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Alarm / Monitoring - SMS Texting

With the wide availability of GSM, many products may benefit from SMS texting eg alarm / monitoring systems etc. Apart from web connectivity, some form of GSM device is needed to enable this.

alarm system

The main interface used for GSM devices is via AT commands which is a standard request / response text based protocol. Using AT commands, it is possible to:

  • Read / write to the phone-book - AT+CPBR...
  • Send a SMS text message - AT+CMGS...
  • Detect ringing - RING +CLIP...
  • Get the date / time - AT+CCLK...

GSM Modem

To integrate a GSM modem into a product, the costs are high because of:

  • Moderate cost of the GSM modem itself
  • High cost of RF related EMC/compliance testing

As dedicated GSM modems are low volume and fairly specialised, they are relatively expensive due to market forces. Current GSM modems interface either via an RS232 cable or an internal, proprietary bus - to date, none have a infrared port. Because a GSM modem is inherently an RF device that is cable-coupled, expensive RF related EMC/compliance testing is required.

GSM Mobile Phone

As GSM mobile phones are high volume, consumer products, they are relatively low-cost and widely available - even second-hand.

phone alarm system - data cable

Using the above design, with a proprietary data-cable, forces high-cost EMC testing - the controller, sensors and phone have to be EMC tested and approved. Because the GSM mobile phone is electrically connected, it forms part of the 'apparatus'.

Most recent phones have an IrDA compliant infrared port that may be used to enable low-cost SMS texting with minimal entry costs because:

  • Any IrDA phone may be used - a phone specific data-cable is not required
  • No RF related EMC/compliance testing is required
phone alarm system - IrDA

Using the above design, with IrDA connectivity, allows minimal EMC testing - only the controller and sensors have to be EMC tested and approved. Because the GSM mobile phone is electrically isolated, it does not form part of the 'apparatus'.

Use of a mobile phone has several other advantages:

  • Non-volatile memory for configuration parameters, ie the SIM phone book
  • In-built keypad / display for viewing and setting configuration parameters
  • Ability to run from integral battery as well as external power
  • Availability of the in-built clock

Re-use a product that would otherwise be scrapped - find a use for that old mobile phone !

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