Cheapest ever 433 Mhz transceiver for PCs

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433 switch 1.JPG

Several shops offer cheap 433Mhz RF remote controlled switches. These are nice, but they introduce a few problems as well:

  • The cheap ones all have different remote controls, using different protocols;
  • We want to control our light switches from our PC, not from the remote controls.

What if you could use a PC to transmit all the RF signals to switch on- and off all devices, and have that PC learn the signals from the remotes? As it turns out, this is very easy to do. Most of these remote control devices use a digital serial protocol of about 1kbaud superimposed on the 433 Mhz signal. That 1kbaud falls well within the range of what a PC sound card can generate or sample. Furthermore, receivers that extract the digital signal and transmitters that turn the digitial signals into an RF signal are readily available.

This page describes how to create an absurdly low-cost USB 433 Mhz transceiver that can serve as a good starting point for your home automation projects.

Contents

Hardware

To build this, all you need is a 433Mhz transmitter/receiver pair (or 315 Mhz if appropriate in your country) and a usb audio-"card" (links are to ebay search results). All in all, with the ever decreasing prices for these devices this is about $2,- in materials. Now give yourself a dollar for the soldering work that is about to happen.

Next: some soldering. The result will surely not win any beauty contests and may constitute criminally inhumane treatment of innocent electronics (AKA 'CITIE', AKA 'hack'):

Receiver (top) and transmitter (bottom) soldered to a USB audio card
Cheapl diagram.png

Obviously, we need to connect the transmitter to the audio output and the receiver to the mic input. The wiring is as follows for the transmitter (see this page for terminology). Bold words refer to pins on the transmitter:

  • GND is connected to the sleeve contact on the socket.
  • data is connected to the tip contact (left audio channel)
  • Vcc to USB 5V, the red wire in the picture.

For the receiver:

  • GND to sleeve
  • Vcc to USB 5V, red wire
  • RxD to tip (for mechanical stability only) and to a bypass (orange wire).

The RxD connection takes some explanation. These USB sound cards have a mic input connector which normally routes the signal throug a high pass filter. Since we want the 'raw' data from the receiver without filtering, we need to bypass that filter. Again, this is not too difficult: we need to find the filtering capacitor. The capacitor in question is found with a volt meter by looking for one that is directly connected to the input (the tip contact). This is the capacitor we need to bypass, and in this particular case I've done it by soldering a wire from the tip connector to the other side of the capacitor.

Note that the exact capacitor to bypass will vary from one soundcard to another. I have found two variations in two different batches bought on ebay. you should definitely look for the right capacitor using the multimeter method.

Also note that you can still get a decent signal without bypass, i.e. by connecting the receiver input to the mic input line. You'll notice that the high-pass filter will push the signal "downwards" to become symmetrical around zero.

Software

Learning remote control signals

After connecting our Frankensteinian contraption, our PC will have another audio 'card'. Mine identifies itself als "Generic USB Audio Device". This device can now be used to record remote signals as if they were audio and to play them back as audio files as well.


Audacity with remote signal.jpg

I use Audacity to record keystrokes on the remote. It's as easy as pressing the Record-button in Audacity, pressing the key that we want to record on the remote and then pressing the stop button. You should see that Audacity has recorded a few repetitions of the same sequence—the image above zooms in on two such repetitions. You can now verify that the signal was correctly recorded by playing it back to our contraption. The switch should behave as if you pressed the key on the remote. Playing back only one occurrence of such a sequence usually doesn't work, so I try to find the minimum amount of repetitions that reproduce the switching behaviour. Then I save those repetitions in a single wav-file. Now repeat the procedure for the other keys of a remote and create a wav-file for each key. You should end up with wav-files with names like onA.wav, offA.wav, onB.wav, etc.

