Sources of 433/434Mhz transmissions? - Page 1
Forum section: electronics / projects

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 14, 2010, 07:33:04 am
content: I've been working on a project that involves receiving 434Mhz transmissions from some consumer equipment.To achieve this, I bought one of these:http://www.coolcomponents.co.uk/catalog/product_info.php?products_id=239and one of these:http://www.coolcomponents.co.uk/catalog/product_info.php?products_id=238I first got them maybe 6 months ago, and briefly tested them both. The receiver periodically showed bursts of data, but on the whole was quiet. I presume this data came from outside my house.The transmitter also works well - if you use 13V (outside of spec), but make sure you keep the duty cycle low, you seem to be able to put out an awful lot of power.Anyway, I moved house a few months back, and have just set up my desk to work on the project again. On setting up the receiver and connecting it to the scope, I saw continuous random data...So I set up the transmitter, and found that this overwhelmed the other signal. It also confirmed that the receiver was at least partially working.I then started turning off anything wireless that could be on 434Mhz - cordless mouse and keyboard, wireless thermostat, etc. Still the random signal.I then started switching pretty much everything off, monitors, TVs, PCs, laptops. Still the random signal.I then tried putting the receiver in a biscuit tin to shield the signal, but found that the transmitter (even at 1.5V supply) could still get through...So, question is, what in the UK operates of 434Mhz? The data is a continuous stream, with some kind of repeating segment every 6-ish seconds (consists of a long 0 with some alternating 0/1 at the end, then a burst of 1).There is the possibility that the repeating data is real, and the rest is noise as a result of some kind of problem with the receiver as well.Anyone got any ideas?

author: DJPhil
author_rank: Frequent Contributor
timestamp: June 14, 2010, 08:08:44 am
content: There's a decent (if slightly older)list herefor UK spectrum allocation. This is the area of concern:QuoteAlso useful,UK Frequency Allocation Table for 2008(still in effect I believe), and a page with links generally describingspectrum use in the UK and Europe.There's a lot of possibilities, as it's a mostly unregulated band. I'd take a walk around the house and see what's around within line of sight. You might be in the path of a repeater, or simply next door to someone with one of those boxes that takes statistics on what you're watching.Hope that helps a little.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 14, 2010, 09:03:21 am
content: Thanks DJPhil - those docs do suggest it could really be anything!I've just also noticed that ebay is full of video senders on this frequency... the nearest shop does appear to have wireless CCTV cameras, there is a council temporary camera on a lamppost nearby...I'm wondering if making a small directional antenna might help track it down.

author: TheDirty
author_rank: Frequent Contributor
timestamp: June 14, 2010, 01:17:38 pm
content: Quote from: shafri on June 14, 2010, 12:40:58 pmThese are 'dumb' ASK receivers/transmitters. They require DC balancing. There are certain instances where you might get away with just transmitting un-encoded serial data over them, but it's not reliable. For the price of a transmitter and receiver pair, you can get a single FSK transceiver for less money and have a far more reliable RF link (though you still need to encode for DC balance).I've used these before with my own library and using a modified version of the Arduino library 'VirtualWire'. VirtualWire is a nice library that includes 2byte->3byte encoding. It was easy to port to another microcontroller by just attaching the ISR to a timer and modifying the hardware specific I/O. You can get source code for VirtualWire by googling it.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 14, 2010, 05:56:38 pm
content: These modules are very simple, especially the transmitters. The DC bias is a problem, one which even commercial designs quite often ignore.The reason I'm using them is that I specifically want to receive transmissions from a system using 434Mhz ASK - it actually uses the same receiver IC (though the one on these particular boards is cloned).I had a poke about just after the IF mixer on the board and there is a signal there - just not sure what it is. The comparator after it seems to be maybe a touch sensitive, so that could be where the problem lies.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 14, 2010, 08:09:07 pm
content: DC bias is something that comes up a lot with the transmission of data.If you transmit a stream of 1/0 with a duty cycle of 50%, the signal is balanced i.e. the average is 0.5.If you transmit more 1 than 0, the average moves up.If you transmit more 0 than 1, the average moves down.You can use an number of encoding schemes e.g. Manchester encoding, which uses transitions rather than levels to ensure that the number of 1s and 0s are balanced, and hence there is no DC bias.Why is this important though? The primary reason is because of the AC coupling and RC filters used in the transmitters and receivers. Most of these ASK receivers have a comparator with an RC filter attached to change the received signal strength into a 1 or 0. The data output itself changes the threshold level - so if the 1s and 0s aren't balanced, you end up making the threshold either too low or too high, and risk receiving noise or nothing!

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 14, 2010, 09:39:18 pm
content: I think that's my issue - there's just a lot on this frequency. It was pure chance that at my old house it was near silent, and here there is a lot.I'm interested in finding out what it is now. Contemplating just sending a random signal using the transmitter and observing to see if anyone tries to find anything nearby.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 14, 2010, 09:44:24 pm
content: it could be a strong transmitter from several miles away

author: Zad
author_rank: Super Contributor
timestamp: June 15, 2010, 04:34:19 am
content: More than several miles, it could be several hundred miles. The 70cm band is used by amateur radio, including many satellites in low earth orbit.http://en.wikipedia.org/wiki/70-centimeter_bandImagine how joyous it feels to have developed expensive and sensitive equipment for that band, only for it to be inundated by cheap, nasty, noisy transmitters

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 15, 2010, 05:37:11 am
content: My old house is less than two miles away - so I'm fairly certain this is something much more localised than a satellite!

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 16, 2010, 07:10:26 am
content: Interesting. I used a microprocessor to send a fairly random stream of data overnight, and now the other transmission has stopped.We'll see if it comes back on during the day!

author: ArtemisGoldfish
author_rank: Regular Contributor
timestamp: June 17, 2010, 06:34:17 am
content: Quote from: Zad on June 15, 2010, 04:34:19 amYeah, could be anyone in that area of the amateur band. I was just running a 150W transmitter up near there a few days ago. ISM bands are ripe for interference.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 17, 2010, 07:09:53 am
content: Well, it came back on yesterday evening, fairly late at about 2300.So I've just had a wander around with the receiver and a very crude RSSI indicator... and it's either coming from my house or one of my two neighbours. Certainly going solely by this, it looks like it is coming from my house - but it doesn't stop, even when I kill the power to my house. So it's either next door, or I have something battery powered in my house doing it.

author: DJPhil
author_rank: Frequent Contributor
timestamp: June 18, 2010, 03:02:56 am
content: Quote from: cybergibbons on June 17, 2010, 07:09:53 amThis made me think of something, though it's unlikely.Where I live they started a few years back changing out residential electrical meters for more modern ones that they could 'read' by driving up the street and using a transceiver to poll and record meter readings. It's cheaper, and avoids problems with the large dogs everyone around here owns. I suppose it's possible that this could be done with water or gas as well. If it's something like this you could rule it out by calling the utility or maybe just having a look at the meter(s).The only other things I can think of are home alarm systems and various 'cordless' electronics like computer mice/keyboards.Hope that helps.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: June 18, 2010, 06:24:54 am
content: Last night I periodically jammed the signal, followed by listening out for the same signal. It went off at 0300 and is still off. Very late for someone to manually turn it off!We're having smart readers rolled out in the UK - these use GPRS to communicate. I'm not sure if there have been any that transmit locally - I might have a dig about. I know our meters are just very standard however.Interestingly, the original project is to do with home alarms. I do think this signal looks alarm like - a signal every 6 seconds, possibly a PIR or something like that? I'm just struggling to see how it turns off now and then as the alarms I have require that the batteries are removed to stop sending the signal.I think it's too high powered to be a cordless mouse - I do have a 434Mhz one sat here, and the signal it puts out is very weak.Anyway, the experiments continue! I'm going to just listen out until it pops back up again today. I would like to try and decode the signal, but one problem is that the AGC on the receiver takes too long to adjust when the signal starts - you need a much longer initial pulse to get it working. I might try and change it to have manual gain control.

author: Psi
author_rank: Super Contributor
timestamp: July 06, 2010, 11:01:52 am
content: Got any CFL's in your room/house?Some of them (cheap brands) tend to put out a crazy amount of noise, my old oscilloscope used to pick it up all the time.Had to build/install some LED room lighting to get away from it.