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MAKE contributing writer Alastair Bland first introduced us to this simple, effective circuit back in 2009. It takes higher-voltage alternating current (AC) from a retro-style bike headlight generator, converts it to direct current (DC), and steps it down to a safe voltage for charging your cell phone or other mobile device.

You may have one of these old bike generators lying around.  If not, they can still be bought new or used online and in some retail bike shops. Technically, this device is an alternator, a very simple and reliable generator that turns rotational momentum into AC electricity.

The phone charger circuit consists of a rectifier, a capacitor, and a voltage regulator.  The rectifier contains four one-way electrical gates called diodes which, working together, convert the back-and-forth wiggling of the charges in AC to a series of DC charge pulses.

The large capacitor connected between the DC terminals of the rectifier smooths out these pulses, charging up when there’s more energy in the system and discharging when there’s less.

Finally, the voltage regulator holds the incoming DC power down at a steady 5V, which is what most cell phones and other mobile devices are designed to accept.  Without the regulator in place, the phone charger might deliver more than 5V to your device, which could damage it.

Steps

Step #1: Prep the box.

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  • Drill a centered 1/4" hole in each end of the box, and another in the lid.
  • Install a 1/4" grommet in each of the holes you just drilled.
  • Mount a large broom clip on the outside of the lid using a 6-32 x 1/2" machine screw secured with a 6-32 hex nut, a #8 split washer, and a #8 flat washer inside the lid.

Step #2: Prep the voltage regulator.

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  • Bend the LM7805's center (ground) lead down 90 degrees, and its lefthand (input) lead up 90 degrees, as shown.
  • Cut two 2" jumpers from 22 AWG solid core hookup wire--one red and one black. Strip about 1 cm of insulation from each end of each jumper. Solder one end of the red jumper to the regulator's input lead, and one end of the black jumper to its ground lead.
  • Trim the excess input and ground lead away from the voltage regulator with a pair of wire cutters.

Step #3: Install the voltage regulator.

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  • Secure the LM7805 in one corner of the box by driving a #6 x 1/4" sheet metal screw through the hole in the regulator's back plate and into one of the box's molded-in mounting holes.
  • Slip an insulated 22-16 AWG #6 ring-tongue lug over the regulator's righthand (output) lead, and secure to the adjacent mounting hole using a second #6 x 1/4" sheet metal screw.
  • Crimp the ring-tongue lug to the regulator lead with electrician's pliers.

Step #4: Prep the rectifier.

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  • The rectifier's positive DC output lead will be indicated with a small + sign on top of the case. The negative DC output lead may or may not be marked, but it will always be directly opposite. Bend the DC output leads up along the sides of the rectifier case as shown.
  • Bend the capacitor leads directly away from each other as shown.
  • The capacitor's negative lead is marked on the side of the case. Making sure the polarities match--positive to positive and negative to negative--stack the capacitor on top of the rectifier and twist the leads together as shown.

Step #5: Connect rectifier and regulator.

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  • Bend the twisted leads up along the sides of the capacitor case, as shown.
  • Solder the free end of the black jumper to the negative side of the capacitor/rectifier combo.
  • Push the capacitor/rectifier combo down into the box and solder the free end of the red jumper to the positive leads, as shown.

Step #6: Mount the rectifier.

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  • Loosely secure a bare 22-16 AWG #6 ring-tongue lug to each of the two remaining mounting holes using a #6 x 1/4" sheet metal screw. The lugs should be free to rotate and tilt up and down.
  • Thread the rectifier's AC input leads into the bare ring-tongue lugs and bend the excess straight up, as shown. Tighten down the sheet metal screws to firmly secure the lugs in place.
  • Use electrician's pliers to crimp the lugs to the rectifier leads. Trim away the excess from each lead with wire cutters.

Step #7: Connect external wiring.

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  • Cut the adaptaplug cable in half and thread the ends into the box through the grommets at each end. Tie a strain-relief knot inside the box about 3" from each end.
  • Separate about the leads from each end of the cable over a distance of about 1-1/2". Strip about 1 cm from each end of each lead, and securely crimp an insulated 22-16 AWG #6 ring-tongue lug to the bare wire.
  • One at a time, remove and reinstall the sheet metal screws from the mounting holes in the box, securing the ring tongue lugs from the adaptaplug cable as shown. The polarity on the rectifier side does not matter; on the regulator side, make sure the lead with the white stripe is connected to the positive output, as shown.

