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My farm have 5 big loads in 2 km radius which i want to turn on or off by Arduino (each load had MCU+GSM).
I have wifi ap in whole farm but it has 5 GHz channel only so drop idea to control it by arduino and ESP8266.
Does anybody have better idea except a GSM module?
If GSM is the only way then is it OK to use SIM900?
I'd be very uneasy controlling a total of 25kVA of distributed agricultural gear using an Arduino that I'd programmed myself and controlled remotely by WI-FI /GSM across 2Km.
My answer may not be strict electrical design within the scope of Help Centre, but I think this detail cannot be ignored. You may have staff /family operating this equipment and are responsible for their welfare, both morally and legally. I'm not being facetious, but this is an Arduino...
It's a small PCB with no connectors nor enclosure nor PSU of any substance. The same will apply to any GSM modules. You'll need to add all those, as well as lots of safety interlocking, or at the very least expensive isolator switches. And on /off indicators - a 5mm LED won't do. And not forgetting your relays which may be just open frame or exposed contacts. Earthed enclosures. And the control equipment at the master end. The list is long. The cost saving of the Arduino will be negligible in the context of the entire project.
The more I think of it, the more complex this project becomes. What's the feed back loop going to be to verify correct and safe operation of 25KVA? If you have a unidirectional wireless control signal, what happens if there is GSM interference and the signal doesn't get through? You could easily get out of synch knowing whether your gizmos are on or off.
If you decide to go home brew, you could start by looking at something like this:-
which is a mini PLC costing ~ £100. At least it's designed for industrial applications and can push out an industry standard 24V for your relays.
If you're the only one using this then have at it. If other people especially employees do too, your legal position changes entirely. If there should happen to be a fault and fire /injury, home brew switch gear might create an opportunity to renege on insurance claims. There's more to this than just the wiring.
Do you want to switch it wirelessly, or wired?
If wired, in a place as big as a farm, you use PLCs not Microcontrollers. they can drive a 24VDC sig, which won't be attenuated like the 5-3.3V signals, they are more rugged.
There are Wireless Communication modules for PLCs, but I haven't checked them.
Please take a look at nRF24 wireless radio and library, http://tmrh20.github.io/RF24/classRF24.html#af2e409e62d49a23e372a70b904ae30e1. Range is less than 2KM but I have just completed a system which uses repeaters in line of site with each other to increase range. The repeaters are made up of one of these radios plus an arduino nano and a battery.
To give you an idea of development cost and recurring cost for your system, my repeaters, would cost about $80 each and my development time for the project was about 800 hours to; Set up arduino development environment. Understand the radio and it's library. Design the hardware. Layout the pcb. Select and acquire parts. Select and test battery options. Write software, write software, and write software. Field testing continues at this time. My software environment includes no debugger which wastes a lot of time. I have only built 9 units to date but my experience with them suggests the radio and nano are suitable for your task.
In my application, repeaters are mounted 2 feet above a frozen lake and scattered over 50 acres. Unit to unit distance is limited to 250 meters.
Three AA cells will power a repeater for a little more than a month.
For your application a battery life goal of 6 months would be reasonable.
