Quantity   Component name
1 × Arduino I used an Arduino Pro Mini because of its small size, but any Arduino such as a Uno or Mega would be acceptable.
1 × SIM800L Cheap GSM/GPRS module.
1 × Power supply (4 volts) The SIM800L needs 4 volts at 2 amps. Luckily the Arduino will also work on 4 volts.
4 × Resistor (10K) Only needed if you are measuring a voltage. Any resistors in the range 10K to 100K should be acceptable, but all of the resistors should be the same value.