Tag: python

The glorious futility of generating NAPLPS in 2023

Yeah! Actual real NAPLPS made by me!

NAPLPS — an almost-forgotten videotex vector graphics format with a regrettable pronunciation (/nap-lips/, no really) — was really hard to create. Back in the early days when it was a worthwhile Canadian initiative called Telidon (see Inter/Access’s exhibit Remember Tomorrow: A Telidon Story) it required a custom video workstation costing $$$$$$. It got cheaper by the time the 1990s rolled round, but it was never easy and so interest waned.

I don’t claim what I made is particularly interesting:

a lo-res red maple leaf in the bottom left corner of a black screen
suspiciously canadian

but even decoding the tutorial and standards material was hard. NAPLPS made heavy use of bitfields interleaved and packed into 7 and 8-bit characters. It was kind of a clever idea (lower resolution data could be packed into fewer bytes) but the implementation is quite unpleasant.

A few of the references/tools/resources I relied on:

Here’s the fragment of code I wrote to generate the NAPLPS:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# draw a disappointing maple leaf in NAPLPS - scruss, 2023-09

# stylized maple leaf polygon, quite similar to
# the coordinates used in the Canadian flag ...
maple = [
    [62, 2],
    [62, 35],
    [94, 31],
    [91, 41],
    [122, 66],
    [113, 70],
    [119, 90],
    [100, 86],
    [97, 96],
    [77, 74],
    [85, 114],
    [73, 108],
    [62, 130],
    [51, 108],
    [39, 114],
    [47, 74],
    [27, 96],
    [24, 86],
    [5, 90],
    [11, 70],
    [2, 66],
    [33, 41],
    [30, 31],
    [62, 35],
]


def colour(r, g, b):
    # r, g and b are limited to the range 0-3
    return chr(0o74) + chr(
        64
        + ((g & 2) << 4)
        + ((r & 2) << 3)
        + ((b & 2) << 2)
        + ((g & 1) << 2)
        + ((r & 1) << 1)
        + (b & 1)
    )


def coord(x, y):
    # if you stick with 256 x 192 integer coordinates this should be okay
    xsign = 0
    ysign = 0
    if x < 0:
        xsign = 1
        x = x * -1
        x = ((x ^ 255) + 1) & 255
    if y < 0:
        ysign = 1
        y = y * -1
        y = ((y ^ 255) + 1) & 255
    return (
        chr(
            64
            + (xsign << 5)
            + ((x & 0xC0) >> 3)
            + (ysign << 2)
            + ((y & 0xC0) >> 6)
        )
        + chr(64 + ((x & 0x38)) + ((y & 0x38) >> 3))
        + chr(64 + ((x & 7) << 3) + (y & 7))
    )


f = open("maple.nap", "w")
f.write(chr(0x18) + chr(0x1B))  # preamble

f.write(chr(0o16))  # SO: into graphics mode

f.write(colour(0, 0, 0))  # black
f.write(chr(0o40) + chr(0o120))  # clear screen to current colour

f.write(colour(3, 0, 0))  # red

# *** STALK ***
f.write(
    chr(0o44) + coord(maple[0][0], maple[0][1])
)  # point set absolute
f.write(
    chr(0o51)
    + coord(maple[1][0] - maple[0][0], maple[1][1] - maple[0][1])
)  # line relative

# *** LEAF ***
f.write(
    chr(0o67) + coord(maple[1][0], maple[1][1])
)  # set polygon filled
# append all the relative leaf vertices
for i in range(2, len(maple)):
    f.write(
        coord(
            maple[i][0] - maple[i - 1][0], maple[i][1] - maple[i - 1][1]
        )
    )

f.write(chr(0x0F) + chr(0x1A))  # postamble
f.close()

There are a couple of perhaps useful routines in there:

  1. colour(r, g, b) spits out the code for two bits per component RGB. Inputs are limited to the range 0–3 without error checking
  2. coord(x, y) converts integer coordinates to a NAPLPS output stream. Best limited to a 256 × 192 size. Will also work with positive/negative relative coordinates.

Here’s the generated file:

maple_nap.zipDownload

Speech from Python with the SYN6988 module

I’ve had one of these cheap(ish – $15) sound modules from AliExpress for a while. I hadn’t managed to get much out of it before, but I poked about at it a little more and found I was trying to drive the wrong chip. Aha! Makes all the difference.

So here’s a short narration from my favourite Richard Brautigan poem, read by the SYN6988.

Sensitive listener alert! There is a static click midway through. I edited out the clipped part, but it’s still a little jarring. It would always do this at the same point in playback, for some reason.

The only Pythonish code I could find for these chips was meant for the older SYN6288 and MicroPython (syn6288.py). I have no idea what I’m doing, but with some trivial modification, it makes sound.

I used the simple serial UART connection: RX -> TX, TX -> RX, 3V3 to 3V3 and GND to GND. My board is hard-coded to run at 9600 baud. I used the USB serial adapter that came with the board.

Here’s the code that read that text:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import serial
import time

# NB via MicroPython and old too! Also for a SYN6288, which I don't have
# nabbed from https://github.com/TPYBoard/TPYBoard_lib/

def sendspeak(port, data):
    eec = 0
    buf = [0xFD, 0x00, 0, 0x01, 0x01]
    buf[2] = len(data) + 3
    buf += list(bytearray(data, encoding='utf-8'))
    for i in range(len(buf)):
        eec ^= int(buf[i])
    buf.append(eec)
    port.write(bytearray(buf))

ser = serial.Serial("/dev/ttyUSB1", 9600)
sendspeak(ser, "[t5]I like to think [p100](it [t7]has[t5] to be!)[p100] of a cybernetic ecology [p100]where we are free of our labors and joined back to nature, [p100]returned to our mammal brothers and sisters, [p100]and all watched over by machines of loving grace")
time.sleep(8)
ser.close()

This code is bad. All I did was prod stuff until it stopped not working. Since all I have to work from includes a datasheet in Chinese (from here: ??????-SYN6988???TTS????) there’s lots of stuff I could do better. I used the tone and pause tags to give the reading a little more life, but it’s still a bit flat. For $15, though, a board that makes a fair stab at reading English is not bad at all. We can’t all afford vintage DECtalk hardware.

The one thing I didn’t do is used the SYN6988’s Busy/Ready line to see if it was still busy reading. That means I could send it text as soon as it was ready, rather than pausing for 8 seconds after the speech. This refinement will come later, most likely when I port this to MicroPython.

More resources:

Raspblocks: Blocks-based Python coding for Raspberry Pi

Update, 2019-01: raspblocks.com appears to be dead, with an “Account Suspended” error from the host

Raspblocks is a new Blocks-based web programming environment for Raspberry Pi. You don’t even need to write the code on a Raspberry Pi, but the Python 3 code it produces will need to be transferred to a Raspberry Pi to run.