Step #8: Attach the box to the bike.

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  • Connect the adaptaplug micro-USB adapter to the DC / voltage regulator side of the adaptaplug cable. Make sure the adapter's + sign is aligned with the word "TIP" on the plug socket, as shown.
  • Install the lid on the project box and secure with the four bundled case screws.
  • Clip the project box to your bike frame with the broom clip. We chose to mount ours underneath the seat, as shown.

Step #9: Install the generator.

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  • Mount the generator to your bike according to its directions.
  • Safely route the AC input cable from the project box along your bike frame to the generator, keeping it well clear of all moving parts. Secure with zip ties.
  • Cut off the plug at the end of the AC input cable at the generator.

Step #10: Complete the wiring.

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  • Separate the leads of the cut cable over a distance of about 1-1/2", and strip about 1 cm of the insulation from each lead.
  • Secure the bare stripped ends of the leads to the generator output terminals. It doesn't matter which lead goes to which terminal.
  • Plug your cell phone into the micro-USB adapter and secure it, with any excess cable, in a zippered saddlebag or other pouch attached to your bike. You're ready to go!
Sean Michael Ragan

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.


  • http://www.facebook.com/PeteGoldstein Peter Goldstein

    Awesome and simple project! I was shocked when I found out that devices like this weren’t mass-marketed yet. Did you run into any issues with voltage variance during starts / stops? I imagine some phones would get grumpy if their voltage varied or power kept turning on / off. Perhaps intercepting this project by putting a lightweight cell phone charger / battery pack (like http://amzn.to/13UiPJ1 )in between the final power-out and the phone would alleviate this? (if it’s a problem at all). Thanks!

    • http://blackheronbikes.com/ James BlackHeron

      This type of charger actually is available right now although it will set you back $70 to buy it so it is not cheap.

      http://www.bike2power.com/LightCharge-Bottle-Dynamo-Bicycle-USB-Charger.html

      The nice thing about this one is that it will work with either a 6v dyno or a 12v dyno. Most dynohubs run at 6v these days and the 12v bike dyno standard is pretty old and outdated. Finding 12v bottle generators is a bit harder these days.

      This project will not work with a 6v generator because the 7805 voltage regulator requires 7v or greater to work. You have to spin a 6v dyno really fast to get it up to 7v although it is possible at really high speeds. It’s no going to work very well most of the time.

      I have a 12v bottle generator and I think I might be making one of these but instead of putting it on my bike (I already have the Bike2power unit) I’m thinking of building a wind-powered generator out of it.

  • Sean Michael Ragan

    Thanks Peter. Actually, no, we had no issues with varying voltages; the LM7805 seems to do a pretty good job of delivering 5V, if it has adequate power, or nothing. My HTC Droid DNA is happy as a clam charging from this thing. Don’t even get the “slow charging” warning that I sometimes get with the cheap micro-USB cable I bought at Walgreen’s.

  • Matt Richardson

    Is that the bike I think it is!? Glad to see it’s still getting the maker treatment!

    • Sean Michael Ragan

      I enjoyed that synchronicity, myself. ;)

  • Haikel

    Where can i get this?

  • http://home.earthlink.net/%7Ejohnrpenner/index-creative.html John Roland Penner

    very very cool project — you may get more power for less pedalling if you use STEPPER MOTORS — since a traditional bottle alternator limits its output with an internal coil (e.g. 14 ohms) to prevent the bulb filament from burning-out at high speed — in other words, you go really fast putting more energy in, without getting any more out! — STEPPER MOTORS can give you almost five times the watts for the same amount of pedalling, and are easy to obtain • http://www.autospeed.com/cms/A_111296/article.html

  • meyer sakkal

    why does it not work with apple products

  • Lena harper

    Do I get all the stuff from radio shack or where???

  • an yu jin

    it’s amazing!! it’s the thing that i have wanted!

  • http://gravatar.com/sagarbhuva sagarbhuva

    great work guys.. keep it up.. :) i want to know how much hours of bicycling is required to fully charge the phone ?

    • Sean Michael Ragan

      Never timed it! But it should be the same amount of time required to charge your phone from the wall. If anyone discovers differently, please let me know!

