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5.16 4 4 keypad matrix showing individual DTMF frequencies
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The low frequencies (R1 to R4) are referred to as the row group.
The high frequencies (C1 to C4) are referred to as the column
group.

DTMF encoding
Any combination of frequencies can be obtained using a 4 4 matrix
of switches or keypad (see Fig. 5.16). Remember we are borrowing
this technology from the telephone industry; it has been designed for
optimum efficiency for less-than-perfect telephone lines.
Standard touch-tone telephones use a 3 4 keypad matrix. This
switch matrix provides coding for all the row frequencies and only
three column frequencies (see Fig. 5.17). A 3 4 keypad matrix
is more readily available and has been used with all the circuits
described here.
Not all telephone keypads are made the same; therefore, some key-
pads on the market will not be suitable in these circuits. For in-
stance, some keypads have different internal switch wiring or
include proprietary ICs. So if you try another keypad, keep that in
mind if the circuit fails to operate properly.
Building a DTMF encoder is simple (see Fig. 5.18). The circuit
only requires a keypad, crystal, and 5089 IC. The pin out of the

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Chapter five