Hi i want to build an alarm that sends a sms to inform me the times that the detection line has been crossed after 24h.
The main problems are:
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The device have to be on for a long time (At least 7 days) using only batteries.
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The cost have to be "Low". The board and the sensors are cheep but the gsm module is expensive, then i thought use gsm of old cellphones but i don't know which ones can be used for that.
Thanks for your help.