big_digital_clock

A big digital clock giving new life to an old 7-segment displays from a petrol station.

Description:

A 7-segment-display from a petrol station has been revived. This is an experiment with the language Python. The controller is a RaspberryPi Zero ; pyboard or ESP32 /esp8266-Boards with Micropython will work with some modifications. Time date comes from a GPS module. The timestamp in UTC ist extracted from GPS-sentence. The communication with the GPS-receiver has readable ASCII-format, so is very simple. A 3.3V to 5V level converter is not essential, as the 3.3V CMOS outputs can also drive TTL. The display works with TPIC6B595 shift registers (similar to 74hc595), which has an SPI Bus (Serial Peripheral Interface); this can be controlled with 3 lines: data, clock and chipselect. The outputs of the 595 directly drive the LEDs.

Brightness control

• The brightness may be controlled via PWM via the gate signal (G) of the 595. This is done here with a squarewave signal from an 555-Timer, because the PWM of the Raspi has too much jitter.
• Hardware-pwm of raspi is a good solution . This is done via pigpio library pigpio daemon. Then the pwm works without jitter. The brightness is set from the command line as integer parameter in percent , see below.

Bill of material:

• RaspberyPi
• GPS receiver Modul
• 4 Seven-segment-displays with SPI-Interface
• Power supply 5V for Raspi, 12V for Display ( depends on the display )

Install and Run

The main python script is "grossuhr_gps.py". You need Python 3.x. Put all files in one directory eg. "/home/pi/clock". Start your Python-IDE ( I use Thonny) and select the file "grossuhr_gps.py", optionally set program argument and select "Run". You can program the board online with Thonny via SSH selecting "Remote Python 3 (SSH) " as interpreter.

Autostart

Edit:
sudo nano /etc/rc.local

  ...
  #start pigpio daemon
  sudo pigpiod
  #start clock with 15% brightness
  python3 /home/pi/clock/grossuhr_gps.py 15 &
  exit 0