GorbaDisplay

Project GorbaDisplay
File:Whyunopicture.jpg
Public transport LED display
Status	Initializing
Contact	bertrik, User:Eloy
Last Update	2023-08-02

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.

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	?	Power or ground
2	?	Power or 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	?	Power or ground
14	?	Power or ground

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