Project Logs

Collapse

• Dimming the array

  Christoph Tack • 06/23/2018 at 19:17 • 0 comments

  The advantage of using the 74HC595 over the MAX7219 and the HT16K33 is that it has an /OE (output enable) pin.  It can easily be used to dim the LEDs from 0 to 100% in as much steps as you like.  Unlike the MAX7219 which is frequently used on LED array modules.  That IC only allows for 4bit dimming control.

  In the video /OE is connected to a 16bit PWM port of the STM32 Nucleo (i.e. pin 6).

• First test

  Christoph Tack • 06/23/2018 at 14:11 • 0 comments

  PCB Assembly

  There's not much to report here.  All parts fit correctly.  I suspect ALLPCB from drilling the holes for the PCB studs a bit too large.

  Pin 1 of the pin headers was not marked in the silkscreen of the PCB.  This will be fixed in the next revision.

  Electronics

  Before connecting power, all connections should be checked.  I felt confident enough to skip this step.  The schematics don't contain any new circuitry to me.

  Unicorn! Revision 0 is fully functional.

  Firmware

  The Adafruit GFX is quite nice.  This will be used to provide high level functionality for drawing lines, character sets etc...  I only need to write a low level driver that can color a pixel at the desired coordinate. 

  It didn't take too long to get this working.

  Next thing is adding dimming functionality.

• Interconnection

  Christoph Tack • 05/10/2018 at 20:02 • 0 comments

  Browsing on LCSC learned me that the cheapest board to board connection is a through hole header/socket connection.

  Pitch

  2.54mm is the most widely used and easy to solder

  Number of contacts

  2x5 & 2&6 pin configuration get you the most bang for the buck when taking into account the cost of the mated connection, i.e. male+female part.  They only cost $0.01 more than the 1x5 or 1x6 configurations.  The benefit of having extra pins is also having extra ground return paths.  This reduces the HF-current loops of the SPI communication.

  PCB Mounting

  Through hole components are cheaper, but require an extra PCB process: wave soldering.

  SMT components are more expensive, but can be reflow soldered with the other SMT parts.  The PCB supports both connector types.

  Mating height

  This might be a bit of a bummer if you really want low profile.  The male header requires 2.5mm height, while the female socket puts 8.5mm on top of that.  This amounts to a 11mm inter-PCB distance.

  Mechanical fixation

  The LED PCB must be supported on both sides to keep it stable.  Adding a few additional header/socket connectors on the corners might do the trick.

  Another option was to use plastic self retaining spacers. Wuerth has these in their product line.  These require a free height of 1.9mm under the LED matrix.

  Instead I opted for SMT studs.  They can be reflow soldered with the rest of the SMT components.  These studs have an internal M3-thread, so they provide for an firm screw connection to the base board.  They're electrically conductive, so can be used to return ground current, a benefit to keep EMI under control.

• Display Driver

  Christoph Tack • 05/10/2018 at 19:34 • 0 comments

  The driver should have SPI or I²C interface and allow dimming.

  SPI

  • MAX7219 : LCSC.com : €2.07 (Digikey >€7)
    • has an internal 8x8bit RAM buffer
  • 74HC595 + 74HC138: 8 outputs, cheap, standard part, PWM-dimming using /OE-pin, Digikey €0.33/pce.  This has the disadvantage that next to SPI, extra control lines for the 74HC138 are required.
  • 2x 74HC595 : One 74HC595 as row controller (only one row should be on at a time).  The other 74HC595 is used as column controller.  Using the same device twice reduces BoM length.
    • no RAM buffer, so each row must be written at least 25 times per second to keep persistence of vision.
  • TLC5925 | STP16CPC26 | CAT4016 | MAX6969: 16 outputs, integrated current source, Digikey €1/pce.  This might be useful for dual color LED matrices.  Here only 8 outputs are needed.

  I²C

  • PCA9685 : LED driver, Digikey €1.80/pce, hard to solder by hand : TSSOP
  • HT16K33 : popular on led matrices, LCSC.com : €0.33, not available on Digikey.

  To avoid that the 74HC595 has to source the current for the LED array, transistors will be used, as switches.  Three transistor types could be used : PNP, NPN and PMOS:

  • PNP would require base transistors.  There's little room for those.
  • PMOS is more expensive than NPN.
  • NPN is used here as a switchable diode.  There's at least 0.7V over the NPN.  In this application it doesn't matter.  It helps for spreading heat over more components, avoiding hotspots.

View all 4 project logs