Introduction: USB Bike Generator

About: I enjoy building things more than actually using the things I build
The USB Bike Generator is a small bike mounted electricity producing device optimized to provide power for two USB ports.  There are so many small electronics that can be powered or charged from a USB connection it only makes since that people might want to do this while riding a bike.  The basic idea for the USB Bike Generator is to use a suitable stepper motor as a generator and a voltage regulator circuit to maintain the 5 volts needed for the USB ports.  In this instructable I will show you how to build this generator and through testing show that it is 70% efficient at converting the power from the generator to the power needed for the USB port. 

This is the third revision of my bike generator project, the first two can be found here and here I would strongly recommend that if you plan on building this USB Bike Generator you at least look over how these past to versions went together.

This third rendition of my bike generator project came about after reading some of the comments made about my previous methods. Specifically, one comment from member ac-dc stated that my decision to use a linear regulator to go from 30 volts from the generator to 3 volts to power the light was at best 10% efficient. Now since one of the interests I've had listed on my profile since I joined this site has been efficiency I decided to read the rest of his comment after wiping the tears from my eyes. Ac-dc suggested that buck switching regulator would be better suited for a bike generator like mine. I had no idea what a switching regulator was so I started to do some research and found out that ac-dc was right and that I could significantly increase the efficiency of the electronics I was using.

In my searching for switching regulators I came across this reference from Dimension Engineering,  They offer a good explanation of the switching regulators and even sell them. 

Step 1: BikeGen Testing

If you only want to learn how I built the USB Bike Generator and not why the BikeGen system wasn't the best, you can skip this step.

In order to set a bench mark for the efficiency of BikeGen system I decided to do some testing.  In my searching for switching regulators one of the best resources I found was this article It explains how switching regulators work and how you can calculate the power lost through heat generation.  The power lost to heat can be calculated using this equation:

Power lost = (Input voltage – output voltage) * load current

So in order to find the power lost you need to know the input and output voltages as well as the load current.  Luckily, I already had the tools for this job which required 3 digital multimeters and a small drill press.  

The test setup was fairly simple, I tightened the drill press chuck onto the shaft of the stepper motor and loosely clamped the motor in a vice.  This was the same stepper motor that I have used for both the Bike Generator and and BikeGen Instructabes.  This actually works out really well for two reasons.  First the speeds that the drill press can spin are labeled and from these speeds I calculated how fast I would need to ride the bike based on how fast the generator is spinning.  So using the tire size, rim size and the diameter of the small wheel mounted on the stepper motor for the BikeGen instructable I calculated the following speeds:

Drill Press RPM            Bike Speed (MPH)
         620                                     4.7
        1100                                    8.3
        1720                                   13.0
        2340                                   17.7
        3100                                   23.4

This range of bike speeds seems very reasonable.  The second reason that using the drill press is a good representation of a bike rider is the power rating of the drill press motor.  This drill press is rated at 1/3 horse power which is roughly 250 watts, this is attainable by an average person while riding a bicycle.  

The next step was to connect 3 digital multimeters into the circuit.  I had cut some of the wires and use a few sets of alligator clips to make this happen.  Check out the picture or the Test Setup pdf to see how I connected the meters.  Basically the current meter was placed inline between the regulator and the charging circuit and the voltage meters connected after the diodes and after the regulator and were grounded at the same spot.  This measures the values I need to calculate the power lost.

Once everything was ready I plugged in the drill and turned it on.  I tested the generator at every speed the drill was capable of and found out that at the first 3 speeds (620,1100,1720 RPM) there was not enough power to consistently charge the batteries.  This was confirmed by the blinking LED on the charger.  At 2340 RPM and 3100 RPM enough power was provided by the generator to charge the batteries.  The values I measured at these two speeds are listed below:

Motor RPM                  2340       3100
Input Volts                    15.2        20.2
Output Volts                   13        12.68
Output Amps                0.29        0.28
Power Lost (W)           0.638     2.106
Output Power (W)       3.770     3.550
Total Power (W)          4.408     5.656
Efficiency                      85.5%    62.8%

The power lost was calculated from the equation I showed earlier, the other values were calculated from the following equations:

output power = output voltage x output current
total power = output power +  power lost
efficiency = output power / total power


So looking at these results I think there is something to be desired.  First there was not enough power to charge the batteries until 2340 RPM which is 17.7 mph on the bike.  This is a high speed and doesn't seem reasonable for a casual bike ride.  Of course this speed is attainable but it would be a lot of effort to maintain the speed for an extended period of time.  The efficiency  seems very good at 2340 RPM (85%) but it drops to 62% at 3100 RPM.  This is because the input voltage goes up at higher speeds which means more power is lost to heat.  So unless I want to ride the bike at and average speed of 18mph everywhere I ride I will not achieve the best efficiency possible.    

Step 2: The Tools and Parts You Will Need

Below is a list of most of the tools and parts I used to build the USB Bike Generator.


Soldering Iron
Solder Sucker
Desoldering Braid
Digital Multimeter (3 of these were used in testing)

Drill Bits
Tap (8-32 screw size)
Tape Measure


USB Car Power Adapter (read more about this in the next step)
Stepper Motor
1N4001 Diodes (8 diodes are needed)

1-1/4" x 1-1/4" x 1/16" Aluminum Angle
1/2" x 1/8" Aluminum Flat
8-32 Machine screws and nuts

Step 3: Build the Electronics

There are three major parts to the electronics of the USB Bike Generator, the stepper motor, the rectifier and the voltage regulator. 

