I’m working on the first revision of the board now. Over the last few months I’ve slowly learned how to engineer a proper electrical design from scratch. Now I’ve got a design to start with and suppliers picked out for almost every piece. To review the major parts of the board:
The brains. Same kind of chip as used in Arduinos(TM), and hopefully the finished board will be compatible for easy hacking (I have a little more research to do on that front). Besides the chip itself, there are decoupling capacitors, a pull down resistor for one of the special pins, a crystal to run it at certain frequency, and a header to program the board with.
In order to keep proper time I need another chip called a real time clock (RTC). This needs it’s own battery backup, some more capacitors, it’s own crystal for timing, and a current limiting resistor and diode on the battery connection.
The meat of the projects. 12 multi (red, green, blue) color LEDs adding up to 36 total! Plus 3 driver chips to run each set of colors. More resistors and caps to control the current and smooth the power supply to each one.
There is also a USB connector for power and the ability to update the pass estimations from a computer connected to the internet. It needs the physical connector plus required resistors on the data lines.
The Power Supply
The part I struggled with the most. There is a super efficient switching power supply that comes in a chip but needs an inductor and capacitors to match the voltages and switching frequency.
I really wanted something better than AA batteries, and so I added a Lithium polymer charging chip, which was surprisingly not as hard to do as I thought. (I also had help picking one out). That has it’s own set of set resistors and a temperature sensor to make sure it doesn’t overheat the battery during recharge. Then there is the battery sensing circuit described last week with the MOSFETs and the resistors.
And that’s it! I think. Here is the combined bill of materials with part numbers. The only thing missing are some resistor values that I’ll have to figure out empirical once I have the first one built — specifically balancing the brightness of each LED color channel to get the best combination of look and power efficiency.
|X1||ATS080BSM-1||8 MHz Crystal||1|
|X2||ABS10-32.768KHZ-7-T||32.768 kHz Crystal 7 pF||1|
|C14/C15||500R15N220JV4T||22 pF capacitor 50 V||2|
|R3/R4||RMCF0805JT1K00||1 kΩ resistor ⅛ W ±5%||2|
|C16/C6||CC0805ZRY5V8BB105||1 μF capacitor 25 V||2|
|C5/C7–13||08055C104KAT2A||0.1 μF capacitor 50 V||8|
|R1/R2||RMCF0805JT22R0||22 Ω resistor ⅛ W ±5%||2|
|J1||TSW-103-23-T-D||AVR ISP Header||1|
|U2||MCP7940N-I/MS||Low power real time clock||1|
|C3||500R15N100JV4T||10 pF capacitor 50 V||1|
|C4||500R15N120JV4T||12 pF capacitor 50 V||1|
|CON1||S8201-46R||Coin battery holder||1|
|D1/D2||MBR120VLSFT1G||Schottky diode 20 V, 1 A||2|
|C17||500R15N101JV4T||100 pF capacitor 50 V||1|
|B1||CR-1216||12.5 mm, 3 V coin cell battery||1|
|U4||MCP73833-FCI/UN||LiPo charging chip||1|
|R10||RMCF0805JT1M80||1.8 MΩ resistor ⅛ W ±5%||1|
|R11/R9||RMCF0805JT820K||820 kΩ resistor ⅛ W ±5%||2|
|R12–14||RMCF0805JT10K0||10 kΩ resistor ⅛ W ±5%||3|
|T1||CMFB3970103JNT||10 kΩ NTC thermister ±5%||1|
|L1||CB2518T3R3M||3.3 μH inductor 1.2 A||1|
|R8||RMCF0805JT2M20||2.2 MΩ resistor ⅛ W ±5%||1|
|U3||TPS63001DRCR||3.3V buck/boost SPS||1|
|URED||TLC59282DBQR||IC LED driver||1|
|UGREEN||TLC59282DBQR||IC LED driver||1|
|UBLUE||TLC59282DBQR||IC LED driver||1|
|LED1–12||HSMF-C114||Surface mount RGB LED||12|
|SW2||EG1224||Slide power switch||1|
|C1||JMK212BJ106KD-T||10 μF capacitor 6.3 V||2|
|C2||JMK212BJ226MG-T||22 μF capacitor 6.3 V||2|
|CON2||10104110-0001LF||USB micro B||1|