Wireless Communication on the Cheap?

Any ideas on how I could send a wireless signal from an Arduino to another device (maybe another arduino)? Basically, I want to turn on a remote "alarm" if a certain criteria is met. I saw the Xbee stuff, but it looks a bit pricey for what I'm trying to accomplish.

Thanks, Dale

What range are you after?

Maybe the Nordic 2.4 GHz chips at http://www.sparkfun.com are good enough? They are only us$ 3-4. Some, if not all, are also available on a breakout board.

only 5-10ft or so…I saw This Post and started poking around but haven’t really found what I’m looking for yet. I’ll order the MiRF kit and see how it works out.

Thanks!

Dale

Sparkfun (and others) have cheap (US$10 to $20) RF modules for short range low speed unidirectional communication. One module will be RX only, another TX only, and you'll need one of each. They usually operate on standard ISM frequencies of 315MHz, 434MHz, 900MHz, etc.

If it's very short range an infrared (IR) LED and IR phototransistor pair may even do the trick, but it could require a bit of thought to keep it from being fooled by a TV remote control.

-j

I have inquired with Nordic about their nRF905/nRF9E5. After some emails they told me that they had a test where they got to 500 meter LOS (Line of Sight), at 1 meter from ground (with a 20cm layer of snow). One of there customers (http://www.flarm.com) use this product for collision-warning system for general aviation and recreational flying. They get to 2km. Of course, in the air, you have better LOS than on the ground.

Oh, the nRF905 is only a few bucks and doesn't need many extra parts.

hey

go to the "dollar store". Get one of the wireless doorbell things. Open it up and put a diode across the switch to determine the required switching polarity. Replace the diode with a transistor driven by the Arduino. Hack the other end in similar fashion. wireless for about $5.

D

Good idea on the wireless doorbell solution!

I read some of the datasheet for the Nordic modules from sparkfun. They look very capable, but the interfacing doesn't seem all that simple - SPI is the physical interface, and there's a protocol as well, IIRC.

-j

I ordered a few items from Sparkfun, but I think I’ll go get a doorbell this weekend for kicks :slight_smile:

Thanks,
Dale

Is it possible to hack a usb bluetooth dongle,would there not be a serial modem inside?

yes the diode hack is my fave. Take any consumer grade piece of gear with a button on it, open it up and put an 1N914/1N4148 diode across the button contacts. Once you know the polarity, substitute an optoisolator, like a TIL117 or a 6N138. That way you have complete electrical isolation from the thing you want to control, and can avoid bad words like noise and high-voltage. It works perfectly on Ipods and the like. Check the "Ipod remote" tutorial linked in the playground , written by two of my students, for more details.

For the receiving end you can do the reverse: connect the LED side of the opto to the doorbell speaker, through a 1K resistor. Use the transistor output to drive the Arduino input.

D

PS: @KG4: do you think we need some kind of disclaimer here about the pain and suffering of working with VHF & UHF, for the non-RF savvy?

PS: @KG4: do you think we need some kind of disclaimer here about the pain and suffering of working with VHF & UHF, for the non-RF savvy?

Maybe just a link to the other thread about getting RF modules to work will do. :)

Seriously, one thing that is definitely worth mentioning is the typical inverse relationship between the cost of a type of module and the ease of use/problems encountered. The cheaper the module, the more work you're likely to have making it work, dealing with noise, and all that fun stuff. Bluetooth, at the more expensive extreme, is pretty much plug & play. On the cheap end of the range, those little US$10 to $20 TX and RX modules are noisy, drift in frequency (especially with temperature), have a short range, are more subject to interference, and all that fun stuff. If you try to save some $$ and use the cheap hardware, that frequently means you've got a lot of work to do in software, like implementing your very own packet formats, checksums, maybe even handshake protocols.

And we're not even talking about RF issues yet...

Leads (wires, board traces, etc) can all act as antennae, so your RF transmitter (or any nearby transmitter) can generate noise back on your digital lines. An antenna is a function of wavelength, and the higher your frequency the shorter the wavelength. This means at 70cm/433MHz, some of the traces (and definitely hookup wires jumpered to a breadboard) are a sizeable fraction of a wavelength. 900 MHz (Xbee, etc) and 2.4GHz (bluetooth, Nordic) has an even shorter wavelength. At the extreme, this means you can see weird problems like every time your transmitter fires up your analog input values go insane, or an interrupt gets falsely triggered.