Stepper Motor

The USB Bike Generator uses a stepper motor as a generator to produce the electricity.  In general any electric motor can be used as a generator but not all motors are well suited as generators.  The stepper motor I used in my last two instructables came from an old printer and was rated at 24 volts.  Through testing I found that this motor provided up to 48 volts when unloaded and spun at 3100 rpm.  People new to electronics should understand that high voltage doesn't always mean high power.  In order to reduce this voltage to the 12 volts needed for the BikeGen instructable the regulator just burned off the extra voltage as heat.  This meant the regulator was inefficient.  

In my searching for a new stepper motor I looked for two important aspects.  First, the voltage rating of the motor need to match the 5 volts required by the USB ports. Second, the amperage of the motor needed to be higher, meaning there was more power potential in the motor.  I found a somewhat local electronics surplus store that sells stepper motors and searched their website,  They had a stepper motor listed at 5 volts and 3.3 amps, this seemed perfect.  I went to the store and after looking at everything they had I got the motor.  I would recommend finding a local surplus store in your area if you plan on building anything electronic, they are a great resource.  


In basic terms a rectifier changes Alternating Current, AC, to direct current, DC.  The coils inside the stepper motor are energized as the motor spins causing the current in the coils to alternate.  This is the alternating current.  The 5 volts need for the USB port need to be direct current.  The rectifier, which is just 4 diodes, changes the alternating current from the stepper motor to the direct current needed for the voltage regulator.    
After doing the testing on the BikeGen regulator circuit I realized that the zener diodes I was using were getting very hot.  One of them even failed because it overheated.  I wanted to use a more suitable diode for this project and after checking back to the instructable that inspired this whole project for me,, I decided to go with a 1N4001 diode which is rated at 50V and 1 Amp.  I got the diodes in a variety pack from Radio Shack.

Voltage Regulator

The voltage regulator is a switching voltage regulator as opposed to linear regulator.  At first my searching led me to the LM2575 switching regulator and I was planning on building the circuit myself.  Just search "LM2575" and the data sheet will show up.  On the data sheet the recommend circuit for a 5 volt output is shown with recommendations for all the components.  I continued searching and found the exact circuit I needed from Lightobject,  This seemed to be a better option for me because I didn't want to have to buy all the components in much higher quantities than I needed from a electronics distributor.  

As a last resort I went to the local dollar store because they always seem to have the things I need.  To my complete surprise they had 12 volt car adapters with power for two USB ports.  So I bought two and went home to take them apart.  They had a switching regulator circuit already!  It uses a MC34063 regulator IC and has all the supporting components.  All of this for a dollar, you can do much better than that.

Follow through the notes on the pictures to see how I connected all the parts together. 


Step 4: Build the Generator Mount

To mount the stepper motor to the bike I used a method very similar to the BikeGen instructable.  A small wheel was attached to the stepper motor and the motor was mounted perpendicular to the bike wheel on the rear rack.  The small wheel on the stepper motor rubs against the braking surface of the bike wheel to spin the motor.  A  spring was used to force the stepper motor toward the bike wheel to ensure constant contact between the two wheels.  

I started by cutting two 6 in. pieces of 1-1/4" aluminum angle.  One of these pieces mounts to the rear rack and the other mounts directly to the motor.  Two sections of each piece were cut out to allow for clearance between the motor, rack and wheel.  I reused the home made u-bolts from the BikeGen instructable to mount to the rack.  Two short pieces of the 1/2" aluminum flat bar were bolted to the angle aluminum to hold the screws that retain the spring and the motor.  The small wheel attached to the motor started out as a few parts from Servocity,  They sell very nice shaft connectors and wheels that make mounting anything to a motor very easy.

While I can't offer exact dimensions for any other mounting situation I do believe this to be a fairly universal method of mounting a stepper motor to any bike with a rear rack.  I have considered many other mounting options for my bike it all my ideas seem to work back to this one.  The parts were fairly easy to make and the precision required was not that high.  I made all of the cuts with the hacksaw and drilled all the holes with my battery drill or the drill press.  Simple tape measure and sharpie to mark everything and a center punch to get all the holes right was enough.  

If anyone has any fresh ideas on the mounting of the stepper motor I want to hear them.  I would like something more permanent and rigid, while still maintaining wheel serviceability.    

Step 5: Final Results

Before I did the final assembly of everything I tested the performance of the new stepper motor, diodes and switching regulator.  I used a dead 5V 1000maH battery as a load and measured the current and voltage input and output of the regulator.  I used the drill press again to turn the generator at its 5 different speeds. The results are listed below.

RPM                           620     1100      1720     2340     3100
Vin (volt)                    5.81      6.61       7.15      7.43       7.67
Ain                              0.15      0.26       0.34      0.38      0.40
Vout                            3.70      4.21       4.55      4.78      5.05
Aout (Amp)                0.15      0.28       0.37      0.42      0.44
Input Power (W)       0.87      1.72       2.43      2.82     3.07
Output Power (W)    0.56      1.18       1.68      2.01     2.22
Efficiency                   64%       69%      69%      71%     72%

As you can see the regulator is right at 70% efficient between 1100 and 3100 rpm.  Also it generated enough power for the USB ports even at 620 rpm.  I also increased the size of the wheel mounted to the generator to slow it down compared to the old BikeGen stepper motor. 

All of the work comes to this final point. The USB Bike Generator can power two USB ports as you ride your bike, to charge your electronics.  The electronics of the system are 70% efficient above 12 mph bike speed and can generate adequate power at 7 mph.  I have tested a rechargeable battery, handheld GPS and an Eton radio with the USB Bike Generator and they all work.  I have also tested a Ipod touch 3gen and have found its is possible to charge but because of the variability of the power source and the pickyness of the Ipod its not very reliable.

I welcome all comments, questions, and suggestions. Thanks 
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