Short range wireless communication - Page 1
Forum section: electronics / beginners

author: king.oslo
author_rank: Frequent Contributor
timestamp: June 30, 2014, 04:32:57 pm
content: Hello there,I want to connect some sensors to an MCU, and when the sensor data meet some criteria I want to broadcast to nearby devices (less than 10 meters, through some concrete) that the criteria has been met. Basically two states, the nearby devices only has to know whether the criteria has been met or not, not what the actual data is.How do I most cheaply do this? I remember as a child that when I short-circuited a 9V battery I would hear a clicking sound in the AM radio (I believe). Could I somehow do this? PWM a transistor to send a high currect through a wire which is picked up by a nearly antenna and amplified to let the nearby devices know that the condition has been met? Would I have all sorts of interferences issues?Is this cost effective? Are there cheaper and better ways to do this?Thank you for your time.Kind regards,Marius

author: katzohki
author_rank: Frequent Contributor
timestamp: June 30, 2014, 04:37:52 pm
content: Just sending PWM isn't going to be reliable, as your receiver will be susceptible to random noise. Better to send something specific so that it can be verified by your receiver. There are modules out there (XBEE?) for exactly this kind of thing. Wireless can be a challenge, but going the modular route can simplify it.

author: tld
author_rank: Regular Contributor
timestamp: July 05, 2014, 12:58:07 am
content: King.oslo? If you're in Oslo, Norway, you can probably buy Xbees over the counter at Elfa.tld

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 05, 2014, 01:36:27 am
content: Some MCUs have radio output stages built on to them.Or for a radio ic, being King.Oslo you may want to look at Nordic.

author: Hideki
author_rank: Frequent Contributor
timestamp: July 05, 2014, 01:44:04 am
content: Quote from: tld on July 05, 2014, 12:58:07 amNope. Elfa shut down their Oslo store quite some time ago.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: July 05, 2014, 04:05:54 am
content: For this purpose, I think your most direct and easy route is to look into On-Off-Keying (OOK) modulation.You can pickup an off-the-shelf oscillator (digital oscillators are cheap and abundant). Something like 24MHz would work. Have the oscillator go into an AND gate. The other pin is your serial data stream. Logic 0? No transmission. Logic 1? output=frequency. Youwillneed to implement a 5th order passive filter at a minimum to round out the harmonics (72MHz, 120MHz...etc). A simple wire of 3m would work for your antenna. It wouldn't be ideal, but it would work.The hard part is the receive.Starting with a similar 3m long wire, you would need to connect it to an LC tank filter. From that, you have a few options. The easiest method possible, is getting a log-amp like the AD8307. That gives you an output proportional to input signal power. No input power? No output. Lots of input power? Max output. It's an RF-to-DC converter (not quite, but close enough). If you can't swing that, the next best bet is to implement a mixer- either a gilbert cell or a diode ring mixer. (All of which are googleable). Use another oscillator of the same frequency to getcloseto the transmitter frequency. (Ideally you would want a PLL).Low pass the output of whichever device you choose at about 5x the data rate (so if you are transmitting 300 bps, your low-pass filter will have a cut-off at 1500 bps). If you're using the AD8307, you should be able to get away with a simple comparator to recover the bits (you'll need to check your polarity - that a 1 comes out to be a 1). If you're using the other methods, you'll need some simple amplifiers to gain up the receive signal enough to drive a comparator with some hysteresis. Again you would recover your digital bitstream this route.Cheers!

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 05, 2014, 05:16:20 am
content: I think these things are absolutely brilliant:http://lowpowerlab.com/moteino/And a previous-generation similar product from EU:http://jeelabs.net/projects/cafe/wiki/I have purchased/used many of each product.They both use HopeRF modules which are quite sophisticated, but very small and inexpensive.They are practically designed for applications as you are describing.

author: tautech
author_rank: Super Contributor
timestamp: July 05, 2014, 05:37:24 am
content: So something like a logic enabled transmitter and a relay output receiver?http://www.rfremotech.com/SomeProducts.htmlThey used to have an Ebay store, haven't looked lately, just deal with them direct.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 06, 2014, 01:31:48 am
content: Quote from: cleaningOut on July 05, 2014, 05:25:14 amWOW! Do you know where we can get LEDs and photodetectors that will operatethrough concrete?

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: July 06, 2014, 01:49:30 am
content: Quote from: Richard Crowley on July 06, 2014, 01:31:48 amYou forgot about the laser ,it can be used to cut a hole through the concrete first

author: katzohki
author_rank: Frequent Contributor
timestamp: July 07, 2014, 04:55:13 pm
content: By the way, make sure you set up to be transmitting only in a frequency that is allowed for unlicensed operation. We have "part 15" here in the US.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2014, 06:47:20 pm
content: "Part 15 Unlicensed Transmission" is rather an old-school scheme for very-low-power transmission in the AM (LW) and FM radio broadcast bands. Typically for AF transmission (speech, music, etc.)Modern data transmission schemes generally use "ISM" bands which are more international (although not completely uniform around the planet.)Ref:http://en.wikipedia.org/wiki/ISM_band

author: katzohki
author_rank: Frequent Contributor
timestamp: July 07, 2014, 07:19:17 pm
content: Quote from: Richard Crowley on July 07, 2014, 06:47:20 pmFrom your linked Wikipedia article:"Part 15 contains the rules for unlicensed communication devices, even those that share ISM frequencies."Anyway, he's not in the US, so our rules don't apply.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2014, 07:44:09 pm
content: Quote from: katzohki on July 07, 2014, 07:19:17 pmYes, since ISM came along decades after the original Part-15 rules, it was incorporated into US Part 15.If you talk to anyone who uses RF spectrum, "Part 15" refers to AM and FM radio broadcast bands.If people mean the ISM bands. we use the term "ISM" because "Part 15" is too broadly-defined (as you have demonstrated).Note, however, that you won't find any of those wireless data modules that use the old Part 15 radio broadcast band.They all use some other unlicensed data bands like ISM, etc.QuoteWhich is another reason to not even bring up Part 15. ISM is a worldwide standard.

author: kingofkya
author_rank: Regular Contributor
timestamp: July 07, 2014, 08:22:56 pm
content: I second the Nordic suggestion There are quite a few library available for the nrf24l01 chips. And you can get breakout boards with all the "hard" rf stuff done.Search ebay for nrf24l01. Generaly $.50-2 depending on how manny you want and the seller.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 08, 2014, 05:19:39 am
content: There are plenty of ICs designed specifically for that kind of applications. For example from ADF7901 (http://www.analog.com/static/imported-files/data_sheets/ADF7901.pdf), then TH72012, Atmel's ATA8203, Si4320 etc. I think they all work in 315MHz, 390MHz or 433MHz ranges and support FSK as well as simple ASK or OOK modulation. I got whole set from DigiKey a while ago for some project - all were very easy to hookup and control from MCU in OOK mode. Only way I tried anyway.All datasheets have info on range and also tell how to extend the range when necessary (like suggested specific PA chip). You can use small helical antenna made from wire and soldered to the board - for that there should be suggested antenna matching circuit in datasheet as well.