Signpost

Hello, frens:

On the wall of my home, I have two NeoPixel 32x8 matrices lashed together, and wired up to a microcontroller. Originally this was set up so that I could flash a "ON AIR" sign automatically when I was on a video call as a warning that to local friends and family that I was in full performative mode and also they should wear clothes when walking behind me.

As these things go, the Signpost design has now spiralled outwards into being a full ulisp-driven graphics system.

Its "design" is mostly just me clumsily lashing together the considered and tidier work of hackers like:

• David Johnson-Davies and frens' ulisp lisp for microcontrollers.
• Lady Ada and frens' Adafruit GFX library.
• Michael Miller and frens' NeoPixelBus library.
• Me-no-dev and frens' ESPAsyncWebServer.
• Ayush Sharma and frens' AsyncElegantOTA.

Signpost by default drives two 32x8 NeoPixel displays, one piled on top of the other to make a 32x16 matrix. The lower display is turned upside-down -- the connecting wires were too short otherwise. The mapping for all this is handled by the NeoPixelBus library, amazingly. It is connected to a Lilygogo (TTGogo) T-Display ESP32 board on pin 17.

LED signs over the web

The easiest way to drive the display is to visit http://signpost.local/screen/?text=YOUR+MESSAGE. You could also use curl -d text="YOUR MESSAGE" 'http://signpost.local/screen/', or manually visit http://signpost.local/ in your browser.

That will display the message in red on the NeoPixel. The font is a basic 3x5 character set: just uppercase, lowercase, numbers, "!" and ".". The text does wrap, but not very prettily.

As an example of signpost's use, onair is a shell script for a Linux PC that will constantly check if a local program, such as Zoom or a web browser, is accessing any attached cameras, and if so, will tell a signpost on the local network to remotely display "ON AIR" in cheery LED letters.

Lisp repl over serial or telnet

The ESP32 can also be controlled from the serial port, or by telnetting into port 1958. You'll get a ulisp repl.

There are currently two additions to the standard ulisp library:

• (led-text string) Output a string in cylon red, to the Neopixel display.

• (led-getpixel x y) Return RGB565 integer value of Neopixel at (x,y).

• (with-led () body) By default, ulisps' graphic extensions are output on the T-Display's built-in TFT screen. Within the with-led special form, ulisp GFX extensions instead write to the Neopixel display.

More power than you can safely use

WARNING: If you're like me, you'll only realise after building your 32x16 signpost, that you have invested in a total of 512 neopixels, each one using a possible maximum of 60ma. That's an insane amount of ampage. Even if you found a power supply that could provide the 30 amps for the full output, the wiring of the matrices would probably melt.

Future versions of Signpost will hopefully include a LED brightness quota to ensure you can't go overboard on power consumption. In the meantime, the led-text function's soothing cylon red and skinny 3x5 font is low in power needs. Just be careful with the graphics extensions -- too much power draw will cause things to halt, if not always catch fire.