      • http://www.Blackheronbikes.com James

        It would really depend on your phone and how large of a battery it has. This charger isn’t going to charge as fast as a wall charger as it only is going to provide about 3w of power at 5v. That is a little over 500 milliamps of current. A typical fast wall charger will have more like 1000 milliamps of current, although many smaller wall chargers like those made for an iPhone (the tiny white wall cube) are 500 milliamps if I remember right.

        It’s best to start with a fully charged phone when you go for a ride anyhow, and just let the charger keep it topped up rather than start with a low phone and expect the bike to charge it.

  • rain.bow

    This is very cool

    Tools link not working – can anyone please provide the list of Tools?

    Parts Link http://makezine.com/projects/pedal-power-phone-charger/#1
    Tools Link http://makezine.com/projects/pedal-power-phone-charger/#2

    • Sean Michael Ragan

      Thanks and sorry for the confusion. Should be fixed now.

      • rain.bow

        Awesome, it works now!! Keep posting gr8 maker stuff :)

  • Igor

    Can i use a 470pF 35v capacitor in the circut.. will it do any harm to it?

  • igor

    Good,thx.Ill add them up in parallel and see how it fits…keep up the great work you doo.

  • http://twitter.com/arkanciscan Jesse Hattabaugh (@arkanciscan)

    I’ve got a generator exactly like that one and you made a mistake in the video. The two terminals on generators like these are actually separate power sources, one for the headlight and one for the tail light. You’ll notice that one says 12v and the other says 2.5v. The bike’s frame acts as the ground wire through the generator’s mounting bracket. I’d be surprised if your demonstration rig actually worked the way you wired it. Since your frame looks to be made of carbon fiber the best solution would be to run the ground wire directly to one of the generator’s mounting bolts.

    Also, I own another generator made by a company called “Tung Lin” that I feel is superior to the one used in the demo. It’s less noisy and seems to be higher quality. You can find it on Amazon for cheap http://www.amazon.com/Bike-Bicycle-Dynamo-Generator-12V/dp/B000OBWMGK/ref=sr_1_12?ie=UTF8&qid=1375562329&sr=8-12&keywords=Tung+lin

    I’m curious why you didn’t use a step-down switching regulator. They take up more space, but they are still cheap. http://www.amazon.com/HOSSEN%C2%AE-Converter-Power-Supply-Module/dp/B00A71CMDU/ref=pd_cp_pc_3 The linear regulator you’re using basically burns off excess energy as heat (you should probably use a heatsink or it might melt the case) but a switching regulator can convert more than 90% of the energy. So instead of getting 300-500mA (which is fine if you’re charging most USB devices) you’d get something more like 750mA-1A. Most tablets can draw up to 2A while charging.

    As you can see I’ve been tinkering with this kind of setup for a while. I plan on running two generators simultaneously. If I wire them in parallel I should be able to get 1.5-2A which will charge my battery pack as quickly as possible. The battery pack I’m using (http://www.voltaicsystems.com/v39.shtml) was designed for solar panels so it’s able to accept a variable current from 6-12vDC. All I really need to do is rectify the AC current, but I’m considering using a capacitor and switching regulator anyway because the battery manufacturer told me the battery pack charges most efficiently at 6v, probably due to a built in regulator.

    • Sean Michael Ragan

      Hi Jesse-

      You may well be right; the only thing you’ve said I’d take issue with is this:

      “I’d be surprised if your demonstration rig actually worked the way you wired it.”

      Be as surprised as you like, but it works that way. Like gangbusters. Otherwise we would not have published it. Haven’t had any problems with heat dissipation yet, either.

      Thanks!
      Sean

      • http://twitter.com/arkanciscan Jesse Hattabaugh (@arkanciscan)

        A saw that the original Instructable which uses the Tung Lin alternator also ran their ground to the tail light output. It’s odd that it works that way. Maybe the 12v of the headlight output overpowers the 2.5v taillight since they share a common ground and you just get a slightly dropped voltage running through your rectifier. Still you’re probably causing more drag and getting less output than if you’d just wire your ground wire to the bracket.

  • http://gravatar.com/joedag32 joedag32

    Has anyone tested this out with an iPhone 5? Just figured I’d ask before I picked up the parts!

  • Michael Cochrane

    OK built and tested. It works! My phone freaks out and needs reboot when riding in stop start traffic. But with sustained rolling, it works really well. Perhaps a rechargeable battery circuit could be incorporated which would hold the voltage while the bike is stopped in traffic. Im cycle touring with it soon, so that will be the ultimate test… can it keep 2 phones, and a USB speaker charged over 10 days? : )

  • Tiago Almeida

    Do you have a schematic for this? Thanks!

