Custom Bicycle Light - Electrical
The two most important electronic components of the light are the LEDs and the battery. The type and number of LEDs were chosen to limit the current draw while maintaining a high maximum light output. Eight WS2812 LEDs were used in the final design. In order to achieve a longer battery life, a 1800 mAH Li-ion battery was chosen with a flat geometry that fit snug into the enclosure. Outside of these two components, the main light PCB includes an nRF52832 Bluetooth microcontroller, various sensors, and a battery charging and filtering circuit for regulating the voltage of the battery to the nRF52 input voltage of 3.3V.
Through-hole pins were added to the board for three data wires required for programming the microcontroller, the two battery terminals and the two charging terminals.
The electrical portion of the project involved designing a PCB board that managed the Bluetooth microcontroller, programming, power management, battery charging and monitoring, and various sensors. The PCB board was hand assembled and an additional USB breakout board was made in-house for charging.
The board and components were ordered separately and hand-assembled with solder paste and a hot air rework station.
The homemade etchant was a mixture of hydrogen-peroxide, white vinegar, and salt. Before etching, a copper-clad board was masked with spray paint and then a negative image of the traces was engraved with a CO2 laser. This way the traces were still masked with paint and the newly exposed copper could be etched away while leaving the traces. A similar process was done for an example project using a 555 timer circuit on the right. In the etchant solution, the copper bubbled and oxidized and had to be scrubbed off intermittently over the course of 20-30 minutes until all of the copper was etched from the board.
The design of the light PCB was done in Autodesk EAGLE and synced with the mechanical enclosure design in Autodesk Fusion to ensure the assembly of every component.
Keeping every electric component on a single side of the PCB would make large-scale assembly easier. However, a light reflector was needed on the top of the board to direct light from the LEDs out of the enclosure. Therefore, the LEDs and the other components could not be on the same side of the PCB.
The only components on the top of the board are the 8 LEDS, the associated current spike limiting capacitors, and a photo-resistor for measuring the amount of ambient near the light to enable automatic shutoff and dimming functionality.
An additional board was needed to break out the SMT pins on the micro-b USB port used to charge the battery. However, to avoid lead-times the board was made in-house using a homemade copper etchant. This was was possible because the board was quite small, making it not worth the shipping costs, and it only needed a single layer of copper. A premade breakout board was not purchased because it needed to be small enough to fit within the light assembly.
Once soldered and tested, both boards worked with the charging, Bluetooth connectivity, device monitoring, and full brightness light-display capabilities.
