Step 3: Build the Electronics
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, http://www.electronicsurplus.com/home.cstm. 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, http://www.instructables.com/id/personal-powerPlant/, 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.
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, http://www.lightobject.com/LM2575-High-Input-6V60V-Switching-5V-Power-Module-Regulator-P417.aspx. 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.