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Radio Frequency Communication (RF)


upneet

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Hi I have to do a major project in three months. I have a idea as follows.
we want to make a unit which measures pH, Conductivty and Temperature of soil then i want to transmit this data using RF to a computer is it possible? please help
i plan to buy good sensors and then using RF i want to transmit it to a computer then i will make a software using java and vb which will plot the graph of all the values and stuff like that
please help i have got a very short deadline
u can contact me at [email protected]
thanks u very much

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Hi upneet,

You will need a transmitter and reciever to do the modulation and demoduation.

What kind of signals are these?

I take it you need 3 channels, one for temperature, conductivitey and PH.

Are these all voltages or currents or frequencies?

How far do you wish to transmit this data?

Radio communication at reasonable power levels in most countries requires a licence are you prepaired for this?

It might not be nessasary to transmit the data at all you could record it and then take it back to the lab for further processing.

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i have completed the pH part the sensors output is in mV range thus i amplified it using op-amp circuit and then displayed it using IC 7107 the o/p of temp sensor is also in vtg now i want to tranmsit this so that the computer withint 100meters can receive this signal (wireless)
i dont want to indulge in licence and all because timeperiod is less so wat do u suggest?
thank u for ur help

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I suggest using the ISM band (probably 900 MHz), the same one used by cordless phones. Texas Instruments offers some ISM RF ICs for transmitting and receiving. I don't know if you will get a 100 meter range though. You could test it if you had a cordless phone. Get someone to call you and then walk out to where you want the system. You may also be able to put a directional antenna to get additional range.

The only other method I can think of is possibly modulating a laser pointed at a photosensor located at the house. Not sure how well that would be received though, might be okay for very low data rates. Or maybe a cell phone with a modem attached to the system which periodically dials the computer (or vice-versa) and updates the info.

The simplest method is to log the data to flash and then pick up the memory card.

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I am not sure of the regulations of your country; however, I found this on a site (see below). ISM2.4 and ISM5.8 are for the 2.4 GHz and 5.8GHz so they don't apply to you. You need to look at the dBm that the device gives (should be variable) and the gain of the antenna you are using.

"Summary of Power Output Rules for ISM bands
The limits are three:
Maximum transmitter output is 1W (30 dBm) (47CFR15.247.b.1)

Maximum EIRP is 4W (36 dBm) (47CFR15.247.b.3) i.e. for every dB of antenna gain above 6dBi, transmitter output must be reduced by 1dBm; per this rule, a 24dBi antenna limits the output power to 12 dBm which is 16mW

For fixed point to point operation in ISM2.4, peak output need only be reduced by 1 dBm for every 3 dBi of antenna gain above 6 (47CFR15.247.b.3.i) i.e. per this rule, a 24 dBi antenna may be fed by 24 dBm or 250 mW

In ISM5.8, you can apply all the antenna gain you want (47CFR15.247.b.3.ii) with no reduction in output power. "

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You are planning on using RS-232 over a distance of 100 meters? I am quite sure it won't be able to do it without difficulty. You should use RS-422 instead. RS422 offers greater noise immunity and greater communication distance than RS232. RS422 wiring can be up to 4,000 feet (1.219 km) in length using 24 AWG cabling.

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I would get fancy with your project. There are a variety of published techniques, all lending the same results. However, depending on your circuit building skills, you will be limited to which ones you can implement. There has to be one that is right for your application. Be aware of all the techniques that exist and realize that many offer more than you need. The technique you choose will need to satisfy your requirements and these requirements only. You have a choice. You can use AM, FM, or PM. You can also use PWM, PPM, and PAM. The difference is that when you convert the signal to a pulse you have a time where you are not sampling the signal. It is much like an A/D where you have the time of the sample which precludes you from getting every instantaneous value of the signal. This results in error when reading the signal. The upside is that you have a stabile voltage reference for the time of the pulse. Which might have something to do with noise immunity and more importantly voltage averaging where you have steady voltage for a longer period of time. I think there are two sides to this story. One side says you must have a steady readable voltage level to represent your signal and the other says you want a real time representation of the signal. So there is a debate here. Maybe it's frequency related, maybe noise related. Anyways, good luck.

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I do not know if you can use 433 MHZ in your area, but in most places, there is no issue with licensing. Also, since you are going to transmit for a short distance, you do not need a lot of power. Have you considered using a dedicated chip? You might want to take a look at www.rfdigital.com to see that there are lots of choices. Many of them require only a short length of wire for an antenna and 5 volts of power. Everything else is already in the chip. These connect directly to the UART connections for RX and TX and act as if you were using RS232.

