GorbaDisplay

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Project GorbaDisplay
File:Whyunopicture.jpg
Public transport LED display
Status Initializing
Contact bertrik, User:Eloy
Last Update 2023-08-04

Introduction

This project is about reverse engineering a former public transport LED display. It consists of Z panels of each X * 16 LEDs. Each panel has 16 amber LEDs vertically.

The plan is to make the display fully addressable as a bitmap display over a network connection, preferably with individually controllable brightness per pixel.

TODO:

  • more reverse engineering by tracing out the connector to the individual lines of the integrated circuits
  • is there a separate power supply line to the board? if so, can we power our new microcontroller from the 14-pin connector?
  • measure clock frequencies (column clock, line update frequency) of original controller board
  • write software to control this thing, e.g. for ESP8266
  • investigate https://github.com/qisun1/ESP8266_LED_64x16_Matrix , seems to be a similar display

Hardware

14-pin header + chips

This thing consists of two main parts:

  • a control board, with logic to receive (for example) text messages over a serial connection and convert them to a bitmap display on the display board
  • a display board, with logic to light up each LED

Between them is a 14-pin connector, probably carrying the low-level LED control signals.

The display board consists of several (4?) panels of LEDs.

It has a light sensor to sense the ambient light level.

Theory of operation

My guess this is probably another row-multiplexed display. The display can light up one row at a time. By quickly lighting up each row in succession, the illusion to a human observer is that all LEDs are controlled simultaneously.

The display board contains the following integrated cicruits:

  • 74HC541, an octal buffer/line driver, probably buffers all signals coming in from the 14-pin connector to the rest of the electronic on the display board
  • 74HC238, a 3-to-8 line decoder/demultiplexer, probably selects which row is currently being lit up
  • group of 4x IRF7425, power MOSFET, probably for driving a row of LEDs with
  • a whole bunch of TPIC6C596, 8-bit shift register, probably drives the columns inside one row of LEDs

The demultiplexer handles only 3 bits, enough for 8 rows. Perhaps there are two of them to handle 16 rows: one for the top 8 rows and one for the bottom 8 rows.

14-pin connector

My guess for the pinout so far is:

Connections
Pin Id Remark
1 GND Ground
2 GND Ground
3 ?
4 ?
5 COL_SEL1 A1 of row multiplexer
6 COL_SEL2 A2 of row multiplexer
7 ?
8 ?
9 ?
10 COL_SEL0 A0 of row multiplexer
11 ?
12 ROW_CLK SRCK of column shift register
13 VCC Power
14 VCC Power


There are possibly pins for:

  • row select, bit 0
  • row select, bit 1
  • row select, bit 2
  • row select, bit 3
  • row-enable
  • shift register data
  • shift-register clear
  • shift-register data
  • analog LDR value
  • remote control input
  • power pin
  • ground pin

Software