volt current may have already been reduced to another voltage, too low to surpass the avalanche point and go past the zener diode.
SECOND MILESTONE POST:
Now that the hardware is nearing completion, I’ve been preparing to case all the parts. This means creating openings in the box for the different parts to go through. The first openings I had to make were for the stepper motor and the attached gears. The idea behind the gears is to make turning the shaft on the motor easier; a handle turns a large gear, which in turn turns a gear a fourth of the size. This smaller gear is affixed to the shaft of the motor; and thus through turning the larger gear via a handle, the shaft on the motor is consistently turned. To attach the gears to the side of the box, however, requires a more delicate mechanism than glue or epoxy, since a dried binding agent won’t allow for rotation. The solution was therefore to drill holes in the box through which a binding post could be inserted, which would act like an axle for rotation. I then had to drill two holes; the first the small gear, which would not turn on a binding post, but instead the motor shaft, and second the large gear. To reduce strain on the box, I had to make sure that the motor could be supported by something other than just its shaft going through the hole. I had to measure a place to drill a hole in the box so that the motor would be close to the bottom of the box. The other hole was drilled accordingly with the size of the gears and location of the first hole.
A hole was drilled on the top of the box for wires from the solar panel to go through; I chose for it to go on top of the box so that it would minimize reflected light by the acrylic box.
My openings for the terminal and switch were much more complex. In order to create an opening large enough to accommodate a switch or terminal, I couldn’t use the small holes I drilled with a drill, I instead used a dremel to widen holes and expand them to fit a switch. The problem with this was that the dremel caused the acrylic to melt, plugging the holes that I had created. The solution to this was to drill a hole in the wood underneath the acrylic to let the molten plastic flow into, thereby avoiding the hole.
This process taught me how to use a power tool properly on a tricky substance like acrylic, which melts at a temperature low enough to come from a dremel or drill.
FIRST MILESTONE! BLOG POST:
Now, two weeks into the program, I have made my first milestone: I built two rectifier circuits and attached the stepper motor to the circuit. I tested it with a digital oscillometer, and did the same with a solar panel. These parts are extremely important because they are the main power sources for the battery; the solar panel and the motor are what generate the electricity. The stepper motor works by using two electromagnetic coils to cause a shaft to turn when power with electricity. Inversely, when the shaft is turned, it generates an electromagnetic alternating current. The alternating current, which is shaped like a typical sinusoid curve, is unstable for battery charging, due to its periodic alternation of current direction; as expressed by the sine wave, the oscillating curve has a crest and a trough, which causes the power to dip upwards and downwards, charging and discharging power to and from the battery. To solve this problem, I introduced two rectifier, one for each of the coils inside the motor. A rectifier is a series of diodes that convert the current into a sinusoid “directed” current, by restricting the curve to omit negative oscillation. The raw alternating current voltage can be graphed as a function of time, as f(x) = Asine(x) + k where A and k are both constants. After passing through the rectifier, the function becomes d(x) = g( f(x)) = |Asine(x) + k| where A and k are constants. All negative values here reflected to be positive. The rectifier is an entire series of diodes directed in a circle, which lead to two terminals and from there the battery charged by that positive current. The solar panel did not require a rectifier, as its current was already DC.
To get to this stage, I had to assemble the diodes solder them, wires, headers, jumpers and terminals. This may sound simple, but I had to start over twice because the first time the solder joints were of poor quality, and the second because I discovered my board was of too small. These process taught me much about the mechanical techniques of soldering, such how and when to clean the soldering iron, and how to recognize poor joints. It also taught me to analyze a data sheet to determine how to align motor coils to rectifiers.
STARTER PROJECT BLOG POST:
During my first two days at BSE, I worked on my starter project, the MintyBoost, which uses AA batteries to charge a phone (or other device) through a USB port.
The design is from Adafruit.com and can be found here: bit.ly/10QVDe3 .
MintyBoost is pretty powerful; it’s providing power to a Verizon Pantech as I write.
Although AA batteries provide only about 1.5-1.6V each, the phone requires approximately 5V when charging. The circuit board relies on a transistor chip (LT1302) to direct the current through an inductor, a coil of wire, which stores current. After milliseconds of the circuit, the transistor breaks the connection and the inductor releases stored current. The loop with the transistor chip is broken, the current from the inductor, flows past it through a diode, which then directs the power to the two electrolytic capacitors. These capacitors are able to store voltage until the voltage exceeds 5V, at which point the transistor opens the circuit again, allowing for the current to be directed back through the inductor, and the voltage stored in the capacitor to dip slightly. When the power level in the capacitor dips below 5V, the transistor breaks the circuit, allowing for the current stored in the inductor to flow again.
Here’s a video of me explaining how it works, with a demonstration: