Transmit Relay Contact Information Wirelessly

Hi,
I seek help for implementing the following:

I wish to transmit Contact Status Information (Close or Open) of four relays to a remote location, about 500 meters - Wirelessly.... The receiving end will have four Bulbs connected to another set of relays which will turn ON or OFF based on the received information....

Since, I have no background in Wireless Communications, I would request the posters to kindly explain in layman terms if that's possible and how...! Would like a DIY kind of an information.... Although any readymade module will also suffice....!

au revoir
Rahul

First you need to know that "a wireless transmission" over 500 meters is not legally allowed in civilized countries without a license. Second, if you need to consult specialists in wireless transmission, I would advise you to locate a radio amateur club in your country. Radio amateurs know what can be done and how to solve your problem, also legally correct way.

If you have four relays and need to know their state on distance, you should either be able to sense the voltage of their loads as input signals to be transmitted. Or you should add one contact to each relay, so you convert the relay state into a voltage (mostly the TTL logic signal is preferred).

The transmitter-receiver system to get the relay information over ~500 meters should operate on a legally usable frequency, with a permitted RF power level; some directional antennas may be needed.

Instead of all such complications, you could use a telephone line if there is one at relay location and where you need the state information indicated by the lamps. If not, you can use a pair of cell phones if the service coverage exists in your area. You can send the information as SMS messages for a low cost.

There are also additional modules available to cell phones allowing data transmission, also without any cost, by using the ringing signal only. Use Google to find companies offering cell-phone modules for such tasks.

Thanks jiripolivka for your detailed reply.... First, I didnt quite know that wireless transmission beyond 500 Meters will involve legal issues.... Will check that out... However, the two locations between which I need the signals to fly may be less than that distance... It was just an offhand guess, that its 500 meters.... The aerial distance may be less by a 100-150 odd meters....

I was just thinking to make a FM Transmitter Circuit - available so freely on internet.... And using that circuit, I was planning to ROUTE the power supply to the circuit thru the contact of the Relay whose status is to be read....

So when the contact is OPEN, there's no power to the FM Transmiiter and the FM Transmitter is 'dead'; so there's no transmission.... When the Relay contact state CLOSES, the FM begins to transmit at a certain frequency....

The receiving side will be tuned to receive these transmissions....! So when there IS a transmission, the receiver instead of sending the received signal to an audio speaker, would send it to a Relay Coil after conversion to DC and signal conditioning and light the Lamp....

Can you guess, if this concept will work or I am just wasting my efforts.... :)...

What minimum time changes must be detected on contact relays?
0.1s? 1s? 1min?

This information is only pure status information or security information?
If security information then your plan isn't good... In this case if on and off state
Is transmitted must information go true wireless....

The relays are going to be operated occassionally - once in a month or even less....! So detection time is not a limiting factor...

---------- Post added at 09:29 ---------- Previous post was at 09:27 ----------

I mean the relay will remain in one state for an extended period of time.... Lets say OPEN state.... Once in a while its state will change to CLOSE, and this state will have to be transmitted wirelessly - or as I am thinking over a FM link...!

I think rfm12 rf12 do the job
http://jeelabs.org/2009/05/06/rfm12-...12b-revisited/
http://www.sparkfun.com/products/9582
But you must work with very low speed baud rates an max transmit power and
Good sensitivite on reseived site!

Thanks js - for a possible solution.... I guess, this will need a microcontroller interface too.... Currently, I dont have any set-up to interface a microcontroller with such a device.... I was looking for a pure hardware solution.... But still, I'll try this one if nothing else works out...

Thanks....

Your thinking of a FM transmission is legally wrong. Once your transmission interferes with legal signals, you would be fined and/or punished. At least in a civilized country.

Others refer to small cheap modules allowing to transmit at 434 MHz, etc. Such solution is legal, but the transmit power and receiver sensitivity are poor. A typical range is ~10-30 meters, like opening garage door or locking a car. To extend the range to your 100-500 meters and to operate reliably, you will need to use a pair of antennas, and you may need >10 dB gain.

I used a similar 435 MHz system to transmit doorbell ring across a concrete building. I used a simple tone-modulated oscillator and a good superregenerative receiver. It worked even without special antennas, but- I had many false responses, as local airport, some 6 miles away, radiated also at 435 MHz.

So be ready to experiment. Consult local radio amateurs, they may have the UHF or VHF equipment ready, licensed, and can help you with experiments. Test one channel first, then decide if you can buy some cheap module, or, have a better module custom-built. Be ready to play with antennas, and learn on the way.