1. CIRCUIT IDEAS
DTMF RECEIVER IC MT8870 TESTER S.C.
DWI
VED
I
RONE POUMAI radio communications, the tone duration The Status of LEDs on Pressing Keys
may differ due to noise considerations. on the Telephone Keypad
T
oday, most telephone equipment use Therefore, by adding an extra resistor and
a DTMF receiver IC. One common steering diode the tone duration can be Key No. LED4 LED3 LED2 LED1
DTMF receiver IC is the Motorola set to different values. (MSB) (LSB)
MT8870 that is widely used in electronic The circuit is configured in balanced-
1 Off Off Off On
communications circuits. The MT8870 is line mode. To reject common-mode noise
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an 18-pin IC. It is used in telephones and signals, a balanced differential amplifier
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a variety of other applications. When a input is used. The circuit also provides an
4 Off On Off Off
proper output is not obtained in projects excellent bridging interface across a prop-
5 Off On Off On
using this IC, engineers or technicians need erly terminated telephone line. Transient
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to test this IC separately. A quick testing protection may be achieved by splitting
7 Off On On On
of this IC could save a lot of time in re- the input resistors and inserting zener di-
8 On Off Off Off
search labs and manufacturing industries odes (ZD1 and ZD2) to achieve voltage
9 On Off Off On
of communication instruments. Here’s a clamping. This allows the transient energy
0 On Off On Off
small and handy tester circuit for the DTMF to be dissipated in the resistors and di-
A On On Off On
IC. It can be assembled on a multipurpose odes, and limits the maximum voltage that
B On On On Off
PCB with an 18-pin IC base. One can also may appear at the inputs.
C On On On On
test the IC on a simple breadboard. Whenever you press any key on your
D Off Off Off Off
For optimum working of telephone local telephone keypad, the delayed steer- Note. 1. LED5 blinks momentarily whenever any key is
equipment, the DTMF receiver must be ing (Std) output of the IC goes high on pressed.
designed to recognise a valid tone pair receiving the tone-pair, causing LED5 (con- 2. On = 1, while Off = 0
greater than 40 ms in duration and to ac- nected to pin 15 of IC via resistor R15) to
cept successive digit tone-pairs that are glow. It will be high for a duration de- The optional circuit shown within dot-
greater than 40 ms apart. However, for pending on the values of capacitor and ted line is used for guard time adjustment.
other applications like remote controls and resistors at pins 16 and 17. The LEDs connected via resistors R11
to R14 at pins 11
through 14, respec-
tively, indicate the
output of the IC.
The tone-pair DTMF
(dual-tone multi-fre-
quency) generated
by pressing the tele-
phone button is
converted into bi-
nary values inter-
nally in the IC. The
binary values are
indicated by glow-
ing of LEDs at the
output pins of the
IC. LED1 represents
the lowest signifi-
cant bit (LSB) and
LED4 represents the
most significant bit
(MSB).
So, when you
dial a number, say,
5, LED1 and LED3
will glow, which is
equal to 0101. Simi-
larly, for every
JUNE 2003 ELECTRONICS FOR YOU

2. CIRCUIT IDEAS
other number dialled on your telephone, switch S2 only if keys A, B, C, and D are or D), the DTMF IC 8870 is correct.
the corresponding LEDs will glow. Thus, a to be used.) Keys A, B, C, and D on the telephone
non-defective IC should indicate proper bi- 4. Now push key ‘*’ to generate DTMF keypad are used for special signalling and
nary values corresponding to the decimal tone. are not available on standard pushbutton
number pressed on your telephone key- 5. Push any decimal key from the tele- telephone keypads. Pin 5 of the IC is
pad. phone keypad. pulled down to ground through resistor
To test the DTMF IC 8870/KT3170, pro- 6. Observe the equivalent binary as R8. Switch on auxiliary switch S2. Now
ceed as follows: shown in the table. the high logic at pin 5 enables the detec-
1. Connect local telephone and the cir- 7. If the binary number implied by tion of tones representing characters A, B,
cuit in parallel to the same telephone line. glowing of LED1 to LED4 is equivalent to C, and D.
2. Switch on S1. (Switch on auxiliary the pressed key number (decimal/A, B, C, This circuit costs around Rs 80.
ELECTRONICS FOR YOU JUNE 2003