For maximum authenticity (and slowness), I fired up  http://www.raspblocks.com/ on a Raspberry Pi Zero over VNC. It took a minute or more to load up the site in Chromium, but creating a simple program was all easy dragging and dropping:

The code it produced was pretty much exactly what you’d write by hand:

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
GPIO.setup(26, GPIO.OUT)

while True:
    GPIO.output(26,True)
    time.sleep(1)
    GPIO.output(26,False)
    time.sleep(1)



And, as you might expect, the code make an LED connected to GPIO 26 turn on and off. Science!


Raspblocks isn’t as polished as its more established rival  EduBlocks, but Raspblocks doesn’t need any software installed. Edublocks installs its own Node.js-based web service, which would be painfully slow on a Raspberry Pi Zero. Raspblocks’ code needs to be run manually from a terminal, but I’d put up with that any day over having yet another Node server distribution installed under /opt.

gpiozero is rather good

gpiozero (‘A simple interface to GPIO devices with Raspberry Pi’) continues to impress me. One of its newer features is a pinout guide, accessed by the pinout command:

Raspberry Pi Zero pinout – click through for PDF

I’m trying to resist running it on every generation of Raspberry Pi that I have (B, A, 2B, 3B, Zero, Zero W) just for these pretty displays.

(ANSI console colours captured using script, then fed through ansi2html [from the Ubuntu colorized-logs package], printed to PDF from Firefox then mucked about a bit with in Inkscape)

maximal annoyance with the BBC micro:bit and MicroPython

I just picked up a micro:bit, the little educational microprocessor board originally from the BBC. It’s a nice little unit, though like all educational resources, it’s sometimes hard to access resources as a non-edu type.

I landed upon MicroPython, a Python language subset that runs directly on the micro:bit’s ARM chip. I rather like the Mu editor:
To give the old microcontroller grumps something real to complain about, MicroPython includes a bunch of very high-level functions, such as a powerful music and sound module. Getting the sound out is easy: just croc-clip a speaker onto the output pads:

(MicroPython warns against using a piezo buzzer as a speaker, but mine worked fine — loudly and supremely annoyingly — with a large piezo element. Some piezos have a fixed-frequency oscillator attached, but this simple one was great.)

This trivial example plays the Nyan Cat theme forever, but every time it loops it gets faster. The beats variable starts at the default 120 bpm, but is increased by one every time:

# nyan but it gets faster
import music
beats = 120
while True:
    music.set_tempo(bpm=beats)
    music.play(music.NYAN)
    beats = beats + 1

This starts out as merely irritating, but quite quickly becomes deeply annoying, and in mere hours become vastly vexing. I’m sure you’d only use this power for good …

VM-CLAP1 👏 sensor + gpiozero on Raspberry Pi

Well, that was easy!

Since the Verbal Machines VM-CLAP1 sensor is an open collector type — that is, it sinks current when triggered — it behaves like a simple button to gpiozero, the Raspberry Pi Python GPIO library. If you attach a callback function to the sensor’s when_pressed event, your Python script will call that function every time it registers a clap.

The wiring is as simple as it could be:

 VM-CLAP1: Raspberry Pi:
 ========= =============
      GND → GND
      PWR → 3V3
      OUT → GPIO 4

This example code just prints clap! when the board picks up a 👏:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

# Raspberry Pi gpiozero test for
# Verbal Machines VM-CLAP1 clap sensor
#   scruss - 2017-06
#
# Wiring:
#
#  VM-CLAP1:    Raspberry Pi:
#  =========    =============
#    GND     →   GND
#    PWR     →   3V3
#    OUT     →   GPIO 4

from gpiozero import Button
from signal import pause

def clapping():
        print("clap!")

clap = Button(4)
clap.when_pressed = clapping
pause()

This is a trivial example, but at least it shows that anything you can do with a button, you can also do with this hand-clap sensor.

all your centroids

As Side Door sign v3 seemed to have fallen off, I needed to make a new one. With access to a laser cutter, I can make really permanent things now, so I designed this:

side_door-cut-lcYes, that’s a pointy thing filled with pointy things (all without thumbs, you’ll notice) and labelled with Cooper Black. Irony, much? Fe!

In order to get the sign to hang correctly, I needed to work out the centroid of the pointy outline. thedatachef/inkscape-centroid: Centroids for Inkscape paths and shapes to the rescue! Well, kinda. First off, the installer had a bug that said a Ruby file was a dependency when the plugin was in Python. So I forked the repo, made the change, tested it, and issued a pull request. So yay, working centroid calculations in Inkscape!

Secondly, the plugin only works well for simple shapes, like these:

Screenshot from 2016-07-13 17-15-54But compound shapes? Not so well:

Screenshot from 2016-07-13 17-42-31I guess it doesn’t like the negative moments generated by the holes, and does its own thing. Oh well.

Pen plotters: not just output devices …

Pen plotters were pretty expensive and complex pieces of electromechanical equipment. While they often earned their keep in the CAD office, they also had a function that’s almost forgotten: they could be used as input devices, too.

As a kid, we sometimes used to drive past the office of Ferranti-Cetec in Edinburgh. They specialized in digitizers: great big desk or wall mounted devices for capturing points from maps and drawings. Here’s one of their 1973 models:

Ferranti EP210 Freescan Digitiser. Source: Grace's Guide, http://www.gracesguide.co.uk/File:Im1973IME-Ferranti.jpg
Ferranti EP210 Freescan Digitiser. Source: Grace’s Guide, http://www.gracesguide.co.uk/File:Im1973IME-Ferranti.jpg

While the technology and size have changed a bit, these huge bits of engineering kit are the ancestors of today’s track pads and touch screens.

Realizing that their plotters had very precise X-Y indexing and that they had two-way communications to a computer, HP made a drafting sight that fitted in place of a pen on their plotters:

HP drafting sight, part no 09872-60066
HP drafting sight, part no 09872-60066

This is a very pleasing piece of kit, all metal, thick plastic and polished optical glass. They show up on eBay occasionally, and aren’t cheap. With a bit of coercion, it fits into my HP plotter like this:

Drafting sight in HP7470A plotter
Drafting sight in HP7470A plotter

The image is very bright and clear:

Drafting sight near an axis label
Drafting sight near an axis label
Drafting sight over a point
Drafting sight over a point, showing cursor dot

If one has a digitizing sight, one needs to find something to digitize post haste … I’m sure everyone can sense the urgency in that. So I found this, a scan from my undergraduate project writeup (centrifugal pump impeller design ftw, or something), which was probably made on an Amiga or Atari ST:

It's a graph, with pointy bits on it
It’s a graph, with pointy bits on it

I printed this as large as I could on Letter paper, as it’s the only size my HP7470A plotter can take. Now all it needed was a small matter of programming to get the data from the plotter. Here’s a minimally-useful digitizer for HP and compatible serial plotters. Although I ran it on my little HP grit wheel plotter attached to a Raspberry Pi, I developed it with my larger Roland plotter. The only fancy module it needs is pySerial.