MP

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thank u kevin and MP for ur support. I was working on both wireless n RS 232 and RS 422 methods the problem with RS 232 is its noise immunity and it cant be used above 50feet and the problem with RS 422 is its cost. The problem with wireless is licencing because the police wireless operates and i have to find about licencing though 900MHz can be used without licencing. As MP told i saw that site and it was very usefull.
I have finalised some main things about my project I want the range of wire or wireless to be around 100 meters even a radius of 50 meters will do. I have to finish my project in 20000 rupees ( around 200$) this includes cost of wireless/wires components and temperature and conductivty sensors. plus all other expenses ( unless someone sponsors my idea)
I know the going will be tough but with ur supports its always going to be easy
thanks everyone for ur support i have gathered lots of info if u want just mail me at [email protected]

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This link might be helpful for RS-232/422. It shows how to daisy-chain RS-232 which might be okay. I'm not sure about your exact requirements.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/723

You could also consider a 4-20 mA current loop. They use these in industrial facilities due to its noise immunity. Here are a few sources of info:
http://focus.ti.com/docs/prod/folders/print/rcv420.html
http://www.allaboutcircuits.com/vol_3/chpt_8/7.html

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  • 2 weeks later...

hey guys/gals i have started working on rf part i was india centric help. I would like to know which frequency will be most suitable for my application.i want to transmit the signal from temp/ph sensor the range is less then 100 meters i wont it as simple and cheap as possible. I dont wont to go in for licencing. Is 900mhz ok?
can I get ready made ics which work as transmitter and other as receiver?
thanks for ur continued support

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So you decided against a wired link?

Texas Instruments has the ICs you are looking for. I don't know if they supply sample parts to India though.

http://focus.ti.com/analog/docs/articles.tsp?articleType=brc&templateId=5266&familyId=367&path=templatedata/cm/brc/data/200210_rf_ismhome&DCMP=RF+NotApplicable&HQS=NotApplicable+OT+ismrf

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hi i saw that site it was helpfull downloaded few stuff which i will study. But the problem with foreign site is that its costly and payment is tough. So here is an indian site www.mdclrf.com its cheap and good and i am planning to use the following ics MDT 4 - 433.42 Mhz High power transmitter 1000 feet and MDR 422 as receiver will it be ok?
it uses serial mode transmission so is it ok or do i need to make any changes?
everyone is against me for using wireless is it sooooo tough?
please help!!

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I don't know if their indian language site works better, but none of their links on their english site work so I can't give you much information on that.

Are you creating a PCB? Wireless, from what I've heard, is difficult to make reliable for a hobbyist. Especially if it isn't on a PCB. I've had difficulty occassionally with a wired system and that should be relatively simple. The problem I think with wireless is, if you don't have a spectrum analyzer you can't really tell if your signal is being transmitted in an easy manner. I've done very little with RF so maybe someone else has a more informed opinion.

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  • 3 weeks later...

No i was not hibernating these days! my project plans have changed drastically NOw i have to interface four sensors ph, temp ( lm35), light sensor ( from optical mouse or something) and humidity sensor if possible to a pic ( mostly pic 16f877) or similiar then i have to display this on led or lcd display and if this gets over then transmit it wireless and next stage is to receive this signal and interface with pc so that it can be displayed using java and VB.
extremely tough project i need all ur help
thanks

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  • 1 month later...

I am getting 4800bps data from my RF ic. Now i want to connect this to my computer I know i will have to use some ic like max 235 or something then i would like to use com port and some vb programming to get a good user interface which will show all the details about the sensed values with bit of graphics like graphs and all
can you please send me some information about it or any sites where i can get the codes and info
thanks u one and all

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Guest Alun

What does the IC output?

If its an RS232 type signal but a logic level you can buy a chip, here's the datasheet: http://www.datasheetcatalog.com/datasheets_pdf/M/A/X/2/MAX232.shtml

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upneet,
All of the rf chips I have seen need a driver for RS232, so what Alun has suggested will work fine. MAX232 has become quite the standard. If you cannot find these dedicated chips in your area, you can make the same thing with a couple of common transistors and discretes.

In addition to the MAX232, the MAX233 is the same but does not need the added capacitors.
My favorite is the DS275 from Dallas Semi. This one is a very low consumption chip which uses some of the line voltage to power the chip. It is great for battery powered devices.

MP

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