LED Spoke POV Kit

Project Description

This kit is basically an array of LEDs that attach to a bicycle wheel.  As the wheel spins, the LEDs cycle on and off in a specific timing pattern, which causes a circular image to appear to the human eye if the wheel is spinning above a certain speed.  A separate connector is required to change or download the images stored on the board’s memory.  All components can be soldered by hand with the tools shown in Figure 1.

Circuit Schematic

A circuit schematic, images of the pre-assembled and assembled board, and a link to a BOM for the connector (USB Dongle) are included in the appendices. 

A rare earth magnet is attached to the frame of the bike.  The magnet is aligned with a Hall Effect sensor (X1) on the end of the board.  With each rotation of the wheel the sensor moves through the magnetic field. When the board is switched on and rotates past the magnet, a signal from the sensor is sent to the microprocessor (U1) which starts a timing cycle. This timing cycle is dependent on the speed of rotation (calculated by the microprocessor) and the parameters set by the user in the software.  In other words, the microprocessor calculates the pattern and speed at which the LEDs (D1-D60) must cycle on and off for the image(s) to appear to persist to the human eye. The microprocessor reads from four banks of memory on the memory chip (U2) and sends out on/off instructions to the serial parallel latches (U3-U10). The power signal is stabilized and regulated by the capacitors (C1, C2). The latches act as distribution gates that regulate and feed power to the LEDs. The four images are cycled through after a number of rotations set in the software.  These images are uploaded to the memory module by an USB dongle that we also assembled Figure 2. 

Functional Diagram

Printable Diagram (PDF)

Persistence of Vision (POV)

POV is an optical phenomenon by which moving light sources cycling on and off in rapid sequence trick the human eye into seeing a ‘persistent’ image.

A bicycle must travel at a minimum speed of 15 miles an hour with one SpokePOV device for the POV effect to fully function. If two devices are used, the minimum is 10 miles an hour and if three are used, the minimum is 7 miles an hour. If the bicycle speeds above the minimum the effect does not change or improve.

Conclusion

We successfully created a circuit schematic and layout for a two-sided PCB based on the Adafruit SpokePOV designs.  We assembled our board and components and verified that the startup routine functions correctly, which indicates that the circuit design and assembly were done correctly.

 

The completed board powers up when two AA batteries are inserted and the switch button is depressed, and the start-up routine (LEDs light up and down the board once and then two status LEDs remain lit) runs correctly. However, we were not able establish a connection through the dongle to the SpokePOV software to modify the images stored on the memory chip.  We tested the dongle with a separate completed kit board and verified that it could establish a connection and transfer data. Thus far we have been unable to transfer images to our project board’s memory. The kit board operates smoothly as seen in Figure 4.

To improve the design, future work should:

• add mounting holes to the board layout to facilitate ease of attachment to bicycle spokes
• purchase preprogrammed microcontrollers from the Adafruit website
• pay extreme attention to detail during design and assembly

Downloads

• BOM (JPEG)
• Project Report (PDF)