I have only played with SMS/GPRS module slightly..
but to touch on some topics (so we're all talking apples here) :)
You can think of your SMS device as a cell phone... in the respect that is will needs its own SIM card..
- (how you get that, and what plan you end up going with I can not suggest anything, as I have only taken my personal SIM card out of my phone and used for some quick text tests, I do NOT have a dedicated SIM card for this, I'm located in the US)
that also means, you only have one (1) phone number to 'communicate' with this device.
to comment on some of your thoughts:
"A light would have to be connected to a certain phone number."
Well.. technically, every 'led' you use will be connected to the Arduino that your SMS/GPRS device is connected to.
"When this phone number sends an SMS (blank preferably) the LED for that number would light up"
huh? Your SMS device (at this point/aspect) is a RECEIVER.... your volunteers would be sending/replying back to this SMS (alert) device....
Do you want them to NOT respond at all if they are not planning on showing up? or do you want them to reply regardless with a 1 = coming 0 = not available?
Either way you'd have to parse the number/message that the SMS device receives.
"This would have to work with around 30 different numbers/LEDs."
- I dont think is an issue.. it can 'received' a text from any phone (no limit outside of your SIM card/contract/data usage..etc)..
I also dont think sending out a response to 30+ numbers will be an issue either.. (although some thought on how you plan on keeping this number list up to date might benefit you in the end.... maybe a microSD card that loads up a list of numbers? otherwise.. you'll be stuck hardcoding this list.. and have to 're-program' the Arduino each time it needs updating.
"The second function would be that when the arduino receives an sms, it would forward that sms to every single of the aforementioned phone numbers."
- I'm not following? (and I dont think you are listening to your own requirements!) (or I'm just confused)
If you (above) stated the INCOMING message to the SMS device would be blank... what exactly do you think you'll be forwarding to this list of numbers? [blank message] that was received?
I dont see any issues with sending out the location either.. you just need to think about how this will work.
Since the location is (more or less) dynamic data, you need a way to get this data TO the SMS device.. so it can be send out to the distribution list.
maybe some sort of code or specific number?
if the SMS device gets a message from a specifically 'tagged' number.. it can know to save this 'location data' to a variable.. and use that to pass on/forward to your list of numbers.
Is that feasible for your scenario? (no clue).. maybe it needs to be able to come from ANY phone number? then perhaps a 'code' or something in the message to declare this is a 'location message' that needs to be forwarded?
xxx-xxx-xxxx lm: 1313 Mocking Bird LN
where the "lm:" portion declares the message as being a 'location message' to be forwarded...
(I['m not clear on how and what data you get from the SMS device.. so parsing maybe easier or harder.. cant say for sure right now)
- need a way to declare the SMS is a 'location message' and hence needs forwarding to distribution list
- need a way to determine an 'alert response' from a member on the recipient list
- need a way to 'light up some matching leds' that correspond to the 'alert responder'
- lastly you want to forward the 'responses' to .....where? how is this effect if you are using an LED system to know who is coming or not?
Sorta sounds like a fun project... but you need to talking things out a bit a decide on a path you want to take for certain questions.