The mobile which is connected to the system is kept in ‘auto answer’ mode. Using DTMF technology, any devices can be switched and controlled by pressing the corresponding buttons during the course of a call. These tones are then decoded by using a DTMF decoder and the corresponding devices are chosen. Initially an authenticated signal is sent from the user’s cellular phone via Global System for Mobile Communication (GSM) network to the phone which is fixed to the equipment.
This signal or code consists of the information about the function or action to be taken place i.e. what appliance should be turned off or turned on. The receiver phone receives the DTMF signal or a SMS message that is send from the user’s phone and then sends it to the DTMF decoder or the GSM modem which in turn sends the output digital signal to the microcontroller. Then the microcontroller, based on the received signal, controls the different relays connected through ULN2003 (Darlington transistor) and triggers the required appliance.