9 Pieces of Pi: Slice 1: The Long Way Round

Introduction

I'm following along with this Design Challenge as an independent participant, not as one of the challengers (that's because I don't agree

with the T&Cs); so I'm ruling myself out of winning any of the prizes, even the finisher prize. I hope that's acceptable to element14

- if not, they can always move the blogs to my personal blog area to get them out of the way.

This is the first in a series of nine, weekly works of electronic art. There won't be time to do anything at all complicated or

elaborate so they'll all be fairly simple and there will probably be quite a lot of LEDs involved. I doubt any of it will be particularly

original from a technical viewpoint, but I'm doing it in my own way so hopefully the resulting art will be interesting or curious. I

need to get my skates on here because some of the actual challengers are already on to their second or third blogs and we are supposed

to blog at least once a week.

This first piece is going to involve LEDs because I'm used to working with them and have all the bits I need to build it.

If you want to see where I started with the Pi 3, have a look at these two blogs:

Getting Started with a Raspberry Pi 3 B+

Using the IO on a Raspberry Pi 3 B+

The Work

Here it is on the wall of my workroom.

The following video shows it working. The 'space' moves to a new, random position and always takes the long way round - hence the

title.

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The Technical Stuff

I decided that I wanted to use canvases for at least some of the pieces. Some of the later slices will also involve paint, though for

some reason the first one has ended up being more spare and conceptual. Conveniently, I've got a few spare canvases that I bought

years ago and never used for anything. Here is what the small one that I've chosen looks like back and front in the packaging (the

wooden pieces in the plastic bag are wedges that allow the canvas to be tensioned). This one is 10" x 10" (25cm x 25cm) which is quite

small but felt about right given the small size of the PI and the diameter of the pixels and the somewhat sparse and minimalist

general feel of the thing.

I initially thought of either having the LEDs projecting light forward onto the back of an acrylic sheet ahead of the canvas or from

the front back onto the canvas but, after a bit of experimenting, discovered that it would work by projecting the LEDs onto the back

of the canvas which would act as a diffuser. The LEDs I used have a narrow viewing angle and produce a nicely defined, not too fuzzy

pixel on the surface. The red light is a bit of a cliche with LEDs, but never mind. Here's a picture of the LED support and the driver

electronics in more detail from when I was testing it on the bench.

The Pi has enough GPIO lines to drive all 16 LEDs directly but I wasn't very sure about powering them from the 3.3V supply generated

on the board so I used a couple of shift register ICs (TPIC6B595) as drivers running on the 5V supply that comes from the power input.

That meant I needed level conversion from the 3.3V GPIO pins, but it's only three lines - clock, data, and latch - so it wasn't too

difficult to do. I used three BSS138 MOSFETs for that.

Here's the circuit diagram. I've only showed one of the level converters, but the other two are the same. They invert, so the code

driving the pins has the levels swapped. The pull-up was a bit on the high side and so the rising edge is a bit slow but the shift

registers don't seem to mind.

Here's a top and bottom view of the circuit board. The MOSFETs are the SOT23 packages [the black blobs] on the righthand side of the

bottom view. The wiring is done with wire-wrap wire.

Here it is wired to the LEDs behind the canvas - not very tidy, but that doesn't matter too much for something like this.

Python seemed to be the quickest way to get going, having previously already followed the simple examples given by Shabaz in his how-

to-use-the-Pi-GPIO blog, so that's what I went for.

Here's the code. I don't think it needs much explanation from me as I should think everyone reading this is better at Python

programming than I am; it's very simple.

import RPi.GPIO as GPIO
import random
# set GPIO directions - all outputs
GPIO.setmode(GPIO.BCM)
GPIO.setup(14,GPIO.OUT) #clock
GPIO.setup(15,GPIO.OUT) #latch
GPIO.setup(18,GPIO.OUT) #data
# outputs are inverted by level convert (True is low)
GPIO.output(14,True)
GPIO.output(15,True)
GPIO.output(18,True)
# start off with something sensible and in range
ledIncrement = 1
currentLed = 2
nextLed = 6
# do forever
while True:
  for x in range(0,16): # do for the 16 LEDs
    if x == currentLed: #set up data
      GPIO.output(18,True)
    else:
      GPIO.output(18,False)
    GPIO.output(14,False) # generate clock
    GPIO.output(14,True)
  GPIO.output(15,False) # after all 16, latch on s/r outputs
  GPIO.output(15,True)
  if currentLed == nextLed:
    time.sleep(3) # at destination, so pause for 3 seconds
    while currentLed == nextLed:
      nextLed = random.randint(0,15)
    if nextLed > currentLed:
      if (nextLed - currentLed) > 8:
        ledIncrement = 1
      else:
        ledIncrement = -1
    else:
      if (currentLed - nextLed) > 8:
        ledIncrement = -1
      else:
        ledIncrement = 1
  else:
  time.sleep(0.5) #not there yet, display for half a second
    currentLed = currentLed + ledIncrement #update position
    if currentLed > 15: # correct for running off either end
      currentLed = 0
    else:
      if currentLed < 0:
        currentLed = 15
GPIO.cleanup()

I set the PI up to run the code from power-up simply by adding this line to cron:

@reboot sudo python /home/pi/slice1.py

Cron can be edited using this command at a command prompt:

crontab -e

The whole thing didn't quite turn out how I imagined it in my head - art rarely seems to - but it will do for the challenge and gets

me off to a reasonably quick start. Originally, I was going to have the Pi on the front, but partway through changed my mind and

tucked it in behind the canvas.

Now to start work on Slice 2. I have rough ideas for all nine now, the next two are planned in detail and underway, so hopefully I can

catch up a bit with the one-a-week schedule.