4.2 Control by DTMF Decoder

  • Dual-tone multi-frequency signalling (DTMF) is used for telecommunication signalling) over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching centre.
  • The DTMF system uses eight different frequency signals transmitted in pairs to represent 16 different numbers, symbols and letters.
  • The DTMF keypad is laid out in a 4×4 matrix in which each row represents a low frequency and each column represents a high frequency. Pressing a single key sends a sinusoidal tone for each of the two frequencies. For example, the key 1 produces a superimposition of tones of 697 and 1209 hertz (Hz). Initial pushbutton designs employed levers, so that each button activated two contacts. The tones are decoded by the switching centre to determine the keys pressed by the user.
  • In the DTMF module, external control can be achieved in case of any untoward incident involving the person on the wheelchair. An outsider can call the mobile phone that is integrated to the wheelchair, and by the press of the button, the wheelchair can be controlled.

Figure 16: DTMF Module

4.2.1 Incorporating DTMF into the wheelchair:

  • In case of any untoward incident involving the wheelchair user, by using a DTMF module, the wheelchair can be externally controlled.
  • A mobile phone is integrated or attached to the wheelchair.
  • An outsider can call this mobile phone, and by the press of the push-buttons, the wheelchair can be controlled.

4.2.2 Working:

The guardian with external control over the chair makes a call to the mobile phone attached to the wheelchair which is set in auto-answer mode (as a result the call gets picked automatically) and connected to HT9170 DTMF IC. The DTMF signals sent by the guardian is accepted by the mobile phone 2 and given to the DTMF IC. The HT9170 powered by a 5V Battery, decodes the DTMF signals to produce four analog signals v0, v1, v2 and v3. These signals are passed to the Raspberry Pi. The ARM11 processes the inputs based on the code written and gives appropriate output signals to the motor driver circuits. The motor driver circuits consist of the PID controllers and the MOSFETs that help in running the motors. They are powered through 12V, 7Ah rechargeable batteries. The motors in turn run the wheels and the desired motion is achieved. As an example, pressing the '5' button generates the tones 770Hz and 1336Hz. The frequencies were chosen to avoid harmonics (such that no frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies). The frequencies generated must be with +/- 1.5%, the highest frequency must be as loud as the lowest frequency and as much as 4db louder. This level difference is referred to as 'twist'.

4.2.3 DTMF Table

Figure 17: Frequency super-positioning in DTMF

Figure 18: Frequency Table for DTMF

4.2.4 DTMF control

  • Press 2 : Forward motion
  • Press 4 : Left turn
  • Press 6 : Right Turn
  • Press 8 : Reverse motion

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