The idea of it is, it will run a Raspberry Pi Zero W with a battery backup, charging module, GSM module and utilize the on board GPIO pins for triggering.

• Raspberry Pi Zero W
• 5v to 3.3v Voltage Regulator
• 2x10uf Capacitor
• SIM800L GSM Module

Basically, we hook a standard 5v wall plug into the box that is then split into 2 legs.

1 goes to the battery charger/battery and powers the Raspberry Pi

1 goes to a voltage regulator (and capacitors) to drop the voltage down to 3.3v, this is now safe and low enough to be read as in input on the raspberry pi GPIO pins.

The system has power, the pi runs off the 1st 5v leg thats also maintaining the battery, and providing 3.3v via leg 2. This tells the Pi that its getting power from the wallwart.

Power drops, the charge circuit swaps to using battery, leg 2 is no longer live and pulls the Pin on the Pi GPIO to LOW thus executing a script on the Pi OS to send a text msg that it is down.

Theres a couple small wiring diagram issues, ie power out from the charging IC goes to the bottom of the pi for power in, not to the GPIO pins 5v power. I still have to test this and make sure a diode isn't needed on the power in to assure theres no reverse voltage back fed from the 5v Input that ends up pulling the GPIO pin back to HIGH when running on battery

And a generic MS Paint early draft, this wiring is till not tested (same as above), as I plan on building it this weekend.
The GPIO wiring for sure is not correct in this image, and do not use it as golden as it was an initial quick brainstorm

Installing Python Dev tools

sudo apt-get update
sudo apt-get install python-dev python-pip
sudo pip install --upgrade distribute
sudo pip install ipython

Installing Raspberry Pi GPIO and serial

sudo pip install --upgrade RPi.GPIO
sudo apt-get install python-serial

3

Python Code for sending a TXT message

import serial
import RPi.GPIO as GPIO 
import os, time

GPIO.setmode(GPIO.BOARD)

# Enable Serial Communication
port = serial.Serial("/dev/ttyS0", baudrate=9600, timeout=1)

# Transmitting AT Commands to the Modem
# '\r\n' indicates the Enter key

port.write('AT'+'\r\n')
rcv = port.read(10)
print rcv
time.sleep(1)

port.write('ATE0'+'\r\n') # Disable the Echo
rcv = port.read(10)
print rcv
time.sleep(1)

port.write('AT+CMGF=1'+'\r\n') # Select Message format as Text mode
rcv = port.read(10)
print rcv
time.sleep(1)

port.write('AT+CNMI=2,1,0,0,0'+'\r\n') # New SMS Message Indications
rcv = port.read(10)
print rcv
time.sleep(1)

# Sending a message to a particular Number

port.write('AT+CMGS="+6xxxxxxxxx68"'+'\r\n')
rcv = port.read(10)
print rcv
time.sleep(1)

port.write('Hello User'+'\r\n') # Message
rcv = port.read(10)
print rcv

port.write("\x1A") # Enable to send SMS
for i in range(10):
rcv = port.read(10)
print rcv