Virtual Wire - Multiple Transmitters, One receiver (1km distance)

Hello, I am looking to build a wireless transmitter and receiver system. Currently I am using two Unos(one RX and one TX) which have the same clock speed of 16 MHz. I am using Virtual wire and have a transmitter and receiver both being 434 MHz.

I would like to have multiple transmitters sending data at the same time to one receiver. I've done research and it seems like if they are all transmitting data at the same frequency, the receiver will not accurately obtain them all. However, I assumed that the receiver and transmitter need to be at the same frequency for data to be sent. How is it possible to have multiple data transmissions to one receiver?

My budget is around $500, max of $1000.

Right now my rx/tx pair does not go far, so im looking into more powerful modules to get that 1km distance. Any recommendations? Xbee, Arduino Bt?
Thank you very mch,

"How is it possible to have multiple data transmissions to one receiver?"

It is not possible - unless you can arrange it so that only 1 trasmitter is on at a time.

Better is to use transceivers - treat the current receiver as the master, let it command the transmitters when to send to maintain order.

Hi Waka:
How many transmitter does the application have?
Actually when working at 433MHz, it is a single channel, so that do not support multiple transmitters working at a time. But we can handle this situation by some flexible step. How large is the data? We can divide one period into different parts and let one transmitter work in certain part. That's called time division. Or Set the transmitters working at different frequencies. The receiver change its frequency to receive signal from different channels. That's called frequency division.
As you mentioned, you need the working distance up to 1Km. Is that RF line-in-sight distance without any obstacle?

Thank you for your insight CrossRoads and Forbelaw.

Yes, I am looking for 500 feet-1km without any obstacles in the way. The transmitters are flow meters which messure the litres per hour. So the data i send is one line long per transmitter. Example of data sent from 1 transmitter is

"200 L/hour 0:0:7"

This last 3 digits is hours/minutes/seconds.

So the data sent is roughly 30 chars in length per transmitter. That frequency division thing sounds interesting. But how can i make the receiver change frequency. I have it at 434 mhz

The device you bought is too simple. The frequency is fixed.

You might consider using an arduino/Ethernet shield/wireless router hardware setup.

The simplest way is polling, have the receiver ask for the data from each of the transmitting stations one at a time, it will then know who it is coming from and there will only be one transmit at a time.

Unless you need to know at the same millisecond what the flow is the timing difference should make little or no difference.

If the measurements are made infrequently, for example once every minute, then each sensor can just send a short "burst" with one measurement, about once each minute. The burst message should include the unique ID number of the sending station. If transmissions from two stations overlap, the message will be garbled but the internal checksum of the VirtualWire package will tell you that the message is invalid (or it will simply drop the message).

To solve the overlap problem, you can have the transmitting stations timed so that they transmit with slightly different pauses between transmissions, for example every 59, 60 and 61 seconds for three stations. Occasionally the transmissions from two sensors will overlap, but the next set of transmissions will not.

This is how home wireless weather stations work, which can have a number of different remote sensors all transmitting using the same radio frequency. The approach is called "time division multiplexing". In principle, if all the transmitters had clocks that could be exactly synchronized, there would be no problem with overlap. But that isn't practical for such a simple system as this, so the tradeoff is to lose some packets.

Can you recommend what type of module is use, with multiple transmitter but one receiver?

This is an old thread. Recommendations this old may not be very useful. Start your own thread, with your own specifications of distance and number of transmitters. That will give you more useful information.