Keypad Dial Tone Frequency Table

The phone system use to be operated by human operator in a telephone exchange room. The caller will pick up the phone, giving instruction to the operator to connect their line to the destination over the other end of the telephone. As more and more people find phone technology a useful communication tools, line connection use human operator would be too tedious.

Then came the dial tone method using electronics and computer to connect, eliminating the needs of manual phone line connection. Basically on the caller side, it is a dial tone generator. When a key is being pressed on the matrix keypad, it generate a unique tone. For example, if the key '1' is being press on the phone, the tone you hear is actually consist of a 697hz + 1209hz sine wave. Pressing key '9' will generate the tone form by 852hz + 1477hz.  The frequency use in the dial tone system is of audible range suitable for transmission over the telephone cable.

On the telephone exchange side, it has a decoder circuit to decode the tone to digital code. For example, the tone of 941hz + 1336hz will be decoded as binary '1010' as the output. This digital output will be read in by a computer, which will then act as a operator to connect the caller's telephone line to the receiving telephone line. The telephone exchange center also generate a high voltage to the receiving telephone, so as to ring the telephone bell, notifying the receiving user.

The project below, focus on building a decoder. This circuit can be interface to a computer, allowing caller to computer interaction. Application can be for example be a computerize call receiving/diverting phone network system. Home/Office electrical appliances remote through telephone. Wireless remote control.

Photos of DTMF circuit built

Output Logic behavior from the DTMF decoder IC.

no button press

TOE: Logic 0

_Q4: Logic 0

_Q3: Logic 0

_Q2: Logic 0

_Q1: Logic 0

'1' press and hold

TOE: Logic 1

_Q4: Logic 0

_Q3: Logic 0

_Q2: Logic 0

_Q1: Logic 1

release from button '1'

TOE: Logic 0

_Q4: Logic 0

_Q3: Logic 0

_Q2: Logic 0

_Q1: Logic 1

'2' press and hold

TOE: Logic 1

_Q4: Logic 0

_Q3: Logic 0

_Q2: Logic 1

_Q1: Logic 0

release from button '2'

TOE: Logic 0

_Q4: Logic 0

_Q3: Logic 0

_Q2: Logic 1

_Q1: Logic 0

'0' press and hold

TOE: Logic 1

_Q4: Logic 1

_Q3: Logic 0

_Q2: Logic 1

_Q1: Logic 0

Schematics

Component Layout (Silkscreen)

PCB Bottom Layer (PCB trace)

Bill of Material (BOM)