I don't want to scare anyone away, but as Daniel hints, there are whole other worlds of complexities that can rear their ugly heads when you throw RF into your circuit. You're not likely to see RF issues with the unlicensed modules that are commonly available, but it's not always quite as simple as it seems.

-j

I picked up one of these on eBay -- http://cgi.ebay.com/RF-Radio-Controller-Wireless-Control-for-Basic-stamp_W0QQitemZ250174795540QQihZ015QQcategoryZ1200QQtcZphotoQQcmdZViewItem

All I was doing was communicating a button press from a location about a hundred feet away. Worked like a charm. I did tie up four of the analog ins as channel sensors, tho (seemed to be the simplest way to integrate the receiver).

I just received my tx/rx from sparkfun http://www.sparkfun.com/commerce/product_info.php?products_id=7815 All I see is garbage on the serial console. Spec says 4800bps I’ve tried both 2400 and 4800 with no luck. All I’m seeing is garbage. Anyone had any luck??

here’s a picture of my setup. I’ve tried it with 2 units but put them on the same board for debugging with the same result.

http://www.flickr.com/photos/39697400@N00/1795577808/

Thanks,
Dale

What kind of garbage? Completely random or semi-random?

Do a test like send ABABABABAB and see what it comes back as.

I was trying to send an incrementing value .. 1 then wait 10 sec then send 2, etc. I was getting What looked like random garbage...at first, a lot of it. Then there seemed to be ABCDEFGHI .... in there somewhere with each "stanza" I'm at work right now, I'll look at it tonight and paste it in.

I read somewhere, that a capacitor might need to be added into the antenna connection. How would I figure out what size cap to use if this is the case?

Thanks, Dale

Antenna caps are usually in series with the antenna, for coupling and DC blocking. Where did you read about that? It's pretty unlikely that one would be required: if it was required, they would have put it on the board.

D

PS: this is pretty much what KG4 was saying about RF modules: 16.95 for the module and $200 for the labour, although sometimes it works right out of the box.

I found the cap comment here http://dma.ucla.edu/senselab/node/389 I've been out of electronics for many years and have forgotten just about everything I learned in college, so forgive me for the lack of knowledge. Killed off too many brain cells over the last 20yrs :)

Thanks, Dale

I picked up one of these on eBay -- http://cgi.ebay.com/RF-Radio-Controller-Wireless-Control-for-Basic-stamp_W0QQitemZ250174795540QQihZ015QQcategoryZ1200QQtcZphotoQQcmdZViewItem

All I was doing was communicating a button press from a location about a hundred feet away. Worked like a charm. I did tie up four of the analog ins as channel sensors, tho (seemed to be the simplest way to integrate the receiver).

I hope this is not too much off-topic, but I would like to know if it is possible to increase (to for example a mile) with an RF amplifier...

I would like to know if it is possible to increase (to for example a mile) with an RF amplifier...

Technically, most likely yes.

Legally, that depends on your jurisdiction. In the US, for example, an amplifier on one of the ISM bands is probably not legal (unless you are a licensed primary user of that band, in which case you aren't operating under ISM rules).

If you can get line of sight, a pair of directional antennas may do the trick, although that's going to increase your ERP and may result in illegal operation.

-j

ISM - Industrial, Scientific, Medical - a class of devices with specific frequencies (in several bands) that have very low RF power output. they are allowed unlicensed operation in the US and other countries.

ERP - effective radiated power. A directional antenna concentrates the RF in a particular direction, resulting in a stronger RF signal in that area.

@jds, perhaps its time to get your ham license if you want to do that. AFAIK, amateur radio operators are the only people licensed to build their own amplifiers and operate them at higher power than a few MW. Before they let you do that you have to pass an exam that shows, among other things, an understanding of harmonics, spurious signals and interference that your amp might generate. The only unlicensed way to increase performance without amplifiers is to increase receiver sensitivity through the use of antennas and antenna positioning. As KG4 says, if you improve the transmitter efficiency, you are in effect increasing the ERP, which is a no-no.

I have seen wireless links that are unlicensed, but generally they're pretty expensive, as they've been carefully engineered to respect the frequency in use and not to interfere with other radio services. The manufacturers also have 50K in test gear to check these things and more $$$ to run the tests required to get FCC etc certification...

The Xbee Pro might be what you want, as I think they rate the range in open space at 1Km.

D