Outline

Used hardware

Panel frontside

Panel backside

• The larger 4 pin connector is for 9 volt power supply. All LED’s lit takes about 3.5-4 Amps!
• The smaller 4 pin connector has: “gnd”, “latch/enable”, “clock”, “data”
• The data pushed in via “clock” and “data” has the following parts: _tail[control byte][bit 89]…[bit 0]_head
  • [control byte] has following bits: [1 bit n/u] [4 bit counter value][3 bit line select]
    • [4 bit counter value] is decremented each clock pulse if ‘enable’ is active. Once the counter reaches ‘0’, the line-select will be suppressed. This is used for built-in brightness control
    • [3 bit line select] the bits at this position dictate which of the 7 lines (traversing all 3 text-rows) is/are lit.

Standalone panel

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• The plexiglas plate was taken from a defective LCD monitor

Involved MCU’s

• ESP8266 : NodeMCU 1.0(ESP-12E Module)
  • USB-serial receives bitmap-packages and forwards these to the Wemos using “ESPNOW”
• ESP32 : Wemos D1 mini
  • Receives bitmap packages and displays it on the LED-panel
  • SPI-pins connect to the LED-panel clock- and data-line
  • Separate output pin connects to the LED-panel latch/enable pin

Notes

• The Wemos D1 mini runs the LED Panel controller software.
  • On CPU0 runs a task which pushes out the data via SPI.
  • CPU1 is used by the WIFI/ESPNOW library to receive the display-packages.
  • A display package consists of [1 byte for payload length][1 byte for brightness 0-15][237 bytes of which 90 bits represent the 90x21 pixels picture].
  • Because of maximum SPI clock speed, the hardware brightness logic, and the fact that data-shifting can only be done while the ‘enable’ is off, the display refresh rate lies at about 60hz.
• The NodeMCU runs a simple package forwarder at 115200 kbps. Receiving via Serial/USB and forwarding via ESPNOW. The first byte read is the length of the payload. Then it reads ‘length’-bytes. Once received, it forwards the package via ESPNOW to the dedicated MAC address of the Wemos. “ESPNOW” maximum package size is 250 bytes, so fits nicely.
• Python is used to provide the NodeMCU with frame-packages. The rendered image is 90x31 pixels. 2 bars of 5 pixels high are removed from the image, so that the LED-less areas are removed from the image and the destination image becomes the required 90x21 pixels.
  • One Python program uses Pillow image library to generate the animation with the time centered, and a rotating 3d-cube.
  • One Python program uses OpenCV to read a .mp4 video, to obtain the frames one by one. Each frame is scaled down to 90-pixels-wide, cropped to 90x31, turned into 2 color B&W before sending.