SpokePOV. Bicycle Wheel Persistence of Vision Device. Jared Myren Patrick Hyland

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1 SpokePOV Bicycle Wheel Persistence of Vision Device Jared Myren Patrick Hyland Materials Engineering Department, Cal Poly SLO Spring Quarter 2011

2 Abstract The objective of this project was to create a circuit schematic and printed circuit board layout from a circuit diagram obtained online from Adafruit Industries, fabricate the board, and assemble the components onto the final product. The resulting PCB attaches to a bicycle wheel and uses LEDs and the persistence of vision phenomenon to display a series of images (uploaded by the user) within the wheel. 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. Figure 1 Partially assembled board and solder setup 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 1 Jared Myren, Patrick Hyland

3 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 Figure 2 - USB dongle used to connect SpokePOV to computer Magnet Reference signal for system -indicates wheel Sensor (X1) -Hall effect sensor detects the magnetic field each time the wheel rotates past Microprocessor (U1) -uses timers to send on/off instructions to serial parallel latches -cycles through 4 images every X rotations Memory (U2) -stores 4 image files Serial Parallel Latches (U3-U10) -distribute on/off timing instructions LED arrays (D1-D60) Cycle on/off to display images Programming Header (P1) -connects to a computer through a separate USB dongle to upload / Figure 3 - Functional diagram of SpokePOV system 2 Jared Myren, Patrick Hyland

4 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. 3 Jared Myren, Patrick Hyland

5 Conclusions and Discussion 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. Figure 4 - Kit board successfully displaying an image of a ring of skulls Figure 5 - Board successfully in standby mode, but not able to connect to computer Originally, the board would not power on reliably or run through the startup routine, and we fixed this issue by resoldering shoddy joints and rechecking the orientation of components (the microprocessor was in backwards). We are unsure of the cause of the problem with computer-board communication at the moment, but we suspect the memory chip on the SpokePOV board and the interboard connectivity. 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 4 Jared Myren, Patrick Hyland

6 SpokePOV - Schematic Sheet 1 5 Jared Myren, Patrick Hyland

7 SpokePOV - Schematic Sheet 2 6 Jared Myren, Patrick Hyland

8 SpokePOV - PCB Layout Top Layout Bottom Layout 7 Jared Myren, Patrick Hyland

9 SpokePOV - Bill of Materials (BOM) 8 Jared Myren, Patrick Hyland

10 USB Dongle - Schematic Image from under the Creative Commons License. 9 Jared Myren, Patrick Hyland

11 USB Dongle - Board Printed Circuit Board Assembled Dongle with case Images from under the Creative Commons License. A bill of materials is available at 10 Jared Myren, Patrick Hyland