I'm going to be working on a project soon for GPS tracking. Primarily this is going to be a lojack type system for an older model Camaro that I've restored. So I plan on having some sort of cellular device, gps, and an arduino board to power it.
The reason I point this one out in particular is it's a lot cheaper than any of the others that I've seen. Just the cellular sm5100b shield on sparkfun is 100 dollars and it doesn't have GPS built in like this one does. Anyone happen to have any experience with this shield and know whether it's any good or not? I mean the one disadvantage I can see on this shield that may make a difference is the antenna is built in so I can't put a remote antenna somewhere to get better reception. Since the device itself will have to be hidden somewhere I'm not sure if this will be a problem or not.
Also I'm wondering about cellular shields in general. I found these sim cards and data plans for sale on embeddedworks.net http://www.embeddedworks.net/psummary.php?mn=wsim&mid=mwsim4205 They are 2 MB a month which I'm figuring should be plenty of data allowance for simple GPS tracking. I could probably go to 1 MB a month and still be fine, but at 2 MB I can transfer up to 24 - 100byte strings per minute for the entire month and still be under the data cap. Anyway, that was sidetracking a little, those sim cards should work in a GSM device correct? Also, they state you get a private / dynamic IP with data plans. If I had the device upload it's IP when uploading it's position, could I initiate a connection back to it in order to send it commands remotely? I'm thinking here if I wanted to wire the device into the primary power to the coil on the car I could use it to disable the car (unless the potential thief was smart enough to find and circumvent my break in the power line). I know I could send SMS to the device and it would be able to receive those as commands, but was trying to avoid that if I can since the sim cards on that site are for data only. SMS cost 20 cents extra each, given I shouldn't really need to use them that often so if I can't make a connection back to the device that is probably the route I'll end up taking.
Has anyone tried the Embeddedworks SIM cards from the link above? Looks like a pretty good deal for $5 a month. Sure you only get 2MB, but for some Arduino applications that's perfect. I'm paying $20 for 1.5GB using 4G,3G, or Edge with Mifi and Wifly chip.
sbright33:
Be careful, some GPS modules work much better than others...
Yeah that is what I'm trying to figure out and hoping some others can post with their experiences. I don't want to spend more than necessary on this project, but I want to be sure it's going to work for my purpose. I like the idea of having the GPS and GSM device on the same shield, saves me space and all, but most of the ones I can find are like 220+ dollars. That is why the one I linked jumped out at me, I was surprised by it's price.
I do not have any experience with this board but i do have a lot of experience with GSM and GPS in general. This board is using a rather small patch antenna for the GPS and I strongly suspect it will have trouble getting a reliable lock if it is tucked away someplace out of view. I see a 50ohm wire running to the GPS antenna and I am willing to bet it would be very easy to just unscrew that top board and get access to the solder pad or connector for the GPS antenna. Just unsolder the existing wire and solder on a lead for your external GPS antenna. I never used this board so I cant say for sure that you have access to the pin/pad or connector but it sure looks like you would. This would allow you to mount the board under the dash and then put the GPS antenna in a proper location.
For a SIM card I have used the GoPhone prepaid SIM from any AT&T store. Make sure to go an an actual AT&T store, not an authorized reseller who will charge more. They have some data and SMS plans that are pretty good and they have always worked well for me. Initiating a data transfer can be done but is harder to do. SMS is much easier to manage and with GoPhone you can buy a $5 plan that gets you 500 SMS per month. GoPhone also has a free E-mail to SMS service so you can just send a free E-mail such as phonenumber@att.txt.com.
The data accounting might not be what you think - to put a 100 byte string on the internet you have to send more than one TCP/IP packet and I'm sure your service provider charges for all the net traffic, not just the payload! 100 byte payload might mean anything from 500 bytes to 5k of overhead depending on packetization and so forth. If you use HTTP over a mobile phone<->internet gateway there will be oodles of headers too. So watch out for that.
MarkT:
The data accounting might not be what you think - to put a 100 byte string on the internet you have to send more than one TCP/IP packet and I'm sure your service provider charges for all the net traffic, not just the payload! 100 byte payload might mean anything from 500 bytes to 5k of overhead depending on packetization and so forth. If you use HTTP over a mobile phone<->internet gateway there will be oodles of headers too. So watch out for that.
I know there is a little overhead that is why I figured 100 byte strings when if I sent a GPS string without any modification it's only 80 bytes. But I'd likely parse out the coordinates and only send those making the actual strings something closer to 20 or 30 bytes. I don't think the overhead would be any where near 500 bytes, maybe more like 200 to 300 bytes if I'm using an http request. But I probably would not do it as http request. I'd probably set up some server somewhere that it would connect to and send the data so that it will have less overhead, if it really were a problem.
But I really don't think it'd be a problem even if I did it as http requests. Most of the time it's not even going to need to update when it's parked and such, unless I send a request for an update. And when it's moving and the location is changing, I'd likely only have it update once every 30 seconds or minute, unless I send it a command to do real time tracking at which point it would update like every 5 seconds.
The real time tracking could probably go over the cap if it ran like that all the time, but this mode would be more for if the car is moving and I'm not in it (stolen).
SMS probably would be easier to implement, but it's also quite a bit more expensive. That embeddedworks website has T-Mobile 400 sms a month for 185 for 12 months, that is 2 1/2 times more expensive than the 2 MB data plan.
I just remembered that I have an old GoPhone SIM that I am not using nor will I ever use. It still has $33 of credit on it and it is active until February 2013. I would be happy if the credit did not go to waste so if you PM with your address i will send it to you.
In terms of cost I still think the GoPhone SIM is better. You pay $100 for 12 months and you then use that $100 to buy "feature packages". 50MB costs $5 a month which takes $60 per year out of your $100. That leaves you with $40 per year for SMS which gives you 200 SMS updates per year. If you are sending more SMS you can pay $5 per month for a 200 message plan. This takes your yearly total up to $120 and I cant imagine you could possibly go over 50MB and 200 MS per month.
I have a similar project up and running with GoPhone and I use the 10c/min plan which has 1c per 5kb of data. This seems useful as I don't use it everyday or extensively even on the days I do use it.
Though be careful about the GoPhone balance. I am not sure if this was just me, but I spent the last 24hours figuring out why I was not being able to connect to any servers through the GPRS device (SIM900), just to come to realize that AT&T might have cut access to data when my balance went below $5 (it was $4.99, I am NOT kidding). When I refilled my account with $10, everything was up and running.
Also, I have just ordered the mentioned shields from DFRobot. Will try them out and let you know how well it goes.
From my understanding of the AT TCP programming commands there is a lot of interaction between a device like an arduino and the cellular device just to register to a network and send a data message. How much data usage does this use up? Lets say that I just want to send a 20 byte data message every 60 seconds via TCP or UDP. Does that mean I'll get charged just for those 20 bytes(the payload) or will I get charged or for the session initiation handshaking and all the communication that goes on behind the scenes? What if the connection goes down and the session has to be reestablished? Will that incur data charges?
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