  • https://plus.google.com/100466886544586541886 Roger Liucci

    Great project! :-)
    My generator has only one terminal. Can I use the bike frame as a ground?
    It’s a very old Traveler brand generator with “CH 1 1407″ embossed on the bottom.
    Thanks

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  • pieter

    I’m using a 6V AC – 3W dynohub
    and it works fine for my old HTC smartphone.
    gives perfect 5V and charges my phone.
    So I tried it out with my Samsung Galaxy S3. The charging connects, so my phone thinks it’s charging, but in reality it is not. It is still using energy and won’t charge at all.
    I’ve read a lot about this problem for iJunk (as i like to call it) and they need a voltage reference on the data lines for it to charge at different speeds. All i can find about this for a galaxy is shorting the data lines instead of leaving them to float. Now shorting the data lines will tell the phone it is plugged into the wall and can take as much power as it wants. So i’m afraid that if I do this it might blow the 7805 or another part of the circuit.
    Or is my smartphone smart enough to take the max amount of current available? (while watching the Vcc usb so it doesn’t fall under lowest limit.)
    Or what should the voltage reference be for my phone so it takes 500mA

  • Molly

    how long does it take/ how hard do you have to pedal to get the phone charging?

  • Kerry Staite

    just a quick note, the 5v reg will eventually die, due to, too much overhead voltage when going fast, the un-loaded dynamo hub or bottle dynamo outputs 1v per 1km/hr…..so if you go fast, like down a hill….you could cook your main 5v Reg…also it won’t charge a iphone unless you have the correct voltage ref on the data lines….other USB devices like Apple products also need correct voltage ref on data pins….if any ones interested, I have some simple circuits that actually do work over time, and offers a voltage clamp and current boosting, for more power output….hope this helps…got got my mag today, with this link…;)

    • http://Mrjoe.uk Joe Leech 红豆 乔

      Hi Kerry,

      I’d love to see your updated circuit.

      joe

    • Dallas

      yes, could you please forward the circuit? Thanks.

    • Colin Rogers

      I would love the circuit. Please forward it!

    • wsanders

      You’re definitely likely to fry a linear regulator like a 7805 (max 25V in). And at 25V and 1A current draw the 25W has to go somewhere as heat…. A more robust alternative might be a buck regulator module like this http://www.seeedstudio.com/depot/Adjustable-DCDC-Power-Converter-125V-35V3A-p-1534.html or this http://www.seeedstudio.com/depot/CPTC5-power-converter-12V24V-switch-to-5V-p-1066.html. (I really like the former for all kinds of applications.)

    • Aacheelektro

      Maybe it would be better with a wide input switching regulator with some voltage clamping thrown in for safety.

    • Leif Burrow

      I think a 7805 is pretty apropriate for the skill level that this article is targeting. We all start somewhere and sometimes it is good to just keep it simple. Sure.. it might not last forever but unless we are talking about a really fast biker I suspect it will last plenty long enough to be worthwile. Besides, it’s not like the 7805 is an expensive component. The user can always replace it now and then.

      A more advanced electronics maker can replace that 7805 with a good switching regulator for better efficiency and add over-voltage protection. But.. that maker doesn’t really need this article to guide him or her.

      The one thing I would change about the article is I would recommend one of those boxes with a metal cover. Then I would have them bolt the regulator to the metal part to act as a heat sync. There might be some additional thought needed regarding preventing the rest of the circuitry from making contact with a conductive cover. I’m sure that can be handled in a simple beginner friendly way though.

      For bonus points IF the maker has access to heat sync compound that would be nice too. Maybe the maker has also assembled a PC at some point and has some lying around from mounting the CPU fan. Or.. if there are babies in the house Desitin is basically the same thing. It may not be maufactured to quite teh same tolerances (greater water content maybe…) but it is good enough to use as a cheap heat sync compound for a beginner project like this.

      Still.. I think we are going for a beginner maker, quite possibly on a kid’s budget here so the heat sync compound should be listed as optional.

