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SPI Interfaced Digit Boards are Back

A project log for Homebuilt LED Displays

This is an attempt to re-create a long obsolete Hex LED display.

BharbourBharbour 10/13/2018 at 23:140 Comments

Bare boards  for the SPI interfaced version of the displays arrived from OSHPark. Using the solder paste stencil for the top side of the board, assembly went pretty fast. I opted for the stencil on the top side because it has the LEDs and QFN packaged 74HC595 chips. The back side of the board was done by hand, and was a little painful because there are no component designators and the parts are all 0402 packages. Anyway the board went together last night, and it occurred to me that the fastest way to hack a test driver together would be to drive it from my PC using an Aardvark (Total Phase Systems) for the SPI interface. The API for the Aardvark supports SPI, I2C, or GPIO. It can be a little fiddly to get the signal polarities configured correctly, but it is easy to change the configuration. The 1.27mm header connectors were too small to get clips onto directly, so I soldered  30 gauge Wire Wrap wire pigtails on for testing.

Here are a few digits:

Board with 0 digit lit.
Board with 0 digit lit.
Board with 4 digit lit.
Board with 4 digit lit.
Board with 6 digit lit.
Board with 6 digit lit.
Board with F digit and Decimal Point Lit.
Board with F digit and Decimal Point Lit.

One thing that I discovered while testing the driver is that the Master Reset signal on the 74HC595 only resets the shift register and not the output latches, rendering it useless in this application. The Output enable will still be useful for a blanking input though.  

The next step will be to lay out a carrier board to hold several digits and make it easier to connect for testing. It also needs a filter to improve the contrast some to make it easier to read.

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