16.1 In this Chapter

This chapter contains the description of a device that interfaces a microcontroller to a telephone line, and is capable of receiving and decoding a series of DTMF signals, and to execute commands received this way.

16.2 Description of the Hardware Solution

Since telephone lines are available almost anywhere, it is interesting to design a device that, when connected to a phone line, is able to answer a call, and to recognize and execute a series of commands generated directly from the keypad of the telephone set that initiated the call.

This device operates according to the following set of rules:

• The human operator initiates a call, using a regular telephone set.

• The relay controller senses and counts the ring tones generated by the central office.

• After a specified number of rings, the relay controller answers the call and transmits a specific audible tone over the telephone line to acknowledge its presence.

• Using the telephone keypad, the operator generates a series of DTMF (Dual Tone Multi Frequency) tones, having a determined structure, which are received and decoded by the controller. The DTMF tones are audio signals, consisting of a combination of two sine waves, having precise frequencies in the range 697 1633 Hz. Each key of the telephone set keypad is associated with a distinct combination of frequencies, so that the receiving device can easily identify the key pressed.

• The relay controller sends back to the operator distinct audible tones after the reception of correct, or erroneous...