My cycle dynamo USB charger - design and principle

Much awaited dream of getting my own mountain bike becomes true last month. :) Last month I bought KHS Alite 500, 2012 model from Pedals and Wheels Malleshwaram. Odo reads around 300 Kms and am loving the rides! First I fell in love with Bullet, then I fell in love with Biking as its even more adventurous and fun. A lot of people get this silly idea in their minds that he is doing a vain attempt to save environment, some say he is over-doing a workout and getting thinner, some even kindle the way I look wearing my cycling attire. But I don't care for what people think because more than all that stuff, I bike just for my own enjoyment :)

Coming back to my original title - USB charger using cycle dynamo. This was my last weekend's plan which I got executed this whole week. My room had become a electronics workshop for a while with smell of burnt solder and also a generator room with the sounds of dragging dynamo over the tire treads. I burned a little of  midnight oil during weekdays in completing this project.

The basic drive for coming up with this charger is to charge my Phone/Camera/GPS device on long touring.
All feature-rich-smartphones have got a weak battery life. Mine is Motorola Defy BF5X 3.7V, 1500mAh which lasts for about max 2 days with CPU underclocking governors of 300MHz and 600MHz in action!

Recently bought a MINI DVR 80, sports camera to shoot my biking adventures which will now be usable if have a all-time-charger on the go!


Having worked on basics of 12V electricals in Bullet it was a pretty easy job to design battery charger for my cycle dynamo. Principles involved are the same and anyone can design it in minutes if you had attended your high school class about non-linear circuits.

AC supply from dynamo -> Full wave rectified (double the AC frequency, using 4 diode bridge-rectifier) pulsating DC -> Apply a capacitor as filter to smoothen out the DC output -> Limit the current by a shunt resistor (simplest electric ballast) -> Regulate the fluctuating DC by putting a IC regulator to appropriate voltage (IC78XX series).

This simple circuit below will explain a lot more:

> The output current varies from 0-1A based on the speeds of the cycling. IC7805 starts to give the output at speeds of 6-7Kmph as the input unregulated DC supply should be more than 8V for IC7805 to kick off.

> The power of dynamo itself restricts the capacity of current it can drive to the IC and thus a decent shunt resistor of rated around 2-3W should be sufficient.

> Capacitor could be more better if its rating is something like 25V, 0.1F. More-the-better. Will hold more charge and thus keeps charging for a while even though you stop for a while during the ride.

> This is not a very efficient circuit but just works for me. I saw better designs on web doing similar stuff. Output current should be ideally 1/10th the Ah rating of the battery being charged from the USB, as there will be chances of over charging and also lessens the life of rechargeable Li-ion batteries. You will need to add voltage comparator and make current trickle when fully charged. Then circuit becomes more complex. But to avoid this I usually connect the USB output to a portable battery power which has got protection inbuilt to cut-off the supply when its fully charged. (Note that, this portable battery power can be charged by other power outlets as well)

Further upgrades would be to put a high rated capacitor like 1F and put some 3W LED lighting for the front and the rear which could lessen the value of current limiting resistor.

Things you need:
1. I found the circuit of full wave bridge rectifier in a damaged wall charger adaptor unit of my WIFI modem at home. It had real good diodes IN5400 which can rectify upto 50V, 3A. A cycle dynamo can never reach that power rating to burn them!
2. Regulator IC costs some 8 Bucks (Courtesy: Famous SP road of Bangalore)/ Or you can find it in some old radios/ Or go to local TV repair shop.
3. Most important of all! Bottle dynamo, Just the generator coil with strong neodymium magnet will cost INR 80. (Hub dynamos will be better as there wont be drag/tire wear)
4. A USB Type A socket (Female pin). (A cheap USB adapter cable which was unused at home)
5. Soldering gun + solder paste + solder wire. (You can get soldering done in nearby electric shop if you are not good at working with PCB)
6. Insulated wires (About 2 meters)
7. A Heatsink(+ metal paste) for regulator. I made my own heatsink with several folds of Aluminium foil as the power dissipation will be less if cycling speeds are around than 20kmph ;) And I don't expect to reach constant speeds of 40kmph during touring.
8. Alligator clips for clamping at the dynamo output terminals. (I use them as its flexible for removal. Saddle bag can hold all of the charger circuit when I want to remove)
9. A digital multimeter to verify that circuit is behaving as expected. (An oscilloscope would be better to view it graphically :P)

With an overall cost of around INR 250 anyone can build this circuit.

Even Nokia has come up with bicycle charger kit but at a whooping cost of INR 1500!!
http://thegadgetfan.com/gadgets/nokia-dc-14-bicycle-charger-now-available.html

http://europe.nokia.com/find-products/accessories/all-accessories/power/chargers/nokia-bicycle-charger-kit/specifications

Some pics of my project:

Final housing of all wiring with charger inside saddle bag.

Had kept my cycle upside down for Pedalling, thus the dynamo reversed!

In my first attempt heat sink was bigger than PCB :P

Rectifier circuit

Constant voltage output of regulator with any output load

Avg output current, 440 mA

USB adaptor PIN

My funny packaging in a organic tea box :P

I shall call this device as Organic USB Charger to Go Green :)

Charging in action, Device getting charged while pedaling

Housing of all cables below the seat.

With just about 20 Mins of cycling at 15kmph you could easily recharge the phone for about 30 mins of talk time / 20 hours of standby time! You can disengage the dynamo from wheel by pulling and pushing a lever when you want to stop charging.