Quick-'n-easy

If you don't like to spend time on configuring existing home automation software and just want to switch something from a command line or script, you only need a command line audio player that is capable to play your sounds to a given audio card. On Windows, vlc might work (I haven't tried). On linux with ALSA installed, aplay fits the bill exactly. In your script, call aplay with the name of the keystroke-file and specify the soundcard, e.g.:

   aplay -D plughw:2,0 onA.wav; 

Fun project: The script below—when run in the background—will switch off a light when my Ubuntu (gnome) screen saver locks the screen and it will switch it on when I log in again. The light is a halogen spot that lights up the wall behind my monitor...

dbus-monitor --session "type='signal',interface='org.gnome.ScreenSaver'" | ( 
	while true; 
	do 
		read X; 
		if echo $X | grep "boolean true" &> /dev/null; then aplay -D plughw:2,0 offC.wav; 
		elif echo $X | grep "boolean false" &> /dev/null; then aplay -D plughw:2,0 onC.wav; 
		fi 
	done )

Integration with home automation software

Instead of writing all the home automation software ourselves, we'd rather create a plugin to integrate this hardware with existing home automation. There seems to be no real widespread, open (and programming language agnostic) protocol for home automation devices (this appears to be a general issue with the internet of things). There is an open protocol though, which is xPL and there is a home automation suite that supports it, domogik. Therefore it seemed like a good idea to implement an xPL bridge for our contraption, and that is exactly what we've done. The source code for our xPL bridge is available on GitHub. This bridge is a small program that runs on linux only (think Raspberry Pi/Beaglebone) and that listens to X10 commands from an xPL hub. This is fairly easy to set up with domogik and it works, as can be seen in the short video below.

Comments? Questions?

nopreview
nopreview
16 September 2013 22:27:43
You sir, redefined 'genius'. Well done.
false
nopreview
10 December 2013 14:33:46
This is probably the best way to start with home automation. There are alarm systems that use these cheap sensors that work on 433mhz, from PIR, through smokede detectors, window/door openning sensors, tilt sensors , flooding sensoers etc etc :-) Just ordered all parts - great idea / post!
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nopreview
10 December 2013 19:46:52
Thanks! I noticed some interest from home automation folks (Domogik specifically), so yesterday I decided I'd go and try to make a linux xPL implementation for these devices. It'll take some time before anything is done though, working my way through ALSA documentation.
false
nopreview
14 January 2014 09:31:53
I have been looking for a simple method to transmit to a PT2272, I was going down the SDR route, but this is simple and elegant, requires no reverse engineering, no GNURadio - 3 bucks in parts - excellent hack!
false
nopreview
20 January 2014 21:24:11
Can this be used as a cheap replacement for RFXCOM RFXtrx433 or TellStick Duo to receive data from ex. a weather station?
false
436f6d6d656e743a4368656170657374206576657220343333204d687a207472616e7363656976657220666f72205043732031333930323533303530313531
nopreview
20 January 2014 23:00:48
I'm not familiar with the RFXCOM or TellStick products, but theoretically, yes. Note that the cheap receivers on ebay are typically not such high quality (transmitters are reasonable), which is fine if you're just using them to record a remote control, but not if you're trying to receive signals from a weather station outside. You should really move to the $8 price point for a receiver with a crystal. The other thing is software: I assume that these products come with some pre-bundled protocols for different systems. For the devices on this page you're on your own (or maybe a bit less once I finally finish the xPL bridge).
false
nopreview
23 January 2014 15:41:15
Wow, very smart project!

May the same procedure (soldering and recording-replaying) be working with an arduino IR kit, too?

I wonder if there is a simple and resource friendly way to convert, store and replay the signals - those seem to be binary. If I find out some answer I'll post it here.
false
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nopreview
24 January 2014 00:35:10
Most arduino IR kits I see are passive only: they have a sensor, but no way to transmit signal from the arduino/usb soundcard. But in essence, yes, this could be done for IR as well as long as you have a way to translate the IR signal into a voltage on the input and to translate a voltage at the output to an IR-output. The signals are indeed binary: officially it is "amplitude modulated", but the amplitude is always either 0 or max. I think that a good compact way to store signals may be the way LIRC (www.lirc.org/) does this: First the waveform for a "1" and for a "0" symbol is described (in terms of how long the signal is up and how long it is down) and then each signal is described as a series of 1s and 0s, binary encoded.
false
nopreview
8 June 2014 02:49:24
Hi

I am interested in making a wireless audio connection between an xlr dynamic microphone and a 3.5mm plug for input into a camera or computer.

