How to Charge Any USB Device by Riding Your Bike




Introduction: How to Charge Any USB Device by Riding Your Bike

To start off, this project was started when we received a grant from the Lemelson-MIT Program. (Josh, if you're reading this, we love you.)
A team of 6 students and one teacher put this project together, and we have decided to put it on Instructables in hopes of winning a laser cutter, or at least a t-shirt.

What follows, is a compilation of our presentation and my own personal notes. I hope you enjoy this Instructable as much as we did.

I'd also like to thank Limor Fried, creator of the MintyBoost circuit. It played an key role in our project.

Jeff Brookins
Divine Child InvenTeam Member

Step 1: Our Original Intention...

Our original project was to develop a product that used the Faraday Principle to allow runners to charge their iPods while they run. This concept would generate electricity the same way those Faraday flashlights do.
However, we had a problem. To quote my team mate Nick Ciarelli,

"At first we considered using a design similar to one of those shake-up flashlights and converting it so that a runner could strap it on for a run and have energy to charge their iPod or whatever device they use. The shake-up flashlight gets its energy from the interaction of the moving magnetic field of the magnet in the flashlight and the coil of wire wrapped around the tube the magnet slides through. The moving magnetic field causes electrons in the coil to move along the wire, creating an electric current. This current is then stored in a battery, which is then available to use for the flashlight bulb/LED. However, when we calculated how much energy we would be able to get from a run, we determined that it would take a 50-mile run to get enough energy to charge one AA battery. This was unreasonable so we changed our project to the bike system."

We then decided to use a bike-mounted system instead.

Step 2: Our Invention Statement and Concept Evolution

We initially theorized the development and feasibility of a regenerative braking system for use on bicycles. This system would create a mobile power source to extend the battery life of portable electronic devices carried by the rider.

During the experimentation phase, the regenerative braking system was found to be incapable of fulfilling its dual functions simultaneously. It could neither produce enough torque to stop the bike, nor generate enough power to recharge the batteries. The team therefore chose to abandon the braking aspect of the system, to focus solely on the development of a continuous charging system. This system, once constructed and researched, proved fully capable of achieving the desired objectives.

Step 3: Design a Circuit

To start off, we had to design a circuit that could take the ~6 volts from the motor, store it, and then convert it to the 5 volts that we needed for the USB device.

The circuit we designed complements the function of the MintyBoost USB charger, originally developed by Limor Fried, of Adafruit Industries. The MintyBoost uses AA batteries to charge portable electronic devices. Our independently constructed circuit replaces the AA batteries and supplies power to the MintyBoost. This circuit reduces the ~6 volts from the motor to 2.5 volts. This allows the motor to charge the BoostCap (140 F), which in turn supplies power to the MintyBoost circuitry. The ultracapacitor stores energy to continuously charge the USB device even while the bike is not in motion.

Step 4: Getting Power

Selecting a motor proved a more challenging task.
Expensive motors provided the proper torque needed to create the braking source, however the cost was prohibitive. To make an affordable and effective device another solution was necessary. The project was redesigned as a continuous charging system, out of all possibilities the Maxon motor would be a better choice due to its smaller diameter.
The Maxon motor also provided 6 volts where as previous motors gave us upwards of 20 volts. For the latter motor over-heating would be a huge issue.

We decided to stick with our Maxon 90, which was a beautiful motor, even though its cost was $275.
(For those wishing to build this project, a cheaper motor will suffice.)

We attached this motor close to the rear brake mounts directly on the bike frame using a piece of a meter stick between the motor and frame to act as a spacer, then tightened 2 hose-clamps around it.

Step 5: Wiring

For the wiring from the motor to the circuit several options were considered: alligator clips for mock up, telephone cord, and speaker wire.
The alligator clips proved to work well for the mock up design and testing purposes but they were not stable enough for the final design.
The telephone wire proved fragile, and difficult to work with.
Speaker wire was tested due to its durability therefore becoming the conductor of choice. Although it was stranded wire, it was much more durable due to its larger diameter.

We then just attached the wire to the frame using zip-ties.

Step 6: The Actual Circuit!

