LichtKrant

Project LichtKrant
350px
Some 3-colour 80x7 pixel moving LED sign
Status	Initializing
Contact	bertrik
Last Update	2019-12-15

Introduction

Found this lichtkrant on the revspace free stuff table.

It is 80 pixels wide, 7 pixels high. Each pixel has two sub-pixels (red and green), so it should be possible to display at least 3 colours (red, orange, green)

Goals:

• investigate if we can drive this with an ESP8266
• investigate whether we can apply some kind of PWM to increase colour depth
• document how it works
• write software for this, so other people can do the same for their display

Hardware

lichtkrantconnector

It consists of a processor board and a LED board.

Processor board

The processor board has a 8052, a flash chip, battery, 32768 Hz crystal, 40 MHz crystal, Altera Max chip. It appears the Altera chip drives two 245 chips which in turn drive the signal to the connector J1 to the LED board.

It takes 5V as input.

The PCB has a text on it: HXITC017_2.PCB

Connector pinout J1

*  J1: pin IC5.A8 -> IC2.A1 -> IC2.B1 -> via ...
                  -> IC1.B1 -> IC1.A1 -> U11.SRCLK
                  -> IC1.B6 -> IC1.A6 -> U1.SRCLK
*  J2: GND?
*  J3: pin IC6.A1 -> IC2.A2 -> IC2.B2 -> via ...
*                    IC1.B2 -> IC1.A2 -> U11.RCLK
*                    IC1.B5 -> IC1.A5 -> U1.RCLK
*  J4: GND?
*  J5: pin IC6.A2 -> IC1.B3 -> IC1.A3 -> U11.SER
*  J6: GND?
*  J7: pin IC6.A3 -> IC1.B4 -> IC1.A4 -> U1.SER
*  J8: GND?
*  J9: pin IC6.A4 -> IC2.A7 -> IC2.B7 -> via ...
                  -> IC4.1A -> IC4.1Y -> IC3.G2B
* J10: VCC?
* J11: pin IC6.A5 -> IC2.A3 (row) -> IC3.A
* J12: VCC?
* J13: pin IC6.A6 -> IC2.A4 (row) -> IC3.B
* J14: VCC?
* J15: pin IC6.A7 -> IC2.A5 (row) -> IC3.C
* J16: ???
* J17: pin IC6.A8 -> IC2.A6 -> IC2.B6 -> IC3.G2A
* J18: ???
* J19: ???
* J20: connected to the "ethernet" dipswitch

IC2.A7 <- IC4.1A <- via
          IC4.1A -> IC4.1Y -> IC3.G2B

• IC1 = 74HC245 (oct buf)
• IC2 = 74HC245 (oct buf)
• IC3 = 74HC138 (3-8 mux)
• IC4 = 74HC04 (hex inverter)
• IC5 = 74HC245 (oct buf) on CPU board
• IC6 = 74HC245 (oct buf) on CPU board
• U1 = lower chain of 595
• U11 = upper chain of 595

There are 8 rows (3 bits). Two chains of 10x595 (each) for the columns, U11-U10 and U11-U20.

In summary.

• rows
  • 3-bits row select
• columns
  • 1 bit shift clock shared for both shift registers
  • 1 bit shift register latch bit shared for both shift registers
  • 1 data bit for 80-bit shift register 1
  • 1 data bit for 80-bit shift register 2

Levels on the bus are 5V (so can we drive it with an ESP?)

LED board

The LED board contains the actual pixels, organized in 16 modules of 5x7 LEDs. Each LED has a red sub-pixel and a green sub-pixel, so it can do colors: red, green, orang The LED modules have the text GYXM-G2357ASRG on them.

The PCB has a text on it: HXBI80070509.PCN, date 2007-12-28

The are already electronics on it, consisting mostly of 595 chips (serial shift registers).

There is an '138 chip (3-to-8 decoder) that drives a set of MOSFETS, 7 in total, so probably the row select.

The MOSFETs are SDM4953, dual P-channel MOSFETs.

Software

The plan is to drive the display using an ESP32, with one core dedicated to the display and the other free to do other stuff (e.g. receiving messages over the network).

Brightness is done using a kind of PWM:

• For each pixel we keep a running brightness count (e.g. 8-bit).
• For each frame we add to it the brightness value from the frame buffer (e.g. 0-256). If the addition results in an overflow, the pixel is lit on the display. If no overflow occurs, the pixel is dimmed on the display.
• Each pixel on the display starts with a random count, so we avoid synchronised flashing of the pixels on the display. In other words, pixels with the same brightness has the same duty cycle, but have random turn-of/turn-off times with respect to each other.

Communication between the cores consists purely of the state of the frame buffer. The main core writes the frame buffer, the real-time core reads the frame buffer.

Links

Possibly useful other info:

• https://medium.com/@tharindu_peiris/p10-led-display-panel-using-esp8266-wemos-d1-mini-nodemcu-1e463ad60722