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# a really crap HP-GL point digitizer
#  scruss - 2016

from time import sleep
from string import strip
import serial

ser = serial.Serial(port='/dev/ttyUSB1', baudrate=9600, timeout=0.5)
lbl = ''
points = []
labels = []
k = 0
retval = 0

ser.write('DP;')                # put in digitizing mode
while lbl != 'quit':
    ser.write('OS;')
    ret = strip(ser.read(size=5), chr(13))
    print ('Retval: ', ret)
    if ret != '':
        retval = int(ret)
    if retval & 4:              # bit 2 is set; we have a point!
        print ('Have Point! Retval: ', retval)
        retval = 0
        ser.write('OD;')
        pt = strip(ser.read(size=20), chr(13))
        print ('OD point: ', pt)
        lbl = raw_input('Input label [quit to end]: ')
        points.append(pt)
        labels.append(lbl)
        k = k + 1
        ser.write('DP;')        # put in digitizing mode again
    sleep(1)
ser.close()

f = open('digit.dat', 'w')
for i in range(k):
    f.write(points[i])
    f.write(',')
    f.write(labels[i])
    f.write('\n')
f.close()

In the unlikely event that anyone actually uses this, they’ll need to change the serial port details near the top of the program.

The program works like this:

  1. Move the drafting sight to the point you want to capture using the plotter’s cursor keys, and hit the plotter’s ENTER key
  2. Your computer will prompt you for a label. This can be anything except quit, that ends the program
  3. When you have digitized all the points you want and entered quit as the last label, the program writes the points to the file digit.dat

I didn’t implement any flow control or other buffer management, so it can crash in a variety of hilarious ways. I did manage to get it to work on the lower trace of that graph, and got these data:

9649,2428,1,300,0
357,2428,1,0,0
357,7217,1,0,0.60
733,3112,1,first
826,3167,1,
968,3256,1,
1122,3334,1,
1290,3405,1,
1588,3583,1,
1891,3725,1,
2215,3880,1,
2526,4051,1,
2830,4194,1,
3143,4280,1,
3455,4516,1,
4077,4767,1,
5008,5229,1,
6543,5954,1,
8067,6548,1,
8740,7195,1,
8740,7195,1,last
8740,7195,1,quit

The first two columns are X and Y, in HP-GL units — that’s 1/40 mm, or 1/1016 inches. The third column will always be 1 if you have the sight down. The last columns are the label; if you put commas in them, opening the file as CSV will split the label into columns. I used it to fudge axis points. You’ll also note that the last three lines of data are my valiant attempts to quit the program …

Assuming the axes are not skewed (they are, very slightly, but shhh) some simple linear interpolation gives you the results below:

 12.1    0.086
 15.1    0.093
 19.7    0.104
 24.7    0.114
 30.1    0.122
 39.7    0.145
 49.5    0.162
 60.0    0.182
 70.0    0.203
 79.8    0.221
 89.9    0.232
100.0    0.262
120.1    0.293
150.2    0.351
199.7    0.442
248.9    0.516
270.7    0.597

Good enough for a demo.

(For prettier things to do with plotter digitizing commands, Ed Nisley KE4ZNU has made some rather lovely Superformula patterns)

If you don’t have a plotter, or even if you do and you don’t have hours to waste mucking about with Python, obsolete optics and serial connections, Ankit Rohatgi’s excellent WebPlotDigitizer (or Engauge, as I found out when this article hit HackerNews in 2021) gets numbers out of graphs quickly. It handles all sorts of graphs rather well.

q absolves data sins and makes CSV queries easy

The cryptically-named q (it also bills itself as being able to “Run SQL directly on CSV files | Text as Data”) is very nifty indeed. It allows you to run SQL queries on delimited text files. It seems to support the full SQLite SQL dialect, too.

I used to frequently query the IESO‘s Hourly Wind Generator Output report (which now hides behind a JS link to obscure the source URL, http://www.ieso.ca//imoweb/pubs/marketReports/download/HourlyWindFarmGen_20160122.csv).  Now that the file has nearly 10 years of hourly data and many (but not all) wind projects, it may have outlived its usefulness. But it does allow me to show off some mad SQLite skills …

The first problem is that the file uses nasty date formats. Today would be 23-Jan-16 in the report’s Date field, which is filled with the ugh. You can fix that, though, with a fragment of SQL modified from here:

printf("%4d-%02d-%02d", substr(Date, 8,2)+2000, (instr("---JanFebMarAprMayJunJulAugSepOctNovDec", substr(Date, 4,3))-1)/3, substr(Date, 1, 2)) as isodate

The above data definition sets the isodate column to be in the familiar and useful YYYY-MM-DD ISO format.

A related example would be to query the whole CSV file for monthly mean generation from Kingsbridge and K2 Wind projects (they’re next to one another) for months after K2’s commissioning in March 2015. Here’s what I did in q:

q -T -O -H -d, 'select printf("%4d-%02d", substr(Date, 8,2)+2000, (instr("---JanFebMarAprMayJunJulAugSepOctNovDec", substr(Date, 4,3))-1)/3) as isomonth, avg(KINGSBRIDGE) as kavg, avg(K2WIND) as k2avg from Downloads/HourlyWindFarmGen_20160122.csv where isomonth>"2015-03" group by isomonth'

which gave the results:

isomonth    kavg    k2avg
2015-04    12.7277777778    37.4569444444
2015-05    8.94623655914    67.6747311828
2015-06    6.05833333333    66.6847222222
2015-07    3.96370967742    45.372311828
2015-08    6.34811827957    67.436827957
2015-09    7.29027777778    79.7194444444
2015-10    14.5658602151    128.037634409
2015-11    15.9944444444    130.729166667
2015-12    17.6075268817    152.422043011
2016-01    19.6408730159    163.013888889

Neat! (or at least, I think so.)

Notes on mini-printers and Linux

miniprinter galleryOver the last few weeks, I’ve been playing with a few small thermal printers. Meant as POS or information booth printers, they make a diverting project for the lo-fi printing enthusiast. While they all have common features — 58 mm/2¼” paper width, 8 pixel/mm resolution, 48 mm print width, serial connection — they all have their quirks. You may have seen these sold as the Adafruit Mini Thermal Receipt Printer or Sparkfun’s Thermal Printer, but there are many others. I’m going to write more on interfacing these directly to Raspberry Pi, Arduino, and (if I can navigate the documentation) a CUPS driver.