Tackling the circuitry was the most difficult challenge of the process. Electricity from the motor first travels through a voltage regulator which will allow up to a continuous five amp current; a larger current than other regulators would pass. From there the voltage is stepped down to 2.5 volts which is the maximum the BOOSTCAP can store and safely handle. Once the BOOSTCAP attains 1.2 volts, it has enough power to allow the MintyBoost to provide a 5 volt source for the device being charged.

On the input wires we attached a 5A diode so that we don't get an "assisted-start effect," where the motor would start to spin by using the stored electricity.

We used the 2200uF capacitor to even out the power flow to the voltage regulator.

The voltage regulator that we used, an LM338, is adjustable depending on how you set it, as seen in our circuit diagram. For our purposes, the comparison of two resistors, 120ohm and 135 ohm, connected to the regulator determines the output voltage. We use it to reduce the voltage from ~6 volts to 2.5 volts.

We then take the 2.5 volts and use it to charge our ultracapacitor, a 140 farad, 2.5 volt BOOSTCAP made by Maxwell Technologies. We chose the BOOSTCAP because its high capacitance will allow us to hold a charge even if the bike is stopped at a red light.

The next part of this circuit is something I'm sure you are all familiar with, the Adafruit MintyBoost. We used it to take the 2.5 volts from the ultracapacitor and step it up to a stable 5 volts, the USB standard. It uses a MAX756, 5 volt boost converter coupled with a 22uH inductor. Once we get 1.2 volts across the ultracapacitor, the MintyBoost will begin to output the 5 volts.

Our circuit complements the function of the MintyBoost USB charger, originally developed by Limor Fried, of Adafruit Industries. The MintyBoost uses AA batteries to charge portable electronic devices. Our independently constructed circuit replaces the AA batteries and supplies power to the MintyBoost. This circuit reduces the ~6 volts from the motor to 2.5 volts. This allows the motor to charge the BoostCap (140 F), which in turn supplies power to the MintyBoost circuitry. The ultracapacitor stores energy to continuously charge the USB device even while the bike is not in motion.

Step 7: The Enclosure.

In order to protect the circuit from external elements, an enclosure was necessary. A "pill" of PVC tubing and end caps was chosen, with a diameter of 6cm and a length of 18cm. While these dimensions are large when compared to the circuit, this made construction more convenient. A production model would be much smaller. The PVC was selected based on durability, nearly perfect weather-proofing, aerodynamic shape, and low cost. Experiments were also performed on containers crafted from raw carbon fiber soaked in epoxy. This structure proved to be both strong and light weight. However, the construction process was extremely time consuming and difficult to master.

Step 8: Testing!

For the capacitors, we test two different types, the BOOSTCAP and a super capacitor.

The first graph depicts the use of the supercapacitor, which is integrated with the circuit so that when the motor is active, the capacitor will charge. We did not use this component because, while the supercapacitor charged with extreme speed, it discharged too quickly for our purposes. The red line represents the voltage of the motor, the blue line represents the voltage of the supercapacitor, and the green line represents voltage of the USB port.

The second graph is the data collected with the BOOSTCAP ultracapacitor. The red line represents the motor's voltage, the blue is the ultracapacitor's voltage, and the green line represents the USB port's voltage. We chose to use the ultracapacitor because, as this test indicates, the ultracapacitor will continue to hold its charge even after the rider has stopped moving. The reason for the jump in USB voltage is because the ultracapacitor reached the voltage threshold necessary to activate the MintyBoost.

Both of these tests were conducted over a period of 10 minutes. The rider pedaled for the first 5, then we observed how the voltages would react for the final 5 minutes.

The last picture is a Google Earth shot of where we did our testing. This picture shows that we started at our school, and then did two laps at Levagood Park for a total approximate distance of 1 mile. The colors of this map correspond to the speed of the rider. The purple line is approximately 28.9 mph, the blue line 21.7 mph, the green line 14.5 mph, and the yellow line 7.4 mph.

