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What's the cheapest way to link a few microcontrollers wirelessly at low speeds over short distances.

I'm looking to keep it ultra-cheap, use common discrete parts and keep it physically small. I don't care about bands and licensing so long as it works.

802.15.4/ZigBee, Bluetooth and WiFi all require an expensive coprocessor, so aren't an option.

Alternatively, are there very cheap radio modules available to hobbyists? The kind of things you find in car keyfobs and wireless thermometers, perhaps?

Would building a simple transceiver on a homebrew PCB even be practical, or will I be plagued by tuning, interference and weirdy analogue stuff?

Could something like this be driven from a microcontroller? What about receive?

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10 Answers

up vote 31 down vote accepted

Infrared emitters and receivers can be used to transmit/receive data. However, interference with other devices like remote controls might be a problem. If the connection needs to be really reliable, this probably wouldn't be that great of an option. However, you might be able to find a really uncommon wavelength emitter and receiver to limit the amount of interference.

These are pretty common, and very cheap. Packs of 20 emitters for $1 and packs of 100 receivers for $13 can be found on eBay (not including shipping). Lots of electronics stores have them as well.

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Yeah, IR is annoying due to the line of sight issue. But it may be an option for some projects –  davr Jun 30 '10 at 16:26
    
@wallacoloo do you have any links to these kinds of devices? I searched ebay for IR emitters and couldn't find anything close to what you describe. Thanks! –  Dave Jan 28 '11 at 15:50
    
@Dave: On eBay, you can do a search for "IR LED pack" or "IR LED" of something similar. Here's [20/$1][cgi.ebay.com/5mm-IR-LED-Emitters-20-Pack-/… (but no data sheet) Searching for "IR receiver diode" or something similar can yield a couple results, like this [100/$15][cgi.ebay.com/100-pcs-5MM-Infrared-receiver-diode-IR-LED-/…. [Futurelec.com][futurlec.com/LEDInfrared.shtml] sells matching 3mm/5mm emitters and detectors for $.16 each. –  Wallacoloo Jan 29 '11 at 19:03
    
@wallacoloo ah ok, thanks, you're talking about the LEDs themselves, and not something that comes on a PCB. Thanks! –  Dave Feb 1 '11 at 6:10
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If you really want low component count, you can use the same LED as emitter and sensor. Simply add a method of reverse biasing the same LED, and it now becomes an inefficient light sensor. See this app note: altera.com/literature/wp/… –  Zuph Mar 21 '11 at 16:18
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You pretty much have to buy pre-made modules, you can't expect to wire up your own transmitter/receiver from a few transistors and a crystal, RF circuit design is unforgiving and all but requires a custom PCB (or custom IC) to do. You could probably build your own RF module on a PCB if you did some work, but at that point if you are making your own PCB's, you're not saving much money versus the very cheap modules that are available.

SparkFun has RF Transmitters & Receivers for $4 and $5 respectively. Since they are just basic parts, you will need to do a little extra logic on your microcontroller to compensate for interference, eg sending error control codes so that missing / flipped bits can be detected and recovered.

I found SeeeeeedStudio sells almost the exact same thing, but even cheaper. It's $4.90 for a pair of a receiver and transmitter.

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your SeeedStudio link doesn't point to a product, but I assume this is what you're referring to? It looks very cool, and I might have to pick up a pair to play with! seeedstudio.com/depot/… –  Dave Jan 28 '11 at 15:52
    
@Dave looks like they re-organized their website, breaking links in the process. –  davr Feb 2 '11 at 22:45
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Hrm, sparkfun ALSO broke their links as well. I fixed both of them. Does nobody understand the importance of long-lived links any more? –  davr Feb 2 '11 at 22:46
    
As a matter of fact, I designed a really inexpensive one-way transmitter/receiver around these basic ASK modules - see node.wickeddevice.com and receivershield.wickeddevice.com respectively meant for wirelessly collecting sensor data and aggregating it on an Arduino (or better yet a Nanode - nanode.wickeddevice.com to publish it to the internet ala Pachube) –  vicatcu Oct 21 '11 at 17:35
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The cheapest radio modules I know are the RFM12 modules, they do frequency shift keying in the 433 or 868 MHz band.

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I've used some of these. The documentation I could get hold of was rubbish! –  Jim Feb 15 '11 at 18:28
    
I used these a couple of times and I am in the middle of making a product with of them. The docs suck but take a look at JeeLabs - the guy is using these modules for all his radio comms) –  jpc Mar 21 '11 at 1:45
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You might take a look at the Jeenodes. They are simple and relatively cheap and provide a nice layer on top of basic RF modules for doing serial transmission and point-multi-point communication.

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And they include the microcontroller, if you like AVR. –  joeforker Feb 3 '11 at 18:35
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I'm not sure if this would count as cheap enough, but the Linx chips are dead simple. I've made a PCB for these chips and successfully run 16 sets at the same time in a 10 x 20 m room without any special PCB trickery.

Future Electronics has the TRM-315-LT modules for £11.33 each in the UK. There might be other modules that are cheaper, but I think those are the ones I used previously (it was ~6 years ago, so my memory is a little fuzzy).

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+1 A radio modem would be simple and fairly cheap. But, I already have a microcontroller so want to avoid adding another expensive chip if possible. –  Toby Jaffey Jun 30 '10 at 0:51
    
How cheap counts as cheap? Say, less than £2? £3? –  pingswept Jun 30 '10 at 0:54
    
Less than £2. In volume, a Bluetooth transceiver is ~$1. The type of radios in car keyfobs must be pennies... –  Toby Jaffey Jun 30 '10 at 1:02
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I don't believe you will get the same price for hobby purposes that a car manufacturer is able to get. The cheapest option I know is the RF Transmitter/receivers from Sparkfun mentioned in the other answer. –  Wouter Simons Jun 30 '10 at 7:46
    
Unless you manage to repurpose a keyfob.. –  XTL Jun 30 '10 at 13:30
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The Nordic Semi nRF24L01+ is easy to use, SFE sells modules.

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The RFM70 (warning: I sell them) modules are even cheaper than the RFM12. The documentation is equally bad but I made a good C library which should make using them a lot easier. You can find the library here:

VOTI RFM70 libraries

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Can you link to your C library? Thanks. –  Toby Jaffey Aug 31 '11 at 19:35
    
Me and my big mouth :( But in the end I finished the library, you can find it at voti.nl/rfm70 –  Wouter van Ooijen Dec 24 '11 at 19:41
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The Nordic nRF24 series. If you order non-amplified versions of these modules, they can be had for under $7 per module. Amplified versions are in the range of $15-$20, so range is a major factor in price. The ones Sparkfun sells are about $23, but are amplified. Unamplified modules have been reported to get around 30 ft of range with trace antennas.

These modules also have the benefit of error detection, acknowledgement and automatic re-transmission.

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If you shop around on ebay, you can get them for $1-$2 per module. –  Connor Wolf Sep 1 '13 at 12:08
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I second the RFM12 and JeeNode suggestions and strongly discourage you from rolling your own using a simple on/off transmitter. It is not so easy to make the transmitter/receiver work without RF experience and even if they work then digital modulation and demodulation of a noisy signal are still not trivial.

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Definatly look into the Bluetooth 4.0 ultra low power profiles. This is said to power devices with simple sensors for months/years.

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