Shallow Thoughts : : Sep

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Thu, 28 Sep 2017

Audio Output from a Raspberry Pi Zero

Someone at our makerspace found a fun Halloween project we could do at Coder Dojo: a motion sensing pumpkin that laughs evilly when anyone comes near. Great! I've worked with both PIR sensors and ping rangefinders, and it sounded like a fun project to mentor. I did suggest, however, that these days a Raspberry Pi Zero W is cheaper than an Arduino, and playing sounds on it ought to be easier since you have frameworks like ALSA and pygame to work with.

The key phrase is "ought to be easier". There's a catch: the Pi Zero and Zero W don't have an audio output jack like their larger cousins. It's possible to get analog audio output from two GPIO pins (use the term "PWM output" for web searches), but there's a lot of noise. Larger Pis have a built-in low-pass filter to screen out the noise, but on a Pi Zero you have to add a low-pass filter. Of course, you can buy HATs for Pi Zeros that add a sound card, but if you're not super picky about audio quality, you can make your own low-pass filter out of two resistors and two capacitors per channel (multiply by two if you want both the left and right channels).

There are lots of tutorials scattered around the web about how to add audio to a Pi Zero, but I found a lot of them confusing; e.g. Adafruit's tutorial on Pi Zero sound has three different ways to edit the system files, and doesn't specify things like the values of the resistors and capacitors in the circuit diagram (hint: it's clearer if you download the Fritzing file, run Fritzing and click on each resistor). There's a clearer diagram in Sudomod Forums: PWM Audio Guide, but I didn't find that until after I'd made my own, so here's mine.

Parts list:

And here's how to wire it: [Adding audio to the Raspberry Pi Zero]
(Fritzing file, pi-zero-audio.fzz.)

This wiring assumes you're using pins 13 and 18 for the left and right channels. You'll need to configure your Pi to use those pins. Add this to /boot/config.txt:

dtoverlay=pwm-2chan,pin=18,func=2,pin2=13,func2=4

Testing

Once you build your circuit up, you need to test it. Plug in your speaker or headphones, then make sure you can play anything at all:

aplay /usr/share/sounds/alsa/Front_Center.wav

If you need to adjust the volume, run alsamixer and use the up and down arrow keys to adjust volume. You'll have to press up or down several times before the bargraph actually shows a change, so don't despair if your first press does nothing.

That should play in both channels. Next you'll probably be curious whether stereo is actually working. Curiously, none of the tutorials address how to test this. If you ls /usr/share/sounds/alsa/ you'll see names like Front_Left.wav, which might lead you to believe that aplay /usr/share/sounds/alsa/Front_Left.wav might play only on the left. Not so: it's a recording of a voice saying "Front left" in both channels. Very confusing!

Of course, you can copy a music file to your Pi, play it (omxplayer is a nice commandline player that's installed by default and handles MP3) and see if it's in stereo. But the best way I found to test audio channels is this:

speaker-test -t wav -c 2

That will play those ALSA voices in the correct channel, alternating between left and right. (MythTV has a good Overview of how to use speaker-test.

Not loud enough?

I found the volume plenty loud via earbuds, but if you're targeting something like a Halloween pumpkin, you might need more volume. The easy way is to use an amplified speaker (if you don't mind putting your nice amplified speaker amidst the yucky pumpkin guts), but you can also build a simple amplifier. Here's one that looks good, but I haven't built one yet: One Transistor Audio for Pi Zero W

Of course, if you want better sound quality, there are various places that sell HATs with a sound chip and line or headphone out.

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[ 15:49 Sep 28, 2017    More hardware | permalink to this entry | comments ]

Sun, 17 Sep 2017

Screen Brightness on an Asus 1015e (and other Intel-based laptops)

When I upgraded my Asus laptop to Stretch, one of the things that stopped working was the screen brightness keys (Fn-F5 and Fn-F6). In Debian Jessie they had always just automagically worked without my needing to do anything, so I'd never actually learned how to set brightness on this laptop. The fix, like so many things, is easy once you know where to look.