Step 9: Future Plans

In order to make the device more economically viable as a consumer product, several improvements must be made in the areas of weather-proofing, circuit streamlining, and cost reduction. Weather-proofing is critical to the long term operation of the unit. One technique considered for the motor was to encase it in a Nalgene container. These containers are known for being waterproof and nearly indestructible. (Yes, we ran over one with a car to no ill effect.) Additional protection was sought against the forces of nature. Expansion foam would seal the unit, however the material has limitations. Not only is it difficult to position properly, but it would also prevent ventilation essential to the overall operation of the device.
As to the streamlining of the circuit, possibilities include a multitasking voltage regulator chip and a custom printed circuit board (PCB). The chip could replace multiple voltage regulators, this would decrease both the product's size and heat output. Using a PCB will provide a more stable base because the connections will be directly on the board and not floating beneath it. To a limited extent it will act as a heat sink because of the copper tracing in the board. This change would decrease the need for excessive ventilation and increase component life.
Cost reduction is by far the most important, and difficult, change that must be made to the design. The circuit itself is extremely inexpensive, however the motor costs $275. A search is underway for a more cost efficient motor that will still meet our power needs.

Step 10: Finish!

Thanks for reading our Instructable, if you have any questions feel free to ask.

Here are some of the pictures from our presentation at MIT.



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    179 Discussions

    Did you consider using a Shimano Nexus hub dynamo (aka motor/generator) as a power source? Output is 6V. Example: (click on the tab that says "more pictures")


    Share did you get the minty boost circuit?

    hello guys;

    i have sun tracker
    project, i wanted anyone can help me to know the specifications of my
    supercapacitors ( my dc source voltage is 20 volt ) i need to have 12 volt and 1.2 amp from my supercapacitor


    try one of thier supplier should be willing todo just that. heres a link 4 them

    That's What the 140F capacitor was used for. In the graph explains that the surplus power is stored in the Ultra capacitor ~Name101

    Such a capacitor would store power, but will not "handle" surplus. When the load current drops the input voltage will rise and the linear regulator will be dropping more voltage, resulting in a higher heat level. You'd need far more than a capacitor to store the energy, something like a battery of a capacity high enough that it never reaches full charge, and yet this would be an additional drag on the bike, it should not be used because the whole circuit is wrong in the first place.

    7.2v RC Battery is Lightweight and ive used one in many applications

    The way I see this design, The Supercapacitor was not meant to store "Large" amounts of energy but enough to keep the power consistent if the rider was to slow down or stop for a short period of time.. I agree a battery would be able to store a lot more energy than a Capacitor. but the capacitor is short term. Correct me if I'm wrong. ~Name101

    Of course, a supercap is short term. The problem is, a supercap doesn't have enough capacity to *buffer* for regular riding. That means that once the supercap is charged the voltage in the system rises and creates even more heat and loss with the linear regulator.

    Let me put it another way. I don't revel in wasting energy but it's not so much a concern when something is AC wall powered. When you are peddling on the other hand, and a design has multiple forms of loss, energy conservation is really good, worth the time to do it right.

    The problem was they didn't look at how to get from point A to point B, point A being a human being producing linear movement of a wheel, and point B, producing the desired charge, then finding the best way to get there.

    Instead, they reused a design not just suboptimal for the purpose but contraindicated for the input and output.

    The way this is set up it would be far better to just strap a battery pack onto a bike to recharge something, or of course to use a proper bike generator, a switching supply circuit that accepts (uses) input over the voltage variations that result from a bike generator, and and output with current regulation and the associated charge control chip complimentary to the battery type being recharged.

    I appreciate this is beyond the ability of someone starting out, but at the same time this is what is great about technology today that we have ready-made ICs to do things difficult or lengthly to do with discrete parts. It is good to experiment but it is also good to see when it is redundant work, that each part of the problem can be seen modularly as how to get from point A to point B and that today we have great custom ICs to do these jobs (since when broken down into units, none of the things being done are new electronically).

    I think it's great if they had no hands-on experience, to learn from building something like this, BUT to put it out there for others as an example of how to get something done, it is a poor one.

    Well Personally am new to electronics.

    I see that stepping down then stepping the voltage up again is a waste of energy. but when the power supply is so close to a LDO linear regulator I thought Providing consistent energy would be a problem.