Could one use the principles you have shown here to use a usb sound card to take some audio, convert it into a digital stream,. One would then tap this and feed it into the wireless transmitter. The stream would be received by the digital receiver and converted into audio by another cheap usb sound card.

Could this be done?
cheers Peter p.berrett@optusnet.com.au
false
nopreview
9 June 2014 18:02:53
Danny deleted this comment on Mon, 09 Jun 2014 20:34:55 +0000
true
nopreview
9 June 2014 18:05:40
Hello! Show please read more connection cards.
false
436f6d6d656e743a4368656170657374206576657220343333204d687a207472616e7363656976657220666f72205043732031343032313935373633363430
nopreview
9 June 2014 20:43:59
Erhm. Digitizing an analog audio stream is exactly what the ebay-bought usb devices do. On the cheap 433 Mhz transmitters you could reach a maximum bitrate of about 1000 bps. This is barely enough to send encoded speech.
false
436f6d6d656e743a4368656170657374206576657220343333204d687a207472616e7363656976657220666f72205043732031343032333337313338353735
nopreview
14 June 2014 23:11:45
I'm not sure I understand what you're asking. Do you want to see more connection diagrams? Trouble is that connections may differ for each USB sound card. Most connections are obvious and direct (audio out to transmitter data, 0V, 5V). The only non-obvious connection is between the receiver data line and the "bypass".

You can actually start by connecting the receiver data line to the microphone input. This should work, but does not give the nice square input signals as shown in the audacitiy picture above. The way to get unfiltered input signals is by using a multimeter to find the capacitor that is directly connected to the mic input and bypassing that one.
false
436f6d6d656e743a4368656170657374206576657220343333204d687a207472616e7363656976657220666f72205043732031333930323538383437343936
nopreview
27 July 2014 21:39:18
Could you probably point a particular (or more) "$8 price point... with a crystal" receiver on ebay?
I'm asking this because I've built the Frankenstein of the article, which is working great along the specifications (good to send, good for recording but not good for receiving), but I'd like to step level towards receiving. I've found many receivers however with better (?, -108/-112/-114/...dBm sensitivity) specs, but have not enough domain knowledge (euphemism; I am far from being an electrical engineer) to choose with confidence.
Thank you in advance!
false
436f6d6d656e743a4368656170657374206576657220343333204d687a207472616e7363656976657220666f72205043732031343036343937313537393731
nopreview
27 July 2014 22:32:36
Ah, You caught me there... I'm also far from being an electrical engineer. Some buying advice is on this webpage:
http://www.martinmelchior.be/2013/12/golden-rules-when-buying-rf-receiver.html
However, that doesn't point you to any ebay results. I think the first ones you get when you search for "super heterodyne 433 ASK" are probably better, but I really couldn't tell you what kind of quality you'd be getting or if these are suitable at all. The following one has a "QC Passed" sticker, so it must be good right :-)
www.ebay.com/itm/Super-heterodyne-ASK-OOK-RF-Wireless-Receiver-Module-433MHZ-112dBm-/280936648330
But again, I don't know if it is at all suited...

One of the first receivers I ordered from ebay happened to be one without the variable inductor (variable inductor ones are the low quality ones) and it was around the $8 price point. Unfortunately, this was more than 3 years ago, so it's not in my ebay history anymore...
false
nopreview
21 August 2014 09:37:27
A while back I wrote a software decoder that decodes a lot of 433 MHz transmissions from remote controls, weather stations and such. I found it useful for exploring what's out there on this radio band. It should work fine with your hardware:

http://www.tablix.org/~avian/blog/articles/am433/
false
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