Disadvantages:
1. Drag when the dynamo is in action. (Quite a humming sound at rear which makes the nearby street dogs chase you at times! :P)
2. Tyre wear (I don't think much as MTB tyres are built for more worse terrain)
3. Weight of dynamo (around 200 gms). Wont be much of effort to pull that weight along on an Aluminum frame bike.

My parabolic-mirror-solar-grill - Story and Design

This weekend, I designed my own solar grill at home using some of the unused articles and also some things were bought at nearby home appliance shop or novelty store.


Background for coming up with this crazy idea was that my android smart phone Motorola Defy was draining out of battery in less than one day with all radios enabled in it esp like mobile data, wifi, GPS. I needed a USB travel charger badly and bought a solar powered + usb powered (4400 mWh) which could charge universal devices like Camera/Ipod/Phone etc., on the go. Its output rating is 5.5V,500mA. It works just good in critical power needs during my trekking/camping. I had underestimated the solar power till then until it helped me really when I needed the most.

A few months back I had been to Lakshadweep islands where the sole energy for the whole island was Solar power. I was really amazed at the self sustained way to renew energy resource even for desalination of water using the temperature difference between the cold water of deep sea and the hotter surface water. Even the entire lighting was done by an array of solar cells.

I was looking at the ways to harness the this power in very small scale and came up with website called GREENPOWERSCIENCE which does build the acrylic parabolic mirrors and extreme powerful fresnel lens which was well suited for my purpose but was not within the reach of my budget anyway. Then I went in search of ways to build my own parabolic mirror. Dan of the GreenPowerScience explains a lot of ways to make a user defined one with casting mortar and using it to prepare a cement mold. Grease the mold, then use 3 layers of fiber glass wool and polyester resin to bind them into a parabolic polyester dish. Well, this preparation was too tedious (but really economical on some scale if we plan to roll out many) but I wanted a better and faster way. The best suited idea for me was given by another youtube video which used the TV antennae that comes with the dish which is of parabolic shape to converge the KU band of TV signals into a focal point for better reception. So I bought a scrap dish from reliance big TV workshop near my place for 350 INR (Found a good quality dish in a stock pile of Defective LNB dishes) Its a 60 cm diameter dish which I bought with the clamps and bolts for fitting.

[If you are not into mathematics of conic section called parabola and want to understand why it works to choose this shape, please refer to parabolic reflector and see here to understand the theory and check out the illustration]

Next step was to prepare a reflecting surface for the entire surface of the parabola. This was done by low cost Aluminum foil of food grade quality. This costs about 40 INR for about 7 mtrs X 30 cms. Not even 1/4th of it was used to cover the dish surface. I cut it into strips of regular sized rectangles (better to make trapezoids for easier coverity with very less wastage) to paste it with a child safe glue. The glue must be water based and it should be non-toxic as it might get heated and may fume in direct sunlight. Even with the application of tin foil all over the surface, the reflectivity is pretty bad about just 35-40% with the wrinkles and overlapping edges. The full sheet as such if stuck on flat surface would give about 55% of reflection. Also I read about Mylar, Foylon and other reflective vinyl sheets which could be used for this but again cost issues exceeded my low budget project. I plan to upgrade to better reflective surface tearing down the current tin foil surface as it is not durable for long and the temperature tin foil reflection generates is very less and is insufficient to boil water. Using a mirror (glass based) would be optimal for me as it will be lesser initial cost but would require a lot of work to cut mirror into several hundreds of pieces to cover entire area of the dish.

I read somewhere that a square meter of earth's surface lit by bright sunlight generates can be equivalent to 1000 watts of power. So a 60 cm dish with good reflective surface will give above 560 watts of energy.
If you have concentrated a beam of light by magnifier lens to burn paper during your school days/for your curiosity then you can imagine the power of the parabolic concentrator.

After the reflective parabolic surface then the problem comes with the proper mount. Mount can be made anything and problem here is to get a grill at the focal point of the sunlight after the dish is mounted on any surface. It can be anything from an unused table fan bottom, a tripod etc. My initial attempt to use a 4 foot long flower vase stand went in vain as it could not withstand the entire weight as its center of gravity went outside the dish when mounted :P

After hunting more and more for unused items at home, I found a cradle (!) in which I used to sleep when I was a baby ;) An antique piece in rust but still standing :P Good thing is that its easily dis-mountable with 4 rivets at the bottom. I shall work on a good paint job sometime next week on it to make it a more durable.
It has got two hooks on the sides which helped me to put up the stainless steel round grill with a wooden handle on top of it with the help of small sized bungee ropes which holds it in place. Good thing about these ropes is that they got good tension retaining ability, they are insulators, and most important of all that I could vary the position and angle of dish for sunlight based on placement of hooks on the side rods of cradle.


Sad that there wasn't much sunlight today as it was rainy and gloomy all day long. So I couldn't test for temperatures it could go with direct sunlight. I shall update it soon after my next series of tests. :)


Things I bought for grill:
Parabolic dish - 350 INR
Aluminum foil + Glue - max 40 INR
Bungee ropes - max 30 INR each (I had some of them already for traveling on bike)
Steel grill - 140 INR

(You may also require a plastic ruler, blade to cut the foil)

Well within 600 INR of cost it is doable and it took me just 2 days with each day spending about 5-6 hours of work all alone on building this. I started building it on Saturday morning and completed on Sunday evening :)

Below are some pics that i took in building my solar grill. Will update more on the mirror upgrade that I plan to do and probably post a video soon of me cooking some good stuff ;)

Check out some FAQs listed here if you have something in mind to start a project something like this.

Cheers!
Karthik