Protest Sign

An oversized scanned-matrix display using 10mm LEDs.

Background

When Steven Colbert and Jon Stewart announced the Rally To Restore Sanity and/or Fear, my politically zealous brother wanted to add some flair to the party. We devised a satirical protest sign that would scroll conservative absurdities on one side of the sign and liberal nonsense on the other, reading such garbage from a hand-curated text file.

What it does

  •  Displays arbitrary text/graphics in a variety of ways.

What it’s made of

  • 74AHC595-series 8-bit shift registers
  • PFET transistors (to source current through columns)
  • NFET transistors (to sink current through rows)
  • LM3150 DC/DC buck convertor
  • 10mm LEDs
  • A bunch of support components
  • Custom PCB designed in EAGLE and fabricated at PCBExpress
  • Firmware written in C using MPLABX, compiled by XC16 and loaded using an ICD3

Theory of operation

Send it bunch of bits over SPI a zillion times a second and you get pretty text! The code linked to above just sets the stage – it’s up to you to send the display what you want to see. When I get some extra time (LOL) I’ll hook it up to a WiFi module and use it as a wall-clock.

Conclusion

It works, yay! Lessons learned:

  • Light-bleed into adjacent LEDs is easily fixed with some opaque heat-shrink tubing.
  • Each row has a considerable amount of capacitance that needs to be discharged with each row switch. Write an all-off sequence to the column drivers to do this.
  • Light output is considerably reduced when you scan the display, essentially reducing it to 1/7 (in this case using 7 rows) of the original brightness. You can overdrive the rows by 7x (in this case using 7 rows) to get around this but you must make sure your OE line is de-asserted until you start switching the rows or you’ll pop the LEDs.

 

Seven Segment Watch

Sometimes you just need a wristwatch, and sometimes you come across a bunch of super-sweet retro LED displays pulled from calculators of yore. I mean, what else am I going to do with my time?

Background

Back in late 2009 my roommate was heckling me about never finishing my projects. At the same time, technologically absurd wristwatches were all the rage in the Maker community. Nixie Tubes, high-res OLEDs and discrete LEDs were the thing to have, battery life be damned.

I wanted a slightly different aesthetic and found exactly what I was looking for.

What it does

  • Displays time in a variety of formats

What it’s made of

Theory of operation

Push the “Display” button and the time glows at you for 3 full seconds.

That’s all the tutorial I have for now – you can read through main.c to see what the setting fields mean.

Conclusion

The watch worked well. One thing to note is the LED segments aren’t very bright when used with a coin-cell battery because those ultra long-life batteries can only source about 2mA sustained and 10mA pulsed. Attach a reasonable battery and things will lighten up.