LDM8E-LED Display Controller

Controller to drive LDM1 or LDM2 modules.

Overview

The LDM8E is designed to drive a string of up to four LDM1 or LDM2 display modules.
It allows individual segment addressing and whole display dimming.
Do not mix different display module types.

Board overview

Dimming

The controller supports three different dimming modes.
Analog dimming, achieved by changing the LED driving voltage,
Split ground PWM using a separate LED ground and
Chip Enable PWM using the shift register enable pins.
Different modes need different components populated.

Analog dimming

When analog dimming, the output voltage of the driving voltage regulator (U2) is adjusted by Omitting the feedback network and driving the regulator adjust pin from MCU DAC1 through an op amp (U3).
Adding R11 connects the LED ground to the supply ground and turns them on constantly, allowing analog dimming.
Populate: R3, R4, R5, R6, R11, C10, C11, U3
Omit: Q1, R13, R14, R26, R27

PWM dimming

Including the feedback network R13, R14 fixes the driving voltage regulator (U2) output voltage to 12.7V.
The brightness is then controlled using PWM by switching the LEDs on and off rapidly.

Split ground PWM dimming

LDM1 modules use a separate LED ground that is switched using Q1 to achieve dimming.
Populate: Q1, R13, R14, R26, R27
Omit: R3, R4, R5, R6, R11, C10, C11, U3, C9

Chip enable PWM dimming

LDM2 modules use the shift register output enable pin to switch the LEDs.
Populate: R11, R13, R14
Omit: R3, R4, R5, R6, R26, R27, C10, C11, Q1, U3, C9

Selecting module logic voltage

The LDM8E has configurable level shifters on the LED logic pins.
The voltage can be set using either R9 or R10. Populating R9 sets it to 5V, R10 to 3.3V.
LDM1 modules work with both, LDM2 modules need 5V logic voltages.

Connectors/Headers

• Power input (CON1): +15V power supply, tip positive, min. 2A
• LED connector (J7): Connect a string of LDM1 or LDM2 modules here.
  The following connectors are strictly 3.3V only.
• UART (J1): Connects to MCU UART pins.
• JTAG (J2): Connects to MCU JTAG pins.
• SPI (J3): Connects to MCU SPI pins.
• I2C (J4): Connects to MCU I2C pins.
• GPIO (J5 and J6): Breakout of MCU GPIO pins and PSU voltages (see schematic for pinout).
  The 3.3V supply is generated using the MCU's onboard regulator. Do not load unnecessarily.

LED Connector pinout

• LED Drive: LED supply voltage.
  The value depends on the selected dimming mode.
• Digital VCC: digital supply voltage. Set using R9 or R10 (R9 = 5V, R10 = 3.3V).
• Data OUT: Serial data sent to modules.
• Register CLK: Module output register clock.
• Serial CLK: Serial data clock.
• AGND: LED ground (connected to GND using Q1 or R11).
  AGND is tied to GND internally on LDM2 modules.
• GND: Digital ground.
• Register CLR: Clear module shift registers (active low).
• Output ENA: Enable module LEDs (unused on LDM1 modules) (active low).

Microcontroller board selection

Any ESP32 board that is pin compatible with the Node MCU should work.
I have only tested a "AZ-Delivery ESP32 NodeMCU" board based on the ESP32-S and CH340.

Arduino library

An Arduino library for easy use of the LDM8E can be found here.

Considerations

Do not plug into the MCU's USB port while it is installed in the LDM8E; high current will be drawn from the USB 5V rail.
With 4 modules using analog dimming, the drive voltage regulator (U2) can dissipate up to 2W!
Ensure adequate cooling of both linear regulators (U1 and U2)!

Other

Published under CERN-OHL-S license.