The 12 keys on a cell phone (0, 1,…, 8,9,*,#) has unique signal associated with itself. This is DTMF signal.When the call is on, the pressing of any numerical key leads to generation of DTMF signal which is audible on the other side.
The DTMF tone for each key is sum of two sinusoidal waves of frequencies as given in following table. Thus each key has unique frequency pair and thus unique DTMF tone. For example, DTMF tone for key 6 is sum of two sinusoidal waves of frequency 1477Hz and 770Hz.
The extra keys A, B, C and D are not present on cell phone. They are actually specially used for special purposes. For example, public payphones that accept credit cards use these additional codes to send the information from the magnetic strip.
Automation Procedure :
We will be setting up a mobile phone permanently at the place to be automated in auto answer mode. The audio output from that phone is fed to the DTMF decoder module which in turn is connected to the microcontroller. When we make a call to this phone now it will automatically answer it, then we will be typing the correct password in our keypad to actuate the target. The numbers we type are sent to destination phone as DTMF signals, which are decoded by the decoder and fed to microcontroller. The microcontroller checks whether the password is correct according to the programmed code and gives us required output(here it opens a garage door). The motor attached to the door is controlled using a motor driver (RIG MOSFET driver).
Problems faced and remedies found:
- DTMF tones where not being decoded correctly by the decoder IC (8870)- we tried changing the gain of the decoder module input but did not have significant effect so we had to buy a decoder module online from hacktronicsindia.com.
- Password detection code was working only once- we used flags in code to overcome this problem.
- Module was not decoding sometimes when it was powered from the arduino board-gave separate power from other source.
Future applications and modifications:
- Complete automation of factories/ home appliances
- Feedback system using GSM for the user.