Radio Frequency Communication (RF)

[upneet]

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

 

[Kevin Weddle]

The quality of the sensor will decide the outcome of your project. You can use a general purpose A/D and apply that directly to the data bus.

 

[upneet]

but the sensors will not be connected to computer they will be on the field then i need to transmit the data using rf to the computer which will demodulate and do the work
my area of intereste is RF transmission

 

[Alun]

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.

 

[upneet]

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

 

[gg4rest]

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.

 

[upneet]

hi friends, I searched for some IC's provided by analog device. Some IC's for modulation/demodulation, receiver, transmitter, transreceiver were real good. I think i will be tring for 900MHz. I live in India. Do u think i will have to get licence and all that stuff?

 

[gg4rest]

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. "

 

[upneet]

if i wanted to use RF there was lots of problem with respect to licencing. then I had to use 900MHz one used in cordless phone but to finish everythiiing in three months is very complex now i plan to use RS 232 hope it works

 

[gg4rest]

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.

 

[upneet]

thanks dude i think ur idea is great will let u know if i need any further help

 

[Kevin Weddle]

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.

 

[MP1]

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

 

[upneet]

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]

 

[gg4rest]

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

 

[somasundaram]

hi
im soma from hydrabad can u send me ur circuit with abstract pls its urgent

 

[upneet]

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

 

[gg4rest]

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

 

[upneet]

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!!