RFXCOM 433MHz Receiver - RS232

whiteduck

New Member
RFXCOM 433MHz Receiver - RS232. Build your own???

All,

I am very sorry if this is the wrong forum to ask such a question... please be gentle! :)

OK here goes - I have seen the RFXCOM devices that allow signals from Electrisave, Owl, Weatherstations, X10 RF etc (using the 433MHz frequency) to be picked up and decoded onto a PC via USB but they are not cheap - i.e. 120 GBP upwards.

Bearing in mind the relatively low cost of 433MHz transceivers I wondered if anyone has attempted to replicate the functionality of the RFXCOM but for a much lower price?

So effectively I am looking for something that plugs into RS232 or USB listening on the 433MHz frequency and capturing any data for use within home automation applications.

Appreciate your help.

BR
Ian
 
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BeanieBots

Moderator
Not so much the wrong forum (although not directly PICAXE related) but the wrong section. I've moved your thread from "finished projects" to the active forum where you are more likely to get a response.

433Mhz is a popular comms frequency with PICAXErs but not my expertise so I'll leave it to others to give feedback.
 

lbenson

Senior Member
Not familiar with your particular equipment, but if the transmissions are ascii or in a format which you know how to translate, it should not be hard. My house monitoring system -- http://www.picaxeforum.co.uk/showthread.php?t=11908 -- uses 315mHz wireless. A receiver attached to an 08M picks up transmissions from wireless sensors (6 at this point) and conveys them through RS232-USB to a controller--in this case a NSLU2 NAS device but it could be a PC.
 

hippy

Technical Support
Staff member
Welcome to the PICAXE forum.

433MHz receiver to PC interfaces have been successfully constructed and often need little more than the receiver module, PICAXE and power supply ( plus USB-to-Serial cable for USB ).

Whether they are suitable for what you need will depend upon the protocols used by the transmitter unit(s) and what you need to send to the PC.
 

whiteduck

New Member
Thanks for the help!

Guys,

Thanks for your help ... yes this is what I am after... some sort of receiver on 433MHz connected through picaxe (if required) and then into RS232... I am a novice at electronics so what components would I need to give this a go?

I gather I need something like an 08M picaxe. Then I'd just need the 433Mhz receiver unit (Perhaps the Maplins transceiver pair) - how does this interface with the 08M - does it need other components?

Then I need to program the picaxe to listen (baud/parity) to 433Mhz on the digital input and repeat this data to the serial port? Perhaps a loop to find the right settings.

I have a Linux box here ready and waiting to start seeing the RS232 signals... but not enough time in the day (or rather evening) to play :)

This looks like a good basis:
http://www.hippy.freeserve.co.uk/picaxewf.htm ( is this the same Hippy as above? )

Cheers
Ian
 

lbenson

Senior Member
Wireless communications between picaxes is very much eased by the presence of the "qualifier" on input. What this means is that the sending picaxe includes a few characters in front of the real data in each transmission, e.g., "abc". The receiver requires this string before it accepts input (see the serin command).

Without this you may see a lot of garbage, generated by rf from such sources as mains wiring and fluorescent lights or your neighbor's garage door opener. Then it becomes a chore to pick out meaningful transmissions. This can be controlled if you are doing the sending and receiving. If you are receiving transmissions from third-party senders, it may be harder--although they have had to face the same issues and may have reached similar solutions.

In theory, the hardware is simple--a 433mHz receiver, a picaxe, and a serial-to-USB link to the PC (if you don't have a serial port on the PC). For receiving serial from the PC, you need the same protective setup as on the picaxe programming pin--22K in series and 10K to ground (not a voltage divider).

If you have Lua on your linux box, then if you substitute in your own tty device, the following Lua program will read from the serial device forever (tty port must be set up--typical picaxe serial is 2400 bit rate, 8 bit, 1 stop bit, no parity, no flow control).


Code:
filein = io.open("/dev/ttyUSB0","r")
while true do
  line = filein:read("*l")   -- read a line or nil
  if line ~= nil then 
    print(line)
  end
end
 

manuka

Senior Member
Quick reply-my experiences with these weather stations etc indicates many do NOT use regular serial protocols, & instead interference immune "house codes" (for want of a better term) are common. One past poster (via no doubt his 'scope) found " differential Manchester encoding with interleaved 0 bytes along with data values"...
 

hippy

Technical Support
Staff member
@ whiteduck : Yes, that is my site, somewhat 'from long ago' and in need of updating.

Connecting a receiver to a PC is the easy part, but unless you know the protocol you are looking for you will likely only see garbage received, if anything at all. As manuka points out, different transmitters use differing protocols and the PICAXE has to be programmed to understand that protocol, decode it and turn it into something which the PC can receive and understand - That software effort is partly what makes commercial units expensive.

The first step is deciding what you will be receiving from, then discovering or determining its protocol. Without that, a PICAXE-based converter may be as ineffective as tying a brick to your PC with a piece of string in an attempt to do the job required.
 

BeanieBots

Moderator
I once had a half-hearted atempt at doing what I think you are trying.
That is, read the data sent by an Orion wireless thermometer.

The first problem is finding exactly what frequency the device transmitted on.
I had the advantage of having a 433Mhz receiver from an old wireless alarm system which had an adjustable core so I could 'tune' to any frequency in the band. The output was manitored using a digital storage 'scope.
It was VERY VERY painful excersise because the thermometer only sent a very breif signal once every thirty seconds. After several hours of extreme patience, I found the frequency (pot setting as apposed to Mhz) and had captured several signal patterns along with their corresponding tempertures.

I was never able to correlate the pattern to the temperature and never even convinced myself that the same temperture even always gave the same pattern. I deffinitely say it was not 'standard' serial data. I gave up in the end.

The irony was that the alarm signals were actually very easy to decode. The task was greatly simplified by being able to transmit a signal on demand. The data was a simple 18-bit manchester encoded data stream. Very easy to capture and reproduce the 'master' key-fob signal used to de-activate the alarm. Be warned if you have a cheap wireless alarm!

Good luck with your project. If you do make any progress, please post your findings, I and and many others would be very grateful of any findings you make.
 

whiteduck

New Member
Thanks guys!

Looks more tricky than I first thought regarding the tx/rx protocols and searching around it seems that RFXCOM have done the hard work with decoding such and charge accordingly for their development time... I do appreciate the help though.

BR
Ian
 
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