The purpose is to have LEDs on the circumference of the clock to show the progress of the time using coloured light.
For that you will need:
- a WS2812B RGB LEDs strip (long enough to go around the clock)
- a development board with a STM32F103 micro-controller equipped with a 32.768 kHz oscillator for the Real Time Clock (such as the [blue pill](https://wiki.cuvoodoo.info/doku.php?id=stm32f1xx#blue_pill)), or using a external [Maxim DS1307](https://www.maximintegrated.com/en/products/digital/real-time-clocks/DS1307.html) RTC module
- a coin cell battery to keep the RTC running (optional)
- a GL5528 photo-resistor to adjust the LED brightness (optional)
- a DCF77 module to set and update the time automatically (salvaged from a radio controlled digital clock)
The brain of this add-on is a [STM32 F1 series micro-controller](http://www.st.com/web/en/catalog/mmc/FM141/SC1169/SS1031) (based on an ARM Cortex-M3 32-bit processor).
To keep track of the time a Real Time Clock (RTC) is used.
If the board includes a 32.768 kHz oscillator (such as a [blue pill](https://wiki.cuvoodoo.info/doku.php?id=stm32f1xx#blue_pill)) the micro-controller will use the internal RTC.
Otherwise connect an external [Maxim DS1307](https://www.maximintegrated.com/en/products/digital/real-time-clocks/DS1307.html) RTC module to the I2C port and set `EXTERNAL_RTC` in `main.c` to `true`.
Also connect the external RTC square wave output in order to have a sub-second time precision.
Connect a DCF77 module (e.g. salvaged from a radio controlled clock) to the micro-controller.
This will allow to automatically get precise time (at least in Europe) when booting.
Since the RTC is drifting, the time will get updated using DCF77 every hour to keep <0.5stimeprecision.
Alternatively set the time using serial over the USB port (providing the CDC ACM profile) or USART port and enter "time HH:MM:SS".
Power the board using an external 5 V power supply (e.g. through the USB port).
This will power the micro-controller, and the LEDs (a single LED consumes more energy than the micro-controller).
To keep the correct time in case the main power supply gets disconnected optionally connect a 3 V coin battery on the VBAT pin for the internal RTC, or in the module for the external RTC.
For the LEDs use a 1 meter LED strip with 60 red-green-blue WS2812B LEDs.
Tape the LED strip along the border/edge of the clock.
Ideally the wall clock has a diameter of 32 cm for a 1 m LED strip to completely fit.
Otherwise change the number of actually used LEDs in the source files.
Connect the 5 V power rail of the LED strip to the 5 V pin of the board.
Connect the DIN signal line of the LED strip to the MISO pin of the micro-controller on PA6.
SPI is used to efficiently shift out the LED colour values to the WS2812B LEDs.
A custom clock is provided for this operation using channel 3 of timer 3 on pin PB0.
Simply connect this clock to the SPI CLK input on pin PA5.
The brightness of the LEDs is dependant on the ambient luminance.
To measure the ambient luminance a GL5528 photo-resistor is used.
Connect one leg of the photo-resistor to ADC channel 1 and the other to ground.
Connect one leg of a 1 kOhm resistor to ADC channel 1 and the other to a 3.3 V pin.
This voltage divider allows to measure the photo-sensor's resistance and determine the luminance.
If you don't want to use this feature, connect PA1 to ground for the highest brightness or Vcc for the lowest brightness.
- USART1_TX; PA9; RX; UART RX; optional, same as over USB ACM
- USART1_RX; PA10; TX; UART TX; optional, same as over USB ACM
- I2C1_SDA; PB7; DS1307 SDA; SDA; optional, when using external RTC
- I2C1_SCL; PB6; DS1307 SCL; SCL; optional, when using external RTC
- TIM2_CH1_ETR; PA0; DS1307 SQ; square wave output; optional, when using external RTC
- ADC12_IN1; PA1; GL5528; photo-resistor + 1 kOhm to 3.3 V; without GL5528 photo-resistor connect to ground for highest brightness or Vcc for lowest brightness
- TIM3_CH3; PB0; PA5; SPI1_SCK; generated clock for WS2812B transmission
- SPI1_MISO; PA6; WS2812B DIN; DIN; WS2812B LED strip data stream
- GPIO; PA2; DCF77 PO; \#EN; DCF77 enable on low
- GPIO; PA3; DCF77 TN; DCF77; DCF77 high bit pulses
There are two firmware images: `bootloader` and `application`.
The `bootloader` image allows to flash the `application` over USB using the DFU protocol.
The `bootloader` is started first and immediately jumps to the `application` if it is valid and the DFU mode is not forced (i.e. by pressing the user button on the board or requesting a DFU detach in the `application`).