Simple data, multiple transmitter, single receiver short range network

[templar]

Hi Everyone,

This is my first post! Yay!

I have had a look around the forum and found a number of different answers, but none of them are really exactly what I am looking for, or in some cases, I'm not sure if they are what I am looking for.

Let me first of all explain what I am after. What I would like to do is have a number of remote sensors that provide a signal between 0 and 5V (or 1-5V) depending on the sensor. I would like these sensors to wirelessly transmit this voltage reading in some way or another to a central receiver. This receiver will be connected to a PC.

From what I have researched so far there are a number of options for wireless modules:

1. Zigbee (Xbee and others)
2. WUSB
3. Radio
4. GSM/Mobile (way overboard)...
5. Others?


The range requirement is only around say 20-30m.

However, because the requirement is "multiple transmitters, single receiver" I believe that the RF or radio option is not going to work. As far as I could tell from my reading, you can have multiple receivers, but multiple transmitters will collide unless there is some communication protocol and timing. I'm assuming I got that right?

Now Zigbee seems to be a very good option. As a prototype, I would be able to get an interface board for about AUS$66, two zigbee chips for AUS$55 each and a simple USB board for AUS$44. The interface takes 5 analog inputs and has a few additional things.

The issues here however is that I dont need anything except analog input. Any extra "features" are an extra cost that I don't need. I am happy to use interface boards as prototypes, but I wonder if there is anything out there that simply has 1 or more analog inputs and a Zigbee module.

It seems overkill to have the cost of a zigbee module to transmit just a single analog input, that's why I wonder if there is something out there that's more economical I guess... In many cases I would have 4 or more analog inputs, but sometimes just one.

How about digital inputs and zigbee modules ... any one seen those?


The other thing I mentioned was Wireless USB, however I don't really see much out there. Comments?


Basically one of my aims is also to make everything as compact as possible. The zigbee analog input and chip board should all be as small as possible so that the analog sensor can simply be wired into it.


Just in case anyone is wondering, this is for a project at home that I am playing around with. I've got some Phidgets and temperature sensors, but I am trying to make a few of them wireless so I don't have to run cables other than supply necessary power.


Any comments are appreciated. My background is in control systems and software development so i understand higher level systems, but not so much on the component level or circuit board level. That is why I will probably just end up buying ready made circuits.



Thanks!

 

[Pommie]

You don't mention how often the data is required to be transmitted or how many sensors are required. If the update rate and the number of sensors are not too high then a simple send and hope scheme may work. Basically you send the data in very short bursts and keep sending it with a random gap in between. If you send at 2400 baud and send ten bytes per packet then each transmission will take ~0.05 seconds. If this is sent every ten seconds plus or minus a random amount then the chance of collision is very small. The odds of not getting a correct packet from each sensor in 5 minutes are phenomenal.

This would of course require a micro at each sensor to control the RF module. The tricky bit is getting identically programmed microcontrollers to generate different random numbers.

Mike.

 

[templar]

Yes, you have a good point there.

I'd say that the number of sensors could be as high as 10-20. I would like data to be available between 1 - 10 seconds.

Because each sensor will probably have different values at some given point in time that could be used as a random seed. So given that the measured voltage will most likely never be the same between any two devices the probability of two transmissions at the same time would be small.

There is one thing I don't quite understand about RF though, if everyone say has a remote controlled garage door or an aircon how do you:

1. Not clog up the radio air waves and collide with other devices.
2. Identify devices so that say someone's garage door opener doesn't open someone elses garage.

Thanks.

 

[Pommie]

Have a read of this page.

Mike.

 

[templar]

Thanks for that.

So after reading that, using the voltage input value as the random delay would work but it wouldn't work for sychronizing any codes and such.

Seems to be getting a bit complicated now that I know you need some way of allowing one set of transmitters to talk to a receiver, but deny another set of transmitters. However, if they are all still transmitting at the same time doesn't that cause a problem? What would happen if say every household on the same street had 10-20 sensors all pulsing every second randomly. Is that even legal?

Sorry if I'm asking stupid questions, but I'm definitely learning more as I read.

Unless there is an off the shelf solution for creating/programming RF transmitters and receivers I'm not sure that I have the skill or experience to do it myself. Maybe Zigbee with a USB development board isn't such a bad idea. C programming is more up my alley.

Cheers!

 

[Pommie]

There are rules regarding how long a transmitter can be active. I think it is something like 5%. They are also of limited range (20M). Hence why you can have 100s of cars in a car park and they all work.

The reason I suggested transmitting for 0.04 of a second and once every 10 seconds was to keep the airway fairly clear. You would of course get collisions and so the 10 bytes is to allow the data to be packetised with CRC etc. I was assuming that something like temperature data would be required on a 1 or 2 minute basis as it changes relatively slowly anyway. If you want to record 20 sensors every 10 seconds then you are probably better of with ZigBee.

Mike.