It turned out the relevant files are in /sys/class/backlight/intel_backlight. cat /sys/class/backlight/intel_backlight/brightness tells you the current brightness; write a number to /sys/class/backlight/intel_backlight/brightness to change it.

That at least got me going (ow my eyes, full brightness is migraine-inducing in low light) but of course I wanted it back on the handy function keys.

I wrote a script named "dimmer", with a symlink to "brighter", that goes like this:

#!/bin/zsh

curbright=$(cat /sys/class/backlight/intel_backlight/brightness)
echo dollar zero $0
if [[ $(basename $0) == 'brighter' ]]; then
  newbright=$((curbright + 200))
else
  newbright=$((curbright - 200))
fi
echo from $curbright to $newbright

sudo sh -c "echo $newbright > /sys/class/backlight/intel_backlight/brightness"

That let me type "dimmer" or "brighter" to the shell to change the brightness, with no need to remember that /sys/class/whatsit path. I got the names of the two function keys by running xev and typing Fn and F5, then Fn and F6. Then I edited my Openbox ~/.config/openbox/rc.xml, and added:

<keybind key="XF86MonBrightnessDown">
  <action name="Execute">
    <execute>dimmer<execute>
  </action>
</keybind>
<keybind key="XF86MonBrightnessUp">
  <action name="Execute">
    <execute>brighter<execute>
  </action>
</keybind>

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[ 19:57 Sep 17, 2017    More linux/laptop | permalink to this entry | comments ]

Mon, 04 Sep 2017

Jumpstarting the Raspberry Pi Zero W: Now Available via Humble Bundle!

[Raspberry Pi Zero W] My new book is now shipping! And it's being launched via a terrific Humble Bundle of books on electronics, making, Raspberry Pi and Arduino.

Humble Bundles, if you haven't encountered them before, let you pay what you want for a bundle of books on related subjects. The books are available in ePub, Mobi, and PDF formats, without DRM, so you can read them on your choice of device. If you pay above a certain amount, they add additional books. My book is available if you pay $15 or more.

You can also designate some of the money you pay for charity. In this case the charity is Maker Ed, a crowdfunding initiative that supports Maker programs primarily targeted toward kids in schools. (I don't know any more about them than that; check out their website for more information.)

Jumpstarting the Raspberry Pi Zero W is a short book, with only 103 pages in four chapters:

  1. Getting Started: includes tips on headless setup and the Linux command line;
  2. Blink an LED: includes ways to blink and fade LEDs from the shell and from several different Python libraries;
  3. A Temperature Notifier and Fan Control: code and wiring instructions for three different temperature sensors (plus humidity and barometric pressure), and a way to use them to control your house fan or air conditioner, either according to the temperature in the room or through a Twitter command;
  4. A Wearable News Alert Light Show: wire up NeoPixels or DotStars and make them respond to keywords on Twitter or on any other web page you choose, plus some information on powering a Pi portably with batteries.

All the code and wiring diagrams from the book, plus a few extras, are available on Github, at my Raspberry Pi Zero Book code repository.

To see the book bundle, go to the Electronics & Programming Humble Bundle and check out the selections. My book, Jumpstarting the Raspberry Pi Zero W, is available if you pay $15 or more -- along with tons of other books you'll probably also want. I already have Make: Electronics and it's one of the best introductory electronics books I've seen, so I'm looking forward to seeing the followup volume. Plus there are books on atmospheric and environmental monitoring, a three-volume electronic components encyclopedia, books on wearable electronics and drones and other cool stuff.

I know this sounds like a commercial, but this bundle really does look like a great deal, whether or not you specifically want my Pi book, and it's a limited-time offer, only good for six more days.

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[ 13:21 Sep 04, 2017    More writing | permalink to this entry | comments ]