More Raspberry Pi adventures: the Pi Zero W and PaPiRus ePaper display

I decided I wanted to have some sort of physical display in the house for the temperature sensors so we wouldn’t need to be taking out our phones to check the temperature on my website if we were already inside at home. After a bunch of searching around, I discovered the PaPiRus ePaper display. ePaper means it’s not going to have any bright glaring light at night, and it also uses very little power.

The Raspberry Pi is hidden away under a side table, and already has six wires attached to the header for the temperature sensors, so I decided to just get a separate Raspberry Pi Zero W — which is absurdly small — and the PaPiRus display.

Setting it up

I flashed the SD card with the Raspbian Stretch Lite image, then enabled SSH and automatic connection to our (2.4GHz; the Zero W doesn’t support 5GHz) wifi network by doing the following:

  1. Plug the flashed SD card back into the computer
  2. Go into the newly-mounted “boot” volume and create an empty file called “ssh” to turn on SSH at boot
  3. Also in the “boot” volume, create a file called “wpa_supplicant.conf” and paste the following into it:

    country=AU
    ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
    update_config=1
    network={
    ssid="WIFI_SSID"
    scan_ssid=1
    psk="WIFI_PASSWORD"
    key_mgmt=WPA-PSK
    }

  4. Unmount the card, pop it into the Pi, add power, and wait 60-90 seconds and it’ll connect to your network and be ready for SSH access! The default username on the Pi is “pi” and the password is “raspberry”.

(These instructions are all thanks to this blog post but I figured I’d put them here as well for posterity).

The PaPiRus display connection was dead easy, I just followed Pi Supply’s guide after soldering a header into the Pi Zero W. If you want to avoid soldering, they also offer the Zero W with a header pre-attached.

Getting the Python library for updating the display was mostly straightforward, I just followed the instructions in the GitHub repository to manually install the Python 3 version.

I wrote a simple Python script to grab the current temperature and humidity from my website’s REST endpoints, and everything works! This script uses the “arrow” and “requests” libraries, which can be installed with “sudo apt-get install python3-arrow python3-requests”.

Next step is to have the Pi 3 that has the sensors run a simple HTTP server that the Zero W can connect to it, so even if we have no internet connection for whatever reason, the temperatures will still be available at home. I’ve updated my Pi Sensor Reader to add HTTP endpoints.

Temperature sensors: now powered by Raspberry Pi

The Weather section on my website is now powered by my Raspberry Pi, instead of my Ninja Block! \o/

Almost exactly three years ago, I started having my Ninja Block send its temperature data to my website (prior to that, I was manually pulling the data from the Ninja Blocks API and didn’t have any historical record of it). Ninja Blocks the company went bust in 2015, and there was some stuff in the Ninja Blocks software that relied on their cloud platform to work and I ended up with no weather data for a couple of days because the Ninja Block couldn’t talk to the cloud platform. I ended up hacking at it and the result was this very simple Node.js application as a replacement for their software. It always felt a bit crap, though, because if the hardware itself died I’d be stuck; yes, it was all built on “open hardware” but I didn’t know enough about it all to be able to recreate it. I’d ordered a Raspberry Pi 3 in June last year, intending on replacing the Ninja Block and it’s sometimes-unreliable wireless temperature sensors with something newer and simpler and hard-wired, but I found there was a frustrating lack of solid information regarding something that on the surface seemed quite simple.

I’ve finally gotten everything up and running, the Ninja Block has been shut down, and I’ve previously said I’d write up exactly what I did. So here we are!

Components needed

  • Raspberry Pi 3 Model B+
  • AM2302 wired temperature-humidity sensor (or two of them in my case)
  • Ethernet cable of the appropriate length to go from the Pi to the sensor
  • 6x “Dupont” female to either male or female wires (eBay was the best bet for these, just search for “dupont female”, and it only needs to be female on one end as the other end is going to be chopped off)
  • 1.5mm heatshrink tubing
  • Soldering iron and solder
  • Wire stripper (this one from Jaycar worked brilliantly, it automatically adjusts itself to diameter of the insulation)

Process

  1. Cut the connectors off one end of the dupont cables, leaving the female connector still there, and strip a couple of centimetres of insulation off.
  2. Strip the outermost insulation off both ends of the ethernet cable, leaving a couple of centimetres of the internal twisted pairs showing.
  3. Untwist three of the pairs and strip the insulation off them, then twist them back together again into their pairs.
  4. Chop off enough heatshrink tubing to cover the combined length of the exposed ethernet plus dupont wire, plus another couple of centimetres, and feed each individual dupont wire through the tubing (there should be three separate bits of tubing, one for each wire).
  5. Solder each dupont wire together with one of the twisted pairs of ethernet cable, then move the heatshrink tubing up over the soldered section and use a hairdryer or kitchen blowtorch to activate the tubing and have it shrink over the soldered portion to create a nice seal.
  6. Repeat this feed-heatshrink-tubing/solder-wire/activate-heatshrink process again but with the cables that come out of the temperature sensor (ideally you should be using the same red/yellow/black-coloured dupont cables to match the ones that come out of the sensor itself, to make it easier to remember which is which).
  7. Install Raspbian onto an SD card and boot and configure the Pi.
  8. Using this diagram as a reference, plug the red (power) cable from the sensor into Pin 2 (the 5V power), the yellow one into Pin 7 (GPIO 4, the data pin), and the black one into Pin 6 (the ground pin).