Update, July 2015: Here’s a CUPS driver: klirichek/zj-58, and my writeup on installing it on a Raspberry Pi — Thermal Printer driver for CUPS, Linux, and Raspberry Pi: zj-58

For now, I’m just leaving you a list of things I’ve found helpful for the DP-EH600 and 701 printers. Note that the similar-looking BTHT-v6 printer uses a completely different command set.

  • Replacement paper is sold as 2¼” × 30′. Staples have a box of 30 rolls for under $25 (item 279096, not on their website). Longer rolls don’t fit.
  • You’ll need a USB→TTL Serial adaptor, preferably one with DTR control. I use one from JY-MCU. In a pinch, you can use a simpler  Debug / Console Cable for Raspberry Pi, but you risk serial overruns and dodgy results. Remember that RX on the adaptor goes to TX on the printer, and vice versa.
  • A good solid power supply is needed; these printers draw ~8 W when printing. Some printers only support 5 V (for which a 3 amp adaptor would be ideal), others 5-9 V. The higher voltage makes text printing faster. You can’t drive these directly from your Raspberry Pi/Arduino power supply.
  • Linux serial ports are set to some defaults which may have been historically useful, but now corrupt 8-bit data. A trick I picked up here is to first issue the command
    stty -F /dev/ttyUSB1 0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0
    which clears all settings, then set the device up as you need it:
    stty -F /dev/ttyUSB1 speed 9600 raw cs8
    (Most of these printers default to 9600 baud. Your device may be called something different to ttyUSB1.)
  • I’ve written a couple of Python driver stubs which take an image and produce the relevant binary output:
    • scruss / esc-pos-image.py – prints an image as a single command. May not work on the SparkFun printer. Does not work on the BTHT-v6.
    • scruss / esc-pos-image-star.py – prints the image in 24 pixel deep bands. Can sometimes cause visible gaps in the printout, but will work on almost all printers, except the BTHT-v6.
  • These Python libraries also work, as long as you address the printer properly (right device, right speed):

Notes/Credits

  1. Reed Zhao (of Tangram Software) lent me a couple of different printers for testing after I bought a different one from him. He’s put a lot of work into sourcing these printers direct from the manufacturers. Thanks, Reed!
    NB: Reed doesn’t sell printers any more. Try eBay.
  2. Image credits for print samples:

Manuals/Docs

Posted more for historical reference:

My Raspberry Pi talks to my Oscilloscope

Hey! This post is completely ancient. It doesn’t even use Python 3. Advice given here might be well out of date.

… it complains that the oscilloscope is always making waves.

DS1EB134907266_0

Ahem. Anyway. I have a Rigol DS1102E 100 MHz Digital Oscilloscope. For such a cheap device, it’s remarkable that you can control it using USB Test & Measurement Class commands. I’d been wanting to use a Raspberry Pi as a headless data acquisition box with the oscilloscope for a while, but Raspbian doesn’t ship with the usbtmc kernel module. I thought I was stuck.

Alex Forencich turned up in the forum with an all-Python solution: Python USBTMC (source: alexforencich / python-usbtmc). I got this working quite nicely today on both the Raspberry Pi and my Ubuntu laptop. Here’s how I installed it:

  1. Check your device’s USB code with lsusb:
    $ lsusb
    Bus 001 Device 002: ID 0424:9512 Standard Microsystems Corp.
    ….
    Bus 001 Device 004: ID 1ab1:0588 Rigol Technologies DS1000 SERIES
  2. Ensure that libusb-1.0 is installed:
    sudo apt-get install libusb-1.0-0
  3. Create a new group, usbtmc:
    sudo groupadd usbtmc
  4. Add yourself to this group:
    sudo usermod -a -G usbtmc pi
  5. As root, create a file /etc/udev/rules.d/usbtmc.rules. You’ll need to put in your device’s ID values:
    # USBTMC instruments
    # Rigol DS1100 – ID 1ab1:0588 Rigol Technologies DS1000 SERIES
    SUBSYSTEMS==”usb”, ACTION==”add”, ATTRS{idVendor}==”1ab1″, ATTRS{idProduct}==”0588″, GROUP=”usbtmc”, MODE=”0660″
    (all of the SUBSYSTEMS to MODE= should be one one line)
  6. Download and install the latest pyusb (Raspbian version is rather old):
    git clone https://github.com/walac/pyusb.git
    cd pyusb
    python setup.py build
    sudo python setup.py install
  7. Now get python-usbtmc:
    git clone https://github.com/alexforencich/python-usbtmc.git
    cd python-usbtmc
    python setup.py build
    sudo python setup.py install
  8. For this simple demo, you’ll need to convert the USB vendor IDs to decimal:
    0x1ab1 = 6833
    0x0588 = 1416
  9. Now, start python as root (sudo python) then type:
    import usbtmc
    instr =  usbtmc.Instrument(6833, 1416)
    print(instr.ask(“*IDN?”))
  10. This should return something like:
    Rigol Technologies,DS1102E,DS1EB13490xxxx,00.02.06.00.01

If you get the status line, congratulations! You now have a fully working usbtmc link. I haven’t had much time to play with this, but I know I can make really nice screenshots to an attached USB drive using the command: instr.write(“:HARDcopy”). Many more commands can be found in the DS1000D/E Programming Guide, available on Rigol‘s site.

I had a couple of problems, though:

  1. The library seems to need root privileges, despite the udev rule thing. After creating the udev rule, you will need to reboot. This is the simplest way of getting it to work without being root.
  2. Reading from the ‘scope’s memory  chokes on non-UTF8 characters. If I do:
    rawdata = instr.ask(“:WAV:DATA? CHAN1”)[10:]
    I get a lengthy Python error which ends:
    …
    File “/usr/lib/python2.7/encodings/utf_8.py”, line 16, in decode
        return codecs.utf_8_decode(input, errors, True)
    UnicodeDecodeError: ‘utf8’ codec can’t decode byte 0x99 in position 10: invalid start byte
    I have no idea what that means, or how to fix it. Alex suggested using ask_raw instead of ask, and the data comes through with no complaints.

I’ve still got to work my way through the Rigol’s data format, but other people have done that before:

  1. Controlling a Rigol oscilloscope using Linux and Python | C i b o M a h t o . c o m
  2. Ken Shirriff’s blog: Four Rigol oscilloscope hacks with Python

I’ll post any updates here, along with the Raspberry Pi forum topic: USB Test & Measurement class (usbtmc) driver?