      • http://wildsong.biz/ Brian Wilson

        All due respect, but something that burns up and fails periodically is a bad design, even if it’s easy to build. But anyone hanging a bottle generator on a bike just to charge a cell phone probably won’t be going fast for very long — down hills only. So it might last a good long time. Bottle generators put quite a bit of drag on the bike; makes the inefficiency in the 7805 moot. Maybe you can use the overheated regulator as a hand warmer in the winter. It’s a fun project idea all the same. I am think a little solar charger makes more sense for real world…

        • Leif Burrow

          Everything fails periodically we are really only discussing the length of the period. I’ve never tried this particular project but I’ve tried other things that are fairly abusive to a 78xx regulator and they really aren’t that easy to kill. I’d be surprised if it broke before the user was ready to upgrade anyway.

          For the beginner what’s the alternative? Get a pre-built regulator module? That isn’t nearly as rewarding. The excitement and pride of getting the simple build to work are what will fuel the learning required to come back and build a better regulator later.

          As for the drag from the bottle generator.. is the phone going to take a whole lot more current than the incandescant light that originally attached to the generator? I used to use one of those all the time when I was a kid and never let it slow me down. If anything my legs got a little stronger and I burnt a few more calories. That is not a bad thing!

          I’m surprised that you know everything that is wrong with the 7805 as a charger and yet you think a bike-mounted solar charger is a good idea! Either you would have a tiny solar cell that doesn’t even produce enough energy to get through the phone’s charging circuitry and to the battery or you would have a large panel that is totally impractical on a bike.

  • Kerry Staite

    DIY USB charger using AA batteries….

    Its a very simple unit….and gets around a unloaded system, with huge voltage variations, buy using the BATTs to lock down the voltage…..

    Dynamo goes via a rectifier and charges up the AA 6 cell (7.2v) NIMH pack (marked DC in pic)

    AA battery’s make the hub lock to use charging voltage of battery…without a booster cap the have a max charge rate of 500mA from hub

    For ease of use, I bought a 5v regulator…it outputs 5v @ max 600mA…you can run 2 of them if needed.
    http://www.pololu.com/product/2107

    when you flick the switch, the battery stops charging and pushes it power to run the 5v reg (for USB output)

    I run a little voltage readout to tell me where the voltage is at…
    http://www.hobbyking.com/hobbyking/store/__7224__Hobby_King_Battery_Monitor_2S_EU_warehouse_.html

    there is a buzzer on board but I remove that, so it just the LED that let me know the voltage for when they are charged…

    7.2v battery pack will reach its empty point, just as the 5v REG falls OUT of regulation and turns OFF, thus stopping the batteries from being over drained.

    you need to keep an eye on the voltage LEDs, for when they are charged…however 4-6hr and they will be done…

    AS the AC from the hub can harm electronics, its made to be CHARGE OR RUN….

    I have made a few PCBs for others to try…its very easy to do, with only basic soldering needed….

    you can also use 4x NI-ZN batterys as the voltage will be very close….

    system can be boosted to over 500mA if you use a bigger battery pack…AA get hot if charged over 500mA….by adding a bi-polar cap in series to the input of the dynamo, you can boost up to 12w of power from a hub, or 7 w from a bottle genny

    got a few PCB is anyone wants to try it out…;)

    • Aacheelektro

      Looks dandy!

  • Adan M

    It would nice to route the charging cable to a weather proof phone holder on the bike bar. But I did like the break down for the adapter box. Thanks

  • Ened Mato

    Hello Kerry! Great Instructions on this Project!. Followed the
    instructions to the point and it worked on a Nokia phone. i am currently
    trying other devices and i would be more than happy to make this work
    for iPhone also. Can you consider any changes or additions in order to
    make this work for iPhone also?? Thank You and Greetings form Albania!!

    • Aacheelektro

      It should work on any phone as long as the input voltage to those phones is 5 V which is the standard anyway. Just be sure your circuit can supply the necessary currents.

  • Aacheelekto

    How about an SMD design for this circuit? I’d prefer to make mine with surface mount parts.

  • Sebastian Lindblom-Taylor

    Can’t I just put a resistor in front of the regulator so it won’t get fried if the voltage is too high?

    • http://wildsong.biz/ Brian Wilson

      A resistor will limit current not voltage. With a constant AC input voltage (like what comes out of the wall at home) you could just put a transformer ahead of the rectifier but that would only work at top speed on the bike generator. A rectifier followed by a step down converter (like the one from Pololu mentioned elsewhere) would be perfect.