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2 radio data transmitters, one receiver protocol

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dr pepper

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I'm putting together a system that monitors the contents of 3 silo's and reports the contents on a webpage.
Each silo has an electromechanical plumb bob type level sensor, and outputs an analogue voltage.
I want 2 of the silo's to have a simple radio tx and a microcontroller, and the 3rd to have an rx and the wifi server.
The thing is comming up with a protocol that works, the 2 transmitters cannot detect if the other is transmitting, I was thinking of just transmitting at a random time for each 'packet', and cope with some transmissions going missing when both transmit at the same time.
Data rate is very slow, even if half of the 'packets'get corrupted it'll still work fine, it takes several minutes for the level to change in these things, they hold a lot.
Any ideas?
 

Nigel Goodwin

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How about what I did? (the simple option) - use HC12 transceivers, the main station (in my case, at the moment, a PC) sends requests for data to each unit in turn, waits for the reply, and then queries the next etc.

So no possibility of clashes, and the HC12's are nice and easy to program.
 

dr pepper

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Interesting.
I havent heard of Hc12's, 5 quid each is Ok, at least for this project, and 500mc is well away from the wifi webserver operating at 2.4gc.
The modules I was thinking of are 433mc and have a 100mw o/p, with a reasonable aerial they'd cut through the noise of an industrial environment, but I see the hc12 is a similar freq and o/p power, so would probably work well, and serial comms is easy.
For anyone interested I wrote some code that attempts to send a packet at random intervals, and monitors its counterpart for collisions, its now sent 250 packets with only 15 collisions, so a checksum calculation would catch the collisions and dump that data, even if 3 or 4 subsequent pavkets were lost it'd still work well.
I didnt rig up radios, its just a pin on one arduino to another arduino and a random number generator.
 

Nigel Goodwin

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Interesting.
I havent heard of Hc12's, 5 quid each is Ok, at least for this project, and 500mc is well away from the wifi webserver operating at 2.4gc.
The modules I was thinking of are 433mc and have a 100mw o/p, with a reasonable aerial they'd cut through the noise of an industrial environment, but I see the hc12 is a similar freq and o/p power, so would probably work well, and serial comms is easy.
The HC12's are just so easy to use, I current have three of them supplying temperature and humidity readings to a Lazarus program running on a second PC.
 

ericgibbs

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hi dr,
As Nigel states the HC12 is easy to use and has a good Tx/Rx range, they use a Serial/RS232 TTL type data signal for TX and RX.
An alternate method to polling your remote TX modules is to have the two remote TX modules on different frequencies, there are 100 channels.
The Base RX then sets its own channel frequency in order to select the remote TX it wants to monitor.

I use Channels 35 and 57 at 9600 baud, without any cross channel pick up.

E
 

dr pepper

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I'm going to see if I can get a few Hc12's.
None the less the random transmit schedule works well, if I need to do something really cheap I'll use it, or if the comms channel has to be on one frequency. The constraint seems to be the packet length has to be a lot less than the average delay time, for a slow changing variable not an issue. Virtual wire for the arduino does all the error checking for you, and a preamble.
 
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