Incidentally, if you’re working with WFM data dumps from the Rigol ‘scopes (and you should, because they make storing data to USB drives quick), mabl/pyRigolWFM is basically magic. Not merely can it describe and decode those binary files, it can do pretty graphics with no thought required:

made by pyRigolWFMHat tip for the mention: MP3 Options & Oscilloscope Interfacing For Raspberry Pi @Raspberry_Pi #piday #raspberrypi « adafruit industries blog

Update, 2013-12-20: I’ve successfully managed to run most of Ken’s examples with Alex’s code. The major modification you have to do is use ask_raw instead of ask. Example code shown below:

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
Download data from a Rigol DS1102E oscilloscope and graph with matplotlib
         using  Alex Forencich's python-usbtmc pure python driver
                https://github.com/alexforencich/python-usbtmc
scruss - 2013-12-20

based on
Download data from a Rigol DS1052E oscilloscope and graph with matplotlib.
By Ken Shirriff, http://righto.com/rigol

which in turn was
Based on http://www.cibomahto.com/2010/04/controlling-a-rigol-oscilloscope-using-linux-and-python/
by Cibo Mahto.
"""

import usbtmc
import time
import numpy
import matplotlib.pyplot as plot

# initialise device
instr =  usbtmc.Instrument(0x1ab1, 0x0588) # Rigol DS1102E

# read data
instr.write(":STOP")
instr.write(":WAV:POIN:MODE RAW")
# first ten bytes are header, so skip
rawdata = instr.ask_raw(":WAV:DATA? CHAN1")[10:]
data_size = len(rawdata)

# get metadata
sample_rate = float(instr.ask_raw(':ACQ:SAMP?'))
timescale = float(instr.ask_raw(":TIM:SCAL?"))
timeoffset = float(instr.ask_raw(":TIM:OFFS?"))
voltscale = float(instr.ask_raw(':CHAN1:SCAL?'))
voltoffset = float(instr.ask_raw(":CHAN1:OFFS?"))

# show metadata
print "Data size:      ", data_size
print "Sample rate:    ", sample_rate
print "Time scale:     ", timescale
print "Time offset:    ", timeoffset
print "Voltage offset: ", voltoffset
print "Voltage scale:  ", voltscale

# convert data from (inverted) bytes to an array of scaled floats
# this magic from Matthew Mets
data = numpy.frombuffer(rawdata, 'B')
data = data * -1 + 255
data = (data - 130.0 - voltoffset/voltscale*25) / 25 * voltscale

# creat array of matching timestamps
time = numpy.linspace(timeoffset - 6 * timescale, timeoffset + 6 * timescale,
                      num=len(data))

# scale time series and label accordingly
if (time[-1] &amp;lt; 1e-3):
    time = time * 1e6
    tUnit = "µS"
elif (time[-1] &amp;lt; 1):
    time = time * 1e3
    tUnit = "mS"
else:
    tUnit = "S"

# Plot the data
plot.plot(time, data)
plot.title("Oscilloscope Channel 1")
plot.ylabel("Voltage (V)")
plot.xlabel("Time (" + tUnit + ")")
plot.xlim(time[0], time[-1])
plot.show()

A Murder of Crows on your Raspberry Pi with Boodler

Boodler is rather fun. It generates ambient music based on user-defined or downloaded ‘soundscapes’. If you’ve got a modern (HTML5/Opus-capable) browser, you can hear a streaming demo here: http://repeater.xiph.org:8000/clock.opus. It’s using the FM3 Buddha Machine samples in this demo, but it can run lots more: a tree full of crows, a thunderstorm, dripping water, …

It’s pretty easy to run on a Raspberry Pi running a recent version of Raspbian. The only technical glitch I had was that there’s something deeply confused about ALSA sound handling on the Raspberry Pi. I’m sure it’ll get fixed soon, but for now, you have to use PulseAudio. (If you want to read about my ALSA woes, go here.)

The installation prerequisites are simple:

sudo apt-get install pulseaudio pulseaudio-utils libpulse-dev python-dev

Now download and configure Boodler:

wget http://boodler.org/dl/Boodler-2.0.4.tar.gz
tar xvzf Boodler-2.0.4.tar.gz
cd Boodler-2.0.4
python setup.py build

It takes a while to do this, but make sure it does something useful when it’s building the various sound drivers. You don’t want it to say:

skipping 'boodle.cboodle_pulse' extension

If it says that, you haven’t installed Pulseaudio. Go back and check your apt-get line.

Once it’s built, now install it:

sudo python setup.py install

Now test it:

boodler --hardware --output pulse --testsound

Not merely should you get some pleasant tones from your Raspberry Pi’s audio, but you sound get some informative and non-threatening terminal output. Mine looks like:

Boodler: PulseAudio sound driver.
 PulseAudio library: 2.0.0.
 Sample rate is 44100 fps.
 Samples are 16-bit little-endian.
 Buffer size is 32768.
 21:37:46 (root) Running "Boodler test sound"

If that works, let’s get those crows a-cawin’. Download the soundscapes you need:

boodle-mgr install http://boodler.org/lib/org.boodler.old.crow.1.0.boop
boodle-mgr install http://boodler.org/lib/com.eblong.zarf.crows.1.0.boop

and run it:

boodler --output pulse com.eblong.zarf.crows/ParliamentOfCrows

Crows everywhere!

I really like the Buddha Machine samples. It’s quite big (> 80 MB), so this next set will take a while to download:

boodle-mgr install  http://boodler.org/lib/com.azulebanana.buddhamachine.1.5.1.boop
boodle-mgr install http://boodler.org/lib/com.azulebanana.buddhaagent.1.5.1.boop

It’s worth the wait:

boodler --output pulse com.azulebanana.buddhaagent/ChangingLoops

Boodler has tons of options, prebuilt packages, and instructions to build your own: Boodler Documentation.

One thing I’ve tried to get working, but failed, is streaming from Boodler via icecast. Sure, I can install and run it, it’s just that the results are, um, undesirable. If you want to have a play, here’s how to install icecast:

sudo apt-get install icecast2 ices2 libshout3-dev

Icecast will configure itself, and ask for a couple of passwords. You’ll have to rebuild and reinstall Boodler for it to catch the new configuration. You can then try streaming:

boodler --output shout --define shout-password=mypassword --define shout-mount='/boodler-buddha.ogg' com.azulebanana.buddhaagent/ChangingLoops

If you open a web browser at this address http://raspberrypi:8000/ you should see a config page listing your boodler-buddha.ogg stream. Click on the M3U link next to it, and your streaming music player should start making a joyful noise …

… except in my case, something went very wrong, and it started to produce industrial ultra-glitch nightmare noise: boodler-streaming_test-fail. I’m sure it’s fixable with some tweaking, but I’m not there yet.

Simple ADC with the Raspberry Pi

Raspberry Pi wearing an MCP3008

Hey! This is a really old article. You should really be using gpiozero these days.

I hadn’t realised it, but the The Quite Rubbish Clock did something that a lot of people seem to have trouble with on the Raspberry Pi: communicating using hardware SPI. Perhaps it’s because everything is moving so fast with Raspberry Pi development, tutorials go out of date really quickly. Thankfully, hardware SPI is much easier to understand than the older way of emulation through bit-banging.