    You said "supercap is charged the voltage in the system rises and creates even more heat and loss with the linear regulator."

    How is this so? I would like to learn from this experience.

    I would like to learn how to make this as efficient as possible.

    (I'm sorry about the short comment I wrote a big one but pressed cancel instead of post =( )


    With a variable input voltage (6V is not constant) an LDO will be a problem, but even if kept above the LDO regulated voltage it's still a very lossy circuit. With the supercap you are improving charging, but putting more drag on the rider to generate that charge, and by keeping the voltage higher it is continual drag instead of the voltage being allowed to drop below the critical level needed to power the rest of the circuit so the rider has no moments of relief. So it does improve the circuit but it does not meet a different goal in use on a bicycle, plus it adds a lot of weight. With a battery on the other hand, or really I mean a pack of series cells, they serve as a crude but effective enough form of regulation to keep the voltage at roughly the sum of the cells. There is some loss in doing this too, but not as much as use of an LDO. One thing that is not really clear is why either would be needed, charging doesn't have to continue at all moments necessarily. It could be good to even tweak the circuit such that applying the brakes bypasses a current limiter right after the motor so that during braking a higher % of power is produced and more motor drag, then less when not, but it would be a secondary issue, the primary one being that the better solution is none of these supercaps or batteries, to just use a wide input voltage, regulated output switching circuit. I'm not suggesting no capacitors are needed for that, but rather less capacitance, a smaller design with only the parts the regulation stage needs, also built more rugged (leaded parts like capactors don't like vibration much as you'd see on a bike, you can cement/glue/etc them in place or you can pick parts more immune to the environment).

    Personally. I want to charge my device a constant as possible(GPS device). Once the Supercap is fully charged the Circuit will only draw the current needed to maintain the charge on the device. Correct? When I was doing the Research with my basic knowledge Something similar to this was perfect. I was aware of the energy loss through heat and converting the voltages 2 times is excess but the way I saw it was the voltages(form power source to USB device) are too close to use the LDO linear regulator. IF the hub generator produced 12v for example having a single Linear regulator would be perfect. Simplifying the circuit drastically. If the supercap was replaced with a battery pack for example would the circuit charge the batteries and would the batteries act like a capacitor? From my understanding capacitors have the advantage of being able to charge or discharge incredibly fast while if a battery had the same current draw the battery it self would be damaged. I completely forgot about the environment with my research. I must thank you so much with helping me here. ~Name101

    The generator voltage is not "close" to the LDO regulator necessarily, it depends on the load on it and of course gearing or friction wheel diameter, how fast it is spinning. A genuine 6V bike dynamo goes quite a bit above and below 6V even being designed for the task. That's why it's normally it's normally rated for wattage like 6W or 12W. I'm not saying it's terribly bad to have the supercap except it puts more drag on the bike at the worst possible time, getting started from a dead stop after having discharged. A constant charge isn't really needed, you only need to limit the max voltage and current within what the battery will accept. For example, suppose 20% of the time it wasn't charging, but the rest of the time it was charging at a high enough rate to compensate. The point that I'm attempting to keep drifting back towards is this needs to be seen as an accessory to bicycling, not the primary detail that matters so much as what factors are present while riding. Yes the batteries would charge like a capacitor, the point would mainly be that you can either: A) Have already charged them before the bike ride so they are a renewable portable power pack. B) Shut off the charging circuit before it drains them too much, then turn on again after having gotten up to speed so they aren't putting a high early drag on acceleration. You really don't want a linear regulator, not even an LDO. The voltage from a motor varies quite a lot, to always have it charging you will have to use something larger and create a great deal of heat as well as extra drag riding. The only way to have a small drop before the linear regulator is if you already had a wide input range switching regulator, but if you did then you wouldn't need the linear regulation stage at all. I have already outlined a much better way to do it, for good reason, that it should not be done a different way for bike powered use.

    Oh. I'm starting to understand. Haha. I suppose I'll keep this in mind when I'm designing future circuits. I did throw myself into the deep end by starting with this projects with very little prior knowledge. Ill Look into what I can do.

    Seriously, Thank you so much. =]