AdaFruit has a Python library for reading data from the sensor, I’m using the node-dht-sensor library for Node.js myself. You can see the full code I’m using here (it’s a bit convoluted because I haven’t updated the API endpoint on my website yet and it’s still expecting the same data format as the Ninja Block was sending).

I’d found a bunch of stuff about needing a “pull-up” resistor when connecting temperature sensors, but the AM2302 page on adafruit.com says “There is a 5.1K resistor inside the sensor connecting VCC and DATA so you do not need any additional pullup resistors”, and indeed, everything is working a treat!

Better Raspberry Pi audio: the JustBoom DAC HAT

I decided that the sound output from the Pi’s built-in headphone jack wasn’t sufficient after all and so went searching for better options (a DAC—digital-to-analog converter).

The Raspberry Pi foundation created a specification called “HAT” (Hardware Attached on Top) a few years ago which specifies a standard way for devices to automatically identify and configure a device and drivers that’s attached to the Pi via its GPIO (General Purpose Input/Output) pins. There’s a number of DACs now that conform to this standard, and the one I settled on is the JustBoom DAC HAT. It’s a UK company but you can buy them locally from Logicware (with $5 overnight shipping no less).

The setup is incredibly simple: connect the plastic mounting plugs, attach the DAC to the Pi, then edit /boot/config.txt to comment out the default audio settings and add three new lines in, then reboot.

To say that I’m impressed would be an understatement! I didn’t realise just how crappy the audio from the Pi’s built-in headphone jack was until I’d hooked up the new DAC and blasted some music out. I’m not an audiophile and it’s hard to articulate, but I’d compare it most closely to listening to really low-quality MP3s on cheap earbuds versus high-quality MP3s on a proper set of headphones.

If you’re going to be hooking your Pi into a good stereo system, I can’t recommend JustBoom’s DAC HAT enough!

Raspberry Pi project: AirPlay receiver

I bought a Raspberry Pi almost exactly a year ago, intending on eventually replacing my Ninja Block and its sometimes-unreliable wireless sensors with hardwired ones (apart from the batteries needing occasional changing, there’s something that interferes with the signal on occasion and I just stop receiving updates from the sensor outside for several hours at a time, and then suddenly it starts working again). To do that, I need to physically run a cable from outside under the pergola to inside where the Raspberry Pi will live and I don’t really want to go drilling holes through the house willy-nilly. I want to eventually get the electrician in to do some recabling so I’m going to get him to do that as well, but until then the Pi was just sitting there collecting dust. I figured I should find something useful to do it with, but having a Linode meant that any sort of generic “Have a Linux box handy to run some sort of server on” itch was already well-scratched.

I did a bit of Googling, and discovered Shairport Sync! It lets you use the Raspberry Pi as an AirPlay receiver to stream music to from iTunes or iOS devices, a la an Apple TV or AirPort Express. We already have an Apple TV but it’s plugged into the HDMI port on the Xbox One which means that to simply stream audio to the stereo we have to have the Xbox One, TV, and Apple TV all turned on (the Apple TV is plugged into the Xbox’s HDMI input so we can say “Xbox, on” and the Xbox turns itself on as well as the TV and amplifier, then “Xbox, watch TV” and it goes to the Apple TV; it works very nicely but is a bit of overkill when all you want to do is listen to music in the lounge room).

Installing Shairport Sync was quite straightforward, I pretty much just followed the instructions in the readme there then connected a 3.5mm to RCA cable from the headphone jack on the Raspberry Pi to the RCA input on the stereo. It’s mentioned in the readme, but this issue contains details on how to use a newer audio driver for the Pi that significantly improves the audio output quality.

The only stumbling block I ran into was the audio output being extremely quiet. Configuring audio in Linux is still an awful mess, but after a whole lot of googling I discovered the “aslamixer” tool (thanks to this blog post), which gives a “graphical” interface for setting the sound volume, and it turned out the output volume was only at 40%! I cranked it up to 100% and while it’s still a bit quieter than what the Apple TV outputs, it doesn’t need a large bump on the volume dial to fix—there’s apparently no amplifier or anything on the Raspberry Pi, it’s straight line-level output. The quality isn’t quite as good as going via the Apple TV, but it gets the job done! I might eventually get a USB DAC or amplifier but this works fine for the time being.

On macOS it’s possible to set the system audio output to an AirPlay device, so you can be watching a video but outputting the audio to AirPlay, and the system keeps the video and audio properly in sync. It works extremely well, but the problem we found with having the Apple TV hooked up to the Xbox One’s HDMI input is that there’s a small amount of lag from the connection. When the audio and video are both coming from the Apple TV there’s no problem, but watching video on a laptop while outputting the sound to the Apple TV meant that the audio was just slightly out of sync from the video. Having the Raspberry Pi as the AirPlay receiver solves that problem too!

UPDATE: Two further additions to this post. Firstly, and most importantly, make sure you have a 5-volt, 2.5-amp power supply for the Raspberry Pi. I’ve been running it off a spare iPhone charger which is 5V but only 1A, and the Pi will randomly reboot under load because it can’t draw enough power from the power supply.

Secondly, the volume changes done with the “alsamixer” tool are not saved between reboots. Once you’ve set the volume to your preferred level, you need to run “sudo alsactl store” to persist it (this was actually mentioned in the blog post I linked to above, but I managed to miss it).