SPI is a synchronous serial protocol, so it needs a clock line as well as a data in and data out line. In addition, it has a Chip Enable (CE, or Chip Select, CS) line that is used to choose which SPI device to talk to. The Raspberry Pi has two CE lines (pins 24 and 26) so can talk to two SPI devices at once. It supports a maximum clock rate of 32 MHz, though in practice you’ll be limited to the rate your device supports.

The device I’m testing here is an MCP3008 10-bit Analogue-to-Digital Converter (ADC). These are simple to use, cheap and quite fast converters with 8 input channels. If you hook them up to a 3.3 V supply they will convert a DC voltage varying from 0-3.3 V to a digital reading of 0-1023 (= 210 – 1). Not quite up there in quality for hi-fi audio or precision sensing, but good enough to read from most simple analogue sensors.

The sensor I’m reading is the astonishingly dull LM35DZ temperature sensor. All the cool kids seem to be using TMP36s (as they can read temperatures below freezing without a negative supply voltage). One day I’ll show them all and use a LM135 direct Kelvin sensor, but not yet.

To run this code, install the SPI libraries as before. Now wire up the MCP3008 to the Raspberry Pi like so:

 MCP 3008 Pin          Pi GPIO Pin #    Pi Pin Name
==============        ===============  =============
 16  VDD                 1              3.3 V
 15  VREF                1              3.3 V
 14  AGND                6              GND
 13  CLK                23              GPIO11 SPI0_SCLK
 12  DOUT               21              GPIO09 SPI0_MISO
 11  DIN                19              GPIO10 SPI0_MOSI
 10  CS                 24              GPIO08 CE0
  9  DGND                6              GND

The wiring for the LM35 is very simple:

 LM35 Pin        MCP3008 Pin
==========      =============
 Vs              16 VDD
 Vout             1 CH0
 GND              9 DGND

The code I’m using is a straight lift of Jeremy Blythe’s Raspberry Pi hardware SPI analog inputs using the MCP3008. The clever bit in Jeremy’s code is the readadc() function which reads the relevant length of bits (by writing the same number of bits; SPI’s weird that way) from the SPI bus and converting it to a single 10-bit value.

#!/usr/bin/python
# -*- coding: utf-8 -*-
# mcp3008_lm35.py - read an LM35 on CH0 of an MCP3008 on a Raspberry Pi
# mostly nicked from
#  http://jeremyblythe.blogspot.ca/2012/09/raspberry-pi-hardware-spi-analog-inputs.html

import spidev
import time

spi = spidev.SpiDev()
spi.open(0, 0)

def readadc(adcnum):
# read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7)
    if adcnum > 7 or adcnum < 0:
        return -1
    r = spi.xfer2([1, 8 + adcnum << 4, 0])
    adcout = ((r[1] & 3) << 8) + r[2]
    return adcout

while True:
    value = readadc(0)
    volts = (value * 3.3) / 1024
    temperature = volts / (10.0 / 1000)
    print ("%4d/1023 => %5.3f V => %4.1f °C" % (value, volts,
            temperature))
    time.sleep(0.5)

The slightly awkward code temperature = volts / (10.0 / 1000) is just a simpler way of acknowledging that the LM35DZ puts out 10 mV (= 10/1000, or 0.01) per °C. Well-behaved sensors generally have a linear relationship between what they indicate and what they measure.

If you run the code:

sudo ./mcp3008_lm35.py

you should get something like:

  91/1023 => 0.293 V => 29.3 °C
  93/1023 => 0.300 V => 30.0 °C
  94/1023 => 0.303 V => 30.3 °C
  95/1023 => 0.306 V => 30.6 °C
  96/1023 => 0.309 V => 30.9 °C
  97/1023 => 0.313 V => 31.3 °C
  97/1023 => 0.313 V => 31.3 °C
  98/1023 => 0.316 V => 31.6 °C
  99/1023 => 0.319 V => 31.9 °C
  99/1023 => 0.319 V => 31.9 °C
 100/1023 => 0.322 V => 32.2 °C
 100/1023 => 0.322 V => 32.2 °C
 100/1023 => 0.322 V => 32.2 °C
 101/1023 => 0.325 V => 32.5 °C
 101/1023 => 0.325 V => 32.5 °C
 102/1023 => 0.329 V => 32.9 °C
 102/1023 => 0.329 V => 32.9 °C
 103/1023 => 0.332 V => 33.2 °C

Note that the sensor had been sitting over the Raspberry Pi’s CPU for a while; I don’t keep my house at 29 °C. I made the temperature go up by holding the LM35.

So, you’ve just (fairly cheaply) given your Raspberry Pi 8 analogue input channels, so it can behave much more like a real microcontroller now. I remember from my datalogging days that analogue inputs can be pretty finicky and almost always return a value even if it’s an incorrect one. Check the chip’s datasheet to see if you’re doing it right, and if in doubt, meter it!

qrclock, the demo reel

classy cable for the Quite Rubbish clock

The video of the Quite Rubbish Clock isn’t running the same code that’s in the listing. Here it is, showing off some of the handy code that’s in bgreat’s nokiaSPI Python class:

#!/usr/bin/python
# -*- coding: utf-8 -*-
# qrmovie

import time
# need to use git://github.com/mozillazg/python-qrcode.git
import qrcode
from PIL import Image, ImageFont
import ImageOps
# uses bgreat's SPI code; see
# raspberrypi.org/phpBB3/viewtopic.php?f=32&t=9814&p=262274&hilit=nokia#p261925
import nokiaSPI

noki = nokiaSPI.NokiaSPI()              # create display device
qr = qrcode.QRCode(version=1,           # V.1 QR Code: 21x21 px
error_correction=qrcode.constants.ERROR_CORRECT_M,
box_size=2, border=1)
bg = Image.new('1', (84, 48))           # blank (black) image background

# intro
noki.cls()
noki.led(0)
time.sleep(3)
for i in range(0,769,32):
    noki.led(i)
    time.sleep(0.04)

# display is 14 columns by 8 rows
noki.centre_word(1, 'scruss.com')
noki.centre_word(3, 'presents')
time.sleep(3)
noki.cls()
noki.centre_word(1, 'qrclock')
noki.centre_word(2, 'the')
noki.gotorc(3,3)
noki.text("[Q]uite")
noki.gotorc(4,3)
noki.text("[R]ubbish")
noki.gotorc(5,3)
noki.text(" Clock")
time.sleep(3)

elapsed=0
start_time = time.time()
while (elapsed<12):
    qr.clear()
    newbg = bg.copy()                   # copy blank background
    s = time.strftime('%Y-%m-%d %H:%M:%S')
    qr.add_data(s)                      # make QR Code of YYYY-MM-DD HH:MM:SS
    qr.make()
    qrim = qr.make_image()              # convert qrcode object to PIL image
    qrim = qrim.convert('L')            # make greyscale
    qrim = ImageOps.invert(qrim)        # invert colours: B->W and W->B
    qrim = qrim.convert('1')            # convert back to 1-bit
    newbg.paste(qrim, (18, 0))          # paste QR Code into blank background
    noki.show_image(newbg)              # display code on LCD
    time.sleep(0.4)                     # pause before next display
    elapsed = time.time() - start_time

noki.cls()
noki.centre_word(1, 'for')
noki.centre_word(2, 'more')
noki.centre_word(3, 'details')
time.sleep(3)
noki.cls()
noki.load_bitmap("blogpost-nokia.bmp", True)
time.sleep(7)
noki.cls()
noki.centre_word(3, 'fin')
noki.centre_word(5, 'scruss, 2013')
time.sleep(1)
for i in range(768,-1,-32):
    noki.led(i)
    time.sleep(0.05)
time.sleep(1)
noki.cls()

