Hacking 433Mhz support into a cheap Carbon Monoxide detector

Skill level:  Easy

My home automation systems use two mechanisms for communication:  Ethernet (both wired and wireless) and 433MHz OOK radio.

433MHz transmitters are readily available and are cheap but unreliable.  Wifi enabled MCUs such as the ESP8266 are also cheap (coming in at around the same cost as an Arduino clone, a 433MHz transmitter and a bag of bits to connect them together), they are reliable enough but extremely power hungry.  If I can plug a project into the mains then I’ll use an ESP8266 and a mobile phone charger for power, if the project needs to run off batteries then a 433MHz equipped Arduino is the way I’ve gone.

Like most people playing with 433MHz radio I found reliability and range of the radio link to be super flaky.  I’ve finally got a more-or-less reliable set-up:

  • A full wave dipole antenna at the receiver
  • A high quality receiver from RF Solutions in place of the cheap ones which are bundled with transmitters. A decent receiver on eBay
  • A big capacitor on the transmitter.  I saw the frequency and amplitude drifting massively during transmission.  Adding a 470µF cap helps.  Allow time for the cap to charge and the oscillator to stabilise, a few seconds delay seemed to do the trick.
  • Using the RCSwitch library on the transmitter:
    • RCSwitch mySwitch = RCSwitch();
    • mySwitch.setProtocol(2); // Much longer pulse lengths = much better range?
    • mySwitch.setRepeatTransmit(20); // Just brute-force it!

With this setup I can get receive a 24bit number from an Arduino running off 2 AA batteries and a coiled 1/2 wave antenna from about 5 meters indoors through walls.  That’s still poor, but it does the job.  Increasing the voltage to the transmitter would probably help.

Once you have a reliable 433MHz receiver setup then you can also buy off the shelf 433MHz enabled home automation gizmos like this smoke alarm or these door sensors.  They have a set of jumpers inside where you can set an ID, which is essentially the same 24bit number that RCSwitch lets you transmit.  For what it’s worth I also have kite-marked smoke detectors in my house, but from the testing I’ve done with a bit of smoldering paper the cheap imports work just fine.

I couldn’t find a cheap Carbon Monoxide which also has 433MHz support so I thought I’d quickly hack one together out of this Carbon Monoxide detector and an Arduino clone and 433MHz radio:

CO Alarm inside











You can barely notice it!








It’s certainly untidy, but it does the job.  If I had PCB facilities at home I’m fairly sure it could be made to fit inside the alarm, along with some more holes in the case for ventilation.

The premise is simple enough.  The Arduino is powered by the 3v3 regulator on the CO alarm PCB.  The cathode of the red alarm LED is connected to pin 2 of the Arduino as an external interrupt.  When the pin goes low the Arduino wakes up and sends it’s 24bit ID number over the radio which is picked up by the receiver which sends an SMS alert, switches the boiler off, etc.  I’ve connected the radio transmitter to directly to the 3 x AA batteries (4.5 volts) via a transistor which is switched by a pin on the Arduino.  In standy-by mode the additional equipment draws a fraction of a milliamp and so I’m not worried about draining the batteries faster.

As with the smoke alarms, this is not my only source of Carbon Monoxide detection.  I’ve yet to test it’s sensitivity.  This is considered to be a “well, if it works, and it turns the boiler off automatically then it’s certainly worth a go, but I’m not relying on it” project.

My first 10 years with Ubuntu


Today I have had a Launchpad account for ten years!

I got started out on this road around 1992.  I remember the day Stuart got a PC and installed Minix on it.  That box was biege, naturally, was about 3 feet square and constructed from inch thick iron plate.  Minix was totally alien when compared to the Acorn MOS and RISCOS powered machines I’d used until then, and absolutely intriguing.

A few years later at university I encountered VAX/VMS and Sun SPARCstations and The Internet and Surfers and Mozilla and a Gopher connected Coke machine.

Then out into the big wide world of work and run-ins with AS400 and RS/6000s running AIX.  During this time I started seeing more and more Red Hat in places where there once would have been the more established players, providing email and web servers.  The fascination with *nix was always there and I started using Red Hat at home for fun.

I quickly ran into frustrations with RPMs and Stuart, always a source of wisdom, suggested I try Debian.

Dpkg made my life a whole lot easier and I started using Debian as my default OS for everything. Pretty soon after that I found myself compiling kernels, modules and software packages because I needed or wanted something in a newer version.  Coupled with the availability of cheap unbranded webcams, sound cards, network cards, TV cards etc and a strong desire to make these things work with Linux meant that I had found a wonderful way to stay up until 4 in the morning getting more and more frustrated.  The phrase “I’m going home to play with the kernel” was frequently questioned by my boss Jeremy.  I wanted these things to work but was endlessly faffing about trying to make it happen.

Better call Stuart.

“You should try this new Debian based distribution called Ubuntu” he said.

So I did, and it just worked.  A box fresh kernel with all the goodies I needed already compiled in and an up-to-date GNOME desktop (I’d set my allegiances before trying Ubuntu so this was another tick in the box), not forgetting one of the brownest themes known to man.

And that was that.  Ubuntu worked for me and I was immediately a fan.

And here I am today, 10 years later, still running Ubuntu.  My servers run Ubuntu, all the desktops in my house run Ubuntu, I have an Ubuntu powered phone and soon I’ll have an Ubuntu powered Mycroft with which I’ll be able to control my Ubuntu powered things while wearing my Ubuntu T shirt and drinking tea (should that be kool-aid?) from my Ubuntu mug.

I salute my Ubuntu brothers and sisters.  Thanks for making all of this possible.