[Design Review]: Charlieplexing LED's using PY32F002 - Page 1
Forum section: electronics / projects

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 09, 2025, 05:00:46 pm
content: Hello all,I am currently working on a cell (CR2032) operated watch project that uses 72 LEDs (60 for seconds and 12 dual color ones for Hours & Minutes). The idea is to operate these via Charlieplexing in the most compact and cost efficient way and also as a personal design challenge for myself as far as Hardware and Firmware is concerned.I have selected the PY32F002AF15P and DS1302S+ as the MCU and RTC for this application since they are pretty cheap on LCSC, and I have also seen some decent reviews for the same.As an additional feature I have also added in a phototransistor (ALS-PT19-315C) for ambient light sensing, that I can probably use for turning on/off the LED's depending on the external light intensity. I have selected 50k loading resistor for this, although its subject to change depending on the light conditions that I want to trigger it at.For LED currenting limiting resistors I have gone for around 91Ohms (180Ohms effective considering 2 port pins). This seems to be good balance considering the Coin voltage will drop from 3V to 2V and the Vf of all 3 leds are in the range of 1.8V to 2.3V. The max worst case current is around 7mA. This value is also subject to change considering the LED brightness and its visibility.There are 2 SPST switches, one for Reset and the other one for setting up the time and other functionalities.The DS1302 will be in a SPI simplex configuration along with the PY32. TBH this is something that I am not fully sure about, hopefully should work fine with the provided SPI drivers.As far as the PCB is concerned, its a 2 layer PCB with most of the traces being around 0.155mm (6.1mils) and Power trace from coin cell around 0.3mm (11.81mils). Current ratings for a temp rise of 5°C seems to be good enough as per Saturn PCB tool. All the passives are 0402 with the exception of phototransistor (0603) and dual led (kind of double 0603 package).Finally, I would appreciate if someone can review the schematic and layout and let me know if I have missed something.Regards,Ashwin619.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 09, 2025, 09:21:16 pm
content: Quote from: Ashwin619 on October 09, 2025, 05:00:46 pmCute design.If you are going to all this trouble, the DS1032 is quite large and rather ordinary and not that cheap.Maybe look at smaller parts, that include the crystal and include DTCXO operation ?These are more common today and will give much better watch performance.eg RX8900CE RX8901CE RV8803 FRTC8900CE all show at lcscAddit : and the RV-3032-C7, press release claims typically ±1.5 ppm 0° to 50°C; equivalent to ±0.13 sec/day

author: Ashwin619
author_rank: Regular Contributor
timestamp: October 10, 2025, 10:21:11 am
content: Cute design.If you are going to all this trouble, the DS1032 is quite large and rather ordinary and not that cheap.Maybe look at smaller parts, that include the crystal and include DTCXO operation ?These are more common today and will give much better watch performance.eg RX8900CE RX8901CE RV8803 FRTC8900CE all show at lcscAddit : and the RV-3032-C7, press release claims typically ±1.5 ppm 0° to 50°C; equivalent to ±0.13 sec/dayHello PCB.Wiz,Thanks for your suggestion.The suggested parts looks really neat. However the cost of one of those (e.g., FRTC8900CE) is close to 10x times of that of DS1302 on LCSC. Also do note that I am planning to hand assemble these (for the 1st batch) in order to keep the cost at bare minimum. The package of these IC's with integrated crystal's seems to be a little bit more challenging, considering that I dont have a hot air gun handy at the moment.There is one more part that looks decent (FRTC8563), is in a MSOP package and supports I2C.Ill probably stick to DS1302 for now, since my layout is complete and changing the pinouts for SPI --> I2C might cause some change.