(This source, plus nokiaSPI class: qrclock-movie.zip)

Lines 43-58 show off the QR clock for a maximum of 12 seconds. Any more, and you’d get really bored.

The screen handling functions I used are:

  • cls() — Clears the screen.
  • led(brightness) — sets the backlight to brightness. For me, full brightness is at 768. A value of zero turns the backlight off. If you don’t have the screen LED connected to one of the Raspberry Pi’s PWM pin, this will either be full on (for any brightness >= 1), or off, for brightness=0. This is used to fade up the screen in lines 24-26, and fade it down far too theatrically in lines 72-74.
  • show_image(PILImage) — display a single bit depth black and white Python Imaging Library object PILImage. This can be no larger than 84×48 pixels.
  • load_bitmap(file, Invert) — load a single bit depth black and white BMP file of maximum size 48×84. If Invert is true, keep the colours as they are, otherwise swap black and white to make a negative image. nokiSPI flips images by 90°, so the image I loaded to show the URL of the blog post looks like this:
    blogpost-nokia
    (I know, I could have generated this in code, but I’d already made the image using qrencode. I couldn’t be bothered working out the image size and offsets.)

The text handling functions I used are:

  • gotorc(row, column) — move the text cursor to row, column. The screen only has 14 columns by 8 rows if you use the standard 6×6 pixel font, so keep your text short to avoid disappointment.
  • text(text) — write text at the current cursor position.
  • centre_word(row, text) — write text centred in row row. Since the text rows are a maximum of 14 columns, text with an odd number of characters will appear slightly off-centre.

There are many more functions in the nokiaSPI class; watch the demo, have a dig through the source and see what you can use.

The Quite Rubbish Clock

Hey! This article is really old and probably doesn’t work any more: things have changed a lot in Raspberry Pi world since 2013 …

Update 3: code for the demo video is here.

Update 2: In which I actually post working code.

Update: Eep! This post was featured on the Raspberry Pi blog today. Thanks, Liz!

And now for something completely different:

… a clock that isn’t human readable. You’ll need a QR code reader to be able to tell the time.

Nokia screen on Raspberry Pi

This, however, is not the prime purpose of the exercise. I was looking for an excuse to try some direct hardware projects with the GPIO, and I remembered I had a couple of Nokia-style surplus LCDs lying about that could be pressed into service. These LCDs aren’t great: 84×48 pixels, 3V3 logic, driven by SPI via an 8-pin header which includes PWM-controllable LED backlighting. They are cheap, and available almost everywhere: DealExtreme ($5.36), SparkFun ($9.95), Adafruit ($10, but includes a level shifter, which you really need if you’re using a 5V logic Arduino), Solarbotics ($10) and Creatron (about $12; but you can walk right in and buy one). Despite being quite difficult to use, helpful people have written drivers to make these behave like tiny dot-addressable screens.

I’d been following the discussion on the Raspberry Pi forum about driving the Nokia LCD from a Raspberry Pi. Only when user bgreat posted some compact code that was supposed to run really fast did I dig out the LCD board and jumper wires. Building on bgreat’s nokiaSPI.py class and a few other bits of code, here’s what I built to make this singularly pointless clock:

#!/usr/bin/python
# -*- coding: utf-8 -*-
# qrclock - The Quite Rubbish Clock for Raspberry Pi - scruss, 2013-01-19

import time
# need to use git://github.com/mozillazg/python-qrcode.git
import qrcode
from PIL import Image
import ImageOps
# uses bgreat's SPI code; see
# raspberrypi.org/phpBB3/viewtopic.php?f=32&amp;amp;amp;amp;t=9814&amp;amp;amp;amp;p=262274&amp;amp;amp;amp;hilit=nokia#p261925
import nokiaSPI

noki = nokiaSPI.NokiaSPI()              # create display device
qr = qrcode.QRCode(version=1,           # V.1 QR Code: 21x21 px
                   error_correction=qrcode.constants.ERROR_CORRECT_M,
                   box_size=2, border=1)
bg = Image.new('1', (84, 48))           # blank (black) image background

while 1:
    qr.clear()
    newbg = bg.copy()                   # copy blank background
    s = time.strftime('%Y-%m-%d %H:%M:%S')
    qr.add_data(s)                      # make QR Code of YYYY-MM-DD HH:MM:SS
    qr.make()
    qrim = qr.make_image()              # convert qrcode object to PIL image
    qrim = qrim.convert('L')            # make greyscale
    qrim = ImageOps.invert(qrim)        # invert colours: B-&amp;amp;amp;gt;W and W-&amp;amp;amp;gt;B
    qrim = qrim.convert('1')            # convert back to 1-bit
    newbg.paste(qrim, (18, 0))          # paste QR Code into blank background
    noki.show_image(newbg)              # display code on LCD
    time.sleep(0.4)                     # pause before next display

(Convenient archive of all the source: qrclock2.zip, really including bgreat’s nokiaSPI class this time …)

To get all this working on your Raspberry Pi, there’s a fair amount of configuration. The best references are bgreat’s own comments in the thread, but I’ve tried to include everything here.

Enabling the SPI kernel module

As root, edit the kernel module blacklist file:

sudo vi /etc/modprobe.d/raspi-blacklist.conf

Comment out the spi-bcm2708 line so it looks like this:

#blacklist spi-bcm2708

Save the file so that the module will load on future reboots. To enable the module now, enter:

sudo modprobe spi-bcm2708

Now, if you run the lsmod command, you should see something like:

Module                  Size  Used by
spi_bcm2708             4421  0

Installing the WiringPi, SPI and other required packages

WiringPi by Gordon is one of the neater Raspberry Pi-specific modules, as it allows relatively easy access to the Raspberry Pi’s GPIO pins. For Raspbian, there are a few other imaging libraries and package management tools you’ll need to install here:

sudo apt-get install python-imaging python-imaging-tk python-pip python-dev git
sudo pip install spidev
sudo pip install wiringpi

Installing the Python QR code library

Finding a library that provided all the right functions was the hardest part here. I ended up using mozillazg‘s fork of lincolnloop‘s python-qrcode module. mozillazg’s fork lets you use most of the lovely PIL methods, while the original hides most of them. Since I had to do some image compositing and colour remapping to make the image appear correct on the Nokia screen, the new fork was very helpful.

