How do I send SMS from an Ethernet shield via SMS Gateway?
The site Clickatell provides free SMS.
Does anyone know how to do this or is there some other way to do it?
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First contact Clickatel's support and ask them how to use their API, make yourself familiar with it.
Then when you know that you connect your Arduino to your ethernet shield, I'm just gonna assume you got Arduino's original shield, because you want to support the arduino foundation.
Open an account and try to implement their API using something like Chrome REST client. Then do the same through the ESP8266. I would personally relay it through my own web service/ url. You could also take a look at: http://www.instructables.com/id/Send-SMS-from-Arduino-over-the-Internet-using-ENC2/
Thanks to the moderator for telling me to give an improved answer, Here it goes->
From your question I infer that you want to send a SMS from your Arduino with an Ethernet Shield.
My suggestion to you is to use a service "CAYENNE". It's a cross platform service for IOT.
You can create a FREE account on it by visiting their website :here
You will be provided with a KEY to use with your devices
Then all you have to do is connect your Ethernet Shield and upload Cayenne's BASIC code to your Arduino so that it would link the key assigned to you to the account.
You can now Control your Arduino from any APP or directly through their website as provided above.
As far as SMS is concerned all you'll have to do is to add a trigger (GO TO OPTIONS -> TRIGGERING AND ALERTS ) and select the notify option(Add SMS here)
Then you just have to enter the country code along with the phone number and when a particular event of your choice occurs you will be notified via SMS.
Hope this helps :) Feel free if you have any further doubts.