Equipment suggestions to build device that

Purpose…
To send message to cell phone based on current sourced analog input 4-20ma
Messages would be code embedded in Arduino mega
Analog values would be 0 - 1023
Possible 100 messages, thus the large memory requirement
Single cell number embedded in the sketch, SMS message

So I was going to use a mega 2560, Arduino GSM, the BlueVia Sim card,a ac power adapter, but not sure what to use for the isolated analog input.
Now I know the GSM will direct fit to the UNO but I am not sure UNO would have enough sketch memory.
Appreciate any input
Thanks

Good luck with that. I can't wait to see the finished project.

Additional info

I can also use a 0-5v input signal, still needs Opto isolation

I can also use a 0-5v input signal, still needs Opto isolation

When you post a block diagram or schematic we can talk about that.

hope this helps

Be sure to use internal pullup on A0, otherwise you may only see 0’s when you read the input no matter what the current loop is doing.
On the input side, you are driving an LED, so current limit resistor in series with the + or - input (one or the other) may be needed as well.

Opto-isolators for digital on/off signals are simple to build or buy. Analog opto-isolators are much more complex and quite expensive for the apparent simple task they perform. Typically they are used in industrial process control systems using PLC or DDC control systems.

http://www.omega.com/pptst/DMD4380.html

If your not required to have galvanic isolation, interfacing a loop powered 4-20 ma current loop to an arduino analog input pin can be as simply as having the loop run through a 250 ohm resistor and measure the resulting voltage drop of 1-5vdc wired to a arduino analog input pin. This does require that one end of the added resistor is wired to the negative terminal of the loop power supply as there must be a common ground between the current loop and the arduino.

CrossRoads: Be sure to use internal pullup on A0, otherwise you may only see 0's when you read the input no matter what the current loop is doing. On the input side, you are driving an LED, so current limit resistor in series with the + or - input (one or the other) may be needed as well.

No, you are thinking of simple 4 terminal optical isolators. Analog optical isolators are much more complex, as they have to not only offer galvanic isolation, they must be very linear over the 4-20ma analog value being measured.