To install it:

git clone git://github.com/mozillazg/python-qrcode.git
cd python-qrcode/
sudo python ./setup.py install

The tiny 84×48 resolution of the Nokia screen doesn’t give you many options for sizing QR codes. For the time display of the clock, a 21×21 module Version 1 code with two pixels per module and one module margin just fits into 48 pixels. Using a medium level of error correction, you can fit the 19-character message (such as “2013-01-19 18:56:59”) into this tiny screen with a very good chance of it being read by any QR code reader.

(In the video, there’s a much larger QR code that’s a link to this blog post. That’s a Version 7 code [45×45 modules] at one pixel per module and no margin. This doesn’t meet Denso Wave’s readability guidelines, but the Nokia screen has large blank margins which seem to help. It won’t read on every phone, but you’re here at this link now, so you don’t need it …)

Wiring it all up

(Do I really need to say that you’ll be messing around with the inner delicate bits of your Raspberry Pi here, and if you do something wrong, you could end up with a dead Raspberry Pi? No? Okay. Just make sure you take some static precautions and you really should have the thing shut down and powered off.)

You’ll need 8 female-female right-angled ones). Note that the thick border of the LCD is the top of the screen. These boards are made who-knows-where by who-knows-whom, and there’s a huge variety of labels and layouts on the pins. My one appears to be yet another variant, and is labelled:

  1. VCC
  2. GND
  3. SCE
  4. RST
  5. D/C
  6. DNK(MOSI)
  7. SCLK
  8. LED
screen labels

This is how I wired it (from comments in bgreat’s code and the GPIO reference):

 LCD Pin       Function      Pi GPIO Pin #   Pi Pin Name
============= ============= =============== =============
 1 VCC         Vcc            1              3.3 V
 2 GND         Ground        25              GND
 3 SCE         Chip Enable   24              GPIO08 SPI0_CE0_N
 4 RST         Reset         11              GPIO17
 5 D/C         Data/Command  15              GPIO22
 6 DNK(MOSI)   Data In       19              GPIO10 SPI0_MOSI
 7 SCLK        Serial Clock  23              GPIO11 SPI0_SCLK
 8 LED         Backlight     12              GPIO18 PWM0
GPIO wiring
back of screen

Wire it up, and fire up the program:

sudo ./qrclock.py

Yes, code that accesses GPIO needs to be run as root. Pesky, but helps you avoid running code that accidentally scrams the nuclear power station you’re controlling from your Raspberry Pi …

python + Arduino + Tk: Toggling an LED

Whoa! This is so old I don’t even know where to start!

  • It’s using Python 2, so if it works at all it probably won’t for much longer, and Tkinter is something completely different under Python 3
    (grrreat planning there, Python guys …)
  • pyfirmata is likely ancient history too.

Phil sent me a note last week asking how to turn an LED on or off using Python talking through Firmata to an Arduino. This was harder than it looked.

It turns out the hard part is getting the value from the Tkinter Checkbutton itself. It seems that some widgets don’t return values directly, so you must read the widget’s value with a get() method. This appears to work:

#!/usr/bin/python
# turn an LED on/off with a Tk Checkbutton - scruss 2012/11/13
# Connection:
# - small LED connected from D3, through a resistor, to GND

import pyfirmata
from Tkinter import *

# Create a new board, specifying serial port
# board = pyfirmata.Arduino('/dev/ttyACM0') # Raspberry Pi
board = pyfirmata.Arduino('/dev/tty.usbmodem411') # Mac

root = Tk()
var = BooleanVar()

# set up pins
pin3 = board.get_pin('d:3:o') # D3 On/Off Output (LED)

def set_led():  # set LED on/off
    ledval = var.get()
    print "Toggled", ledval
    pin3.write(ledval)

# now set up GUI
b = Checkbutton(root, text = "LED", command = set_led,
                variable = var)
b.pack(anchor = CENTER)

root.mainloop()



This is explained quite well here: Tkinter Checkbutton doesn’t change my variable – Stack Overflow. I also learnt a couple of things about my previous programs:


    

  • You don’t really need to set up an Iterator unless you’re reading analogue inputs
    • 

  • My “clever” cleanup-on-exit code actually made the script hang on Mac OS.
    •

Servo Control from pyfirmata + arduino

Hey! This article is really old! So old, in fact, that I clearly thought that saying (ahem) “w00t w00t” was a good idea. Information here may be misleading and possibly wrong. You probably want to be using a newer client library and you definitely want to use an Arduino IDE ≥ 1.6 and not the ancient one that comes with Raspbian.

pyFirmata‘s documentation is, to be charitable, sparse. After writing Raspberry Pi, Python & Arduino *and* a GUI (which should be making an appearance in The MagPi soon, w00t w00t yeet!), I looked at pyFirmata again to see what it could do. That pretty much meant digging through the source.

Firmata can drive hobby servos, and if you’re not driving too many, you can run them straight from the Arduino with no additional power. I used a standard cheapo-but-decent Futaba S3003, which gives you about 180° of motion. The particular one I tried started to make little growly noises past 175°, so in the example below, that’s hardcoded as the limit.

#!/usr/bin/python
# -*- coding: utf-8 -*-
# move a servo from a Tk slider - scruss 2012-10-28

import pyfirmata
from Tkinter import *

# don't forget to change the serial port to suit
board = pyfirmata.Arduino('/dev/tty.usbmodem26271')

# start an iterator thread so
# serial buffer doesn't overflow
iter8 = pyfirmata.util.Iterator(board)
iter8.start()

# set up pin D9 as Servo Output
pin9 = board.get_pin('d:9:s')

def move_servo(a):
    pin9.write(a)

# set up GUI
root = Tk()

# draw a nice big slider for servo position
scale = Scale(root,
    command = move_servo,
    to = 175,
    orient = HORIZONTAL,
    length = 400,
    label = 'Angle')
scale.pack(anchor = CENTER)

# run Tk event loop
root.mainloop()

The code above makes a slider (oh, okay, a Tkinter Scale widget) that moves the servo connected to Arduino pin D9 through its whole range. To set the servo position, you just need to write the angle value to the pin.

I haven’t tried this with the Raspberry Pi yet. It wouldn’t surprise me if it needed external power to drive the Arduino and the servo. This might be a good excuse to use my Omega-328U board — it’s Arduino code compatible, runs from an external power supply, and has Signal-Voltage-Ground (SVG) connectors that the servo cable would just plug straight into.