Harnessing energy from a bicycle

I enjoy cycling not only because it is one of the most efficient and cheapest forms of transportation, but also it is very satisfying. Yeah hills can be a pain but what goes up must come down, eventually.

When I ride my bike I like to have my phone with me so that I can have a map, a list of directions and also see how fast I am going. Couple of years ago I created an electronic dashboard for cyclists called Smart Turn. It was designed to help cyclists navigate safely through urban roads.

Smart Turn consisted of a mobile phone app and a safety vest with lights. The lights can be activated wirelessly to indicate turn signals and braking. I successfully created a working prototype of Smart Turn. However, it had several issues:

  1. The cell phone battery dies fast
  2. The turn signal lights cannot be seen during the day
  3. The vest is cumbersome to wear, specially in hot weather

The biggest problem it had is with the cell phone. The Smart Turn app uses GPS, Bluetooth and keeps the screen on all the time. This makes the cell phone battery drain in a couple of hours. So to fix this issue I created a cell phone charging system for my bicycle.

The cell phone charging system uses both solar and pedal power to charge the phone. While building this I decided to also include a battery pack, a front lamp, and a rear lamp which could also be powered by the same charging system.

The following is a schematic of the charging system:

Schematic of bicycle charging system

The Dynamo

For the dynamo I used a stepper motor. This Nema17 stepper motor was rated at 12 V and 1.2 A per phase. Generally the higher the amp rating of the motor, the more power it can generate. The first thing I needed to do is to verify that this motor can generate enough power to charge the cell phone and accessories. To do that I had to attach the motor to the bicycle wheel. Thanks to my 3D printer I was able to make a holder to attach the motor to the bicycle. Then I also 3D printed a contact wheel for the motor.

Dynamo motor attached to the bicycle
Dynamo motor attached to the bicycle
Contact wheel of the dynamo
Contact wheel of the dynamo

This stepper motor has two pairs of windings. Each pair of windings generates an alternating current when the motor is rotated. To convert the AC to DC, I used a multi-phase rectifier bridge as shown in the schematic.

During initial testing I found that the motor can generate up to 70 V when hand cranking the pedal. So it was obvious that I needed a regulator before connecting it to a cell phone. As shown in the schematic I made a simple regulator using a 5.6 V Zener diode and a TP129 transistor. The transistor needed to have a heat sink since it will be generating a fair amount of heat. I put the regulator and the mode select switch in a control box under the back of the seat. The six position mode switch selects which power source is connected to which device.

Six position mode switch for selecting which power source is connected to which device
Six position mode switch for selecting which power source is connected to which device

Solar Panel

The Solar Panel I found at a local electronics store is rated to produce 200 mA at 5 V in direct sunlight. The only thing I had to do for this is to design a holder to attach it to the bicycle.

Solar panel attached to the front handle bar
Solar panel attached to the front handle bar

The Battery Pack

The battery pack consists of six Ni-Cd batteries arranged in two rows with three batteries per row connected in series. These batteries are salvaged from old solar garden lights. Ni-Cd batteries are safer and easier to charge compared to Li-Ion batteries. Also a special charging circuit is not required for Ni-Cd batteries as long as the charge current is limited.  The two rows of batteries power the front and rear lamps when neither the solar power or pedal power is available. The battery pack can be recharged by either the solar panel or the dynamo. I made a case for the battery pack using the 3D printer and sealed the case using a glue gun.

Rechargeable Ni-Cd battery pack made using solar garden light batteries
Rechargeable Ni-Cd battery pack made using solar garden light batteries

Front & Rear Lights

Both the front and the rear lights consist of two LEDs that blink alternatively. The blinking circuit is simply an astable multivibrator made with a couple of transistors. For the rear light I designed and printed a case from  scratch. But the front light is hacked from a purse light. The lights can be power from either the battery pack, solar panel or the dynamo. Although powering the lights with the solar panel is somewhat useless since there is no need for lights when it is sunny.

Front and rear lights
Front and rear lights

This charging system will significantly extend the battery life of the cell phone and uses energy that is freely available. The next step is to make turn signal indicators that are visible during the day. Unlike the front and rear lights the turn signals will be attached to the rider instead, making them more effective. More on wearable turn signal indicators will be covered in the next post.

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