Spoke POV
Abstract
The objective of this project was to use the schematic provided by SPOKE POV to create a Spoke POV board. A bill of material will be generated from the list of parts needed. Those parts will be modeled in dip trace and will be used to recreate a schematic. A layout will be designed from the schematic and sent out for fabrication. Once assembled, the board will create a persistent image inside the wheel.
Project Description
The goal of this project was to recreate the Spoke POV board designed by Adafruit Industries. The original design can be found at SPOKE POV.
The Spoke POV attaches to the spoke of a bicycle and creates an image in the spoke once the wheels spin fast enough. The image is created because of the “persistence of vision” phenomenon. The board consists has 60 LEDs, 30 on each side, and because of their brightness, they create streaks of light when in motion. When timed correctly, the streaks of light can be manipulated to form images.
The functional diagram is shown in Figure 1.
Figure 1. Functional diagram for Spoke POV
The 60 LEDs are controlled by 8 serial to parallel latches that cycle the power on and off. The serial to parallel latches receives a signal from the microprocessor and the speed of the signal is determined by pulses sensed by the Hall Effect sensor from the magnet. The image displayed is stored on the memory chip and those images can be altered from a computer through the programming header with the use of an “USBTiny”.
This board was designed for use with yellow LEDs, 27 ohm resistor networks and powered with 2 AA batteries.
The microprocessor used is an ATTINY2313V-10PUand comes preprogrammed by Adafruit. If the ATTINY2313V-10PU is purchased from another source, the firmware can be downloaded at SPOKE POV.
The memory used for this project came preprogramed with stock images from Adafruit. To upload your own images to the board, an USBTiny must be used. The USBTiny can be found at SPOKE POV.
Designing the Board
The schematic and layouts were designed with DipTrace. The bill of materials, schematic, and layouts, are attached in the appendix.
While assembling the board, two design flaws were discovered.
Switch
The through holes for the switch leads were designed too small. The switch leads are rectangular and have two different dimensions: .5mm and .8mm. The hole on the board was only sized for a .5mm lead. In order for the switch to fit, the leads had to be filed down. After thinning the leads by .3mm, it was soldered into the board.
LED
For the component patterns, pad 1 is supposed to be designed as a rectangle. For the LEDs, however, all the pad 1’s were ovals while the pad 2’s were squares. Figure 2 shows the incorrect and correct pattern for the LEDs.
Figure 2. Incorrect and correct LED pattern.
Spoke POV Yellow Layout Rev A Top Side and Bottom Side reflects the incorrect LED Pattern. Spoke POV Yellow Layout Rev B Top Side and Bottom Side reflects the correct LED pattern. The schematic remains unchanged.
Without knowing this beforehand, all the LEDs were initially soldered into the board backwards. The LEDs were desoldered and resoldered into the correct position, but it seems that the desoldering processes may have burned the board and caused some of the LEDs to short. As seen in Figure 3, the top image shows that the voltage drop across the LEDs mounted backwards was 2.45V. The middle image shows that when a LED and resoldered in the correct orientation and working, the voltage drop was 1.9V. In the bottom image, you can see that even when the LED was resoldered in the correct orientation, the voltage difference drops to 1.8mV.
Figure 3. Voltage difference for backwards, working, and shorted LED.
Thankfully two boards were fabricated. There were not enough components to make another yellow LED board, but a kit with blue LEDs was available. The schematic for the yellow and blue LED versions are the same. The only differences are three components: the LEDs themselves, the resistor network required, and the battery case. The new bill of materials for the blue version is attached in the appendix.
For the blue LEDs to work, the resistor networks must be changed from 27 ohms to 56 ohms. The original battery pack only holds 2 AA batteries and totals 3V. Because the forward voltage required for the blue LEDs is 3.3V, the battery case must be changed to increase the number of batteries. The replacement battery case was found at RadioShack and holds 4 AAA batteries.
The redesigned layout is attached in the appendix as Spoke POV Blue Layout Rev A: Top Side and Bottom Side. The only change is on the silk screen layer where the “27 ohm” text has been replaced with “56 ohms.”
Conclusion
This project resulted in the successful fabrication of a Spoke POV board designed by Adafruit Industries. Figure 4 shows the top and bottom side of the bare board.
Figure 4. Top and bottom side of bare board.
Figure 5 shows the top and bottom side of the completed board.
Figure 6. Top and bottom completed with LEDS lit up.
Figure 7 shows the some of the preprogrammed images when attached to a bicycle.
Figure 7. Images from Spoke POV (left to right: hazard symbol, double square, “Spoke POV”
Though the board was completed in the end, a major oversight caused excessive rework and rendered a board useless. To avoid such an event from happening again, the designer must pay particular attention to the way he or she designs the component patterns.
Considerations for Future Designs
Battery connection: In this design, the leads from the battery cases were soldered into the board. For improved strength, a recommendation would be to use headers instead. This would make it easier to disconnect the battery for prolonged storage.
Batter case: In this design the battery case was a perfect fit. Be sure the measure the clearance on your bicycle if you plan to use a thicker case as it might interfere with the frame.
Downloads
- Presentation (PDF)
- BOM (PDF)
- Schematics (PDF)
- Layouts (PDF)