The past few weeks have gone by in a blur of insomnia, stress and a bunch of random entropy that has triggered my penchant for radical simplification, so I turned off nearly all notifications on work apps, set some recurring reminders to go offline earlier in the day, and decided to spend some time tying up loose ends (as well as building Docker containers to run R, but that’s another story).

In summary, I’m exhausted, and the amount of typos in the first version of this post is ample evidence of that. So I’m cutting back and doing satisfying little hacks to relax.

There Can Only Be OneDrive

Even though this site is still managed via Dropbox (instant publishing and trivial updates are addictive), I’m planning to change that someday.

The trigger, ironically, was their move to restrict filesystem support on Linux. There’s an ingenious fix that injects a different filesystem check into the client, but since their CLI daemon is a nightmare to keep running (it insists on taking up 100% CPU without apparent reason now and then) and I’ve had an Office 365 family plan since well before joining Microsoft, the extra expense of paying for Dropbox and not being able to use it properly on two of the machines where it actually made a difference (my VM and my Linux laptop) doesn’t really pay off for me.

So I’m preparing to cancel it next Summer, and trying out abraunegg/onedrive, which has the great side benefit of being easy to run inside Docker and get running on ARM–although, to be fair, it being written in D, of all things, is a major turn-off.

Someone really ought to rewrite the entire thing in Go because it’s single-threaded and slow, but it seems promising for my use cases, and this post was actually drafted on my Linux laptop while it synced my Arduino development tree across.

Limiting OMS Agent CPU Usage on Ubuntu 18.04

Another thing that has been bugging me is the amount of CPU omsagent uses in my Azure VMs, so I decided to deal with that too by using raw cgroups to limit its CPU usage to roughly 10%.

Limiting CPU usage for a single user is relatively trivial (but hard to piece together for Ubuntu 18.04), so after installing cgroups-bin I created a few files that create a limitcpu group that limits usage to around 10% of CPU shares, and that apply that policy to anything running under the omsagent user:

$ cat /etc/cgconfig.conf
group limitcpu {
  cpu {
    cpu.shares = 100;

$ cat /etc/cgrules.conf
omsagent cpu limitcpu/
*:dropbox cpu limitcpu/

$ cat /etc/rc.local
cgconfigparser -l /etc/cgconfig.conf

You can check if it’s working by poking around inside /sys/fs/cgroup and checking what PIDs are included inside the limitcpu group. Limiting RAM may require a bit more tweaking, but I did’t see a need for it just yet.

This was a bit of a surprise because I had no idea rc.local was still actually usable under systemd (it’s run at startup if it exists and marked as executable), but there you go. There are still bits of the Azure agents running as root, but at least now I don’t have the bloated Ruby components taking up resources.

The World’s Most Roundabout Doorbell Extender

I’ve got a weirdly-shaped flat with reinforced concrete walls, which besides being a bane for wireless coverage is also a problem for package deliveries, since it’s hard to hear the doorbell when you’re vacuuming or cleaning around the house. Over the past few weeks I came to appreciate that first hand as I worked from home for a while, and decided to do something about it.

My doorbell is a classic Friedland Type 4, which means it’s about the most analog thing in the house except for a couple of clocks, and getting a modern Honeywell doorbell “converter” kit is unaccountably hard (it’s been in and out of stock).

Plus I have a feeling a single extra doorbell isn’t likely to be enough, and it’s more fun to try to “fix” things yourself.

Sensor Hardware

I started by looking at building a powered ESP8266 sensor that could work off the 8-12VAC it’s powered with and was putting together a rectifier bridge when it hit me that the doorbell has a coil and that I could do something that wasn’t going to have to reboot at every bell push.

At first I thought I didn’t have any magnetic reed switches handy, but then the second round of that particular epiphany hit me–I grabbed one of my Xiaomi Aqara contact sensors, removed it from its casing, and taped it directly to the coil.

This Xiaomi marketing image is better than the crummy photo I took of the naked sensor

Sure enough, I now had a Zigbee-enabled doorbell. Which, by the magic of MQTT, I soon tied to HomeKit, after I fixed the irritating quirk of it sending out multiple messages a second.

Why? Because doorbells run on AC, and thus the reed switch actually vibrates every time the doorbell rings1. So I had to ignore new events after the first within a given time window (easily done with a trigger node in Node-RED acting as a software debouncer), and then it was just a matter of turning on notifications on the Home app (which are instant on my Mac).

Remote Buzzers

That having been dealt with, I started working on the hardware to cater to resident humans–a set of small, noisy devices to scatter about the house. Since I have a bunch of ESP-01S devices2 left over from my initial shenanigans with Azure IoT and a handful of passive buzzers, those were the obvious choice, although it doesn’t really feel like a finished solution and I’d have preferred something able to carry a tune:

The simplistic approach

The buzzer is driven by GPIO2 going low, and the 100Ω resistor limits the current so as to not fry the ESP-01S. I should have made it something like 200Ω to keep the sink current under 12mA3, but the buzzer isn’t visibly rated and my multimeter is MIA, so I chanced it and it has been working fine.

I had to borrow one of my kids' breadboards for this.

The ESP-01S is going to be mounted on a DIP connector on the final board in order to make it easily removable and re-flashable, and that will probably help keep the layout compact by moving the buzzer fully under it.


I initially meant to write everything from scratch, but as I started putting together the Wi-Fi Manager library, an MQTT client and all the rest, I decided I wasn’t going to spend an afternoon in C++ land and took another look at Sonoff-Tasmota, which is what my smart sockets use anyway, and which has support for extra sensors and GPIO control over MQTT.

As it turns out, I was able to flash the ESP-01S with it, set it up as a Generic device and have it control the buzzer as an “inverted” relay, which was just great and saved me a lot of time, because:

  • It has the same MQTT topic structure as my smart sockets
  • I had to do pretty much zero extra work to get it working under Node-RED

Or, that is, it would have saved me a lot of time if both the Arduino libraries for the ESP8266 weren’t riddled with Wi-Fi bugs, which meant I spent around six hours flashing and re-flashing a couple of ESP-01S and a WeMos D1 with various firmware variants built atop three versions of the ESP8266/Arduino core–2.3.0, 2.4.2, and 2.5.0b3, of which only the latter actually worked for me (and isn’t easy to find, since it’s not directly linked from the project page).

I had previously reflected upon this firmware being a work in progress, and in retrospect I’m very happy I didn’t try to upgrade my smart sockets in the meantime. But I might end up doing just that, since it’s perfectly possible to place one of these passive buzzers inside a Sonoff S20 socket and have a dual-purpose smart socket and doorbell extender…

I might even go all the way and try out the baked-in MP3 player.

But first, I want to make sure we don’t miss any deliveries.

  1. I don’t expect that sensor’s battery to last as long as the others, but the trade-off seems reasonable. ↩︎

  2. I also have plenty of WeMos D1 Minis lying around, but since I have micro-USB connectors and discrete voltage regulators aplenty, I thought I’d use those too. But the D1 is a great alternative for this kind of thing, and you can just stick the resistor and buzzer onto it call it a day. ↩︎

  3. Another option would have been to use a 2N7000 MOSFET to drive the buzzer, but I’m all out of those at the moment. ↩︎