DTMF Encoder and Decoder
Objectives:
1.
2.
3.
4.
To design DTMF Encoder circuit
To display DTMF Tone using Digital Oscilloscope
To design DTMF decoder circuit.
To display the decoded output using set of LEDS
Equipment and Components:
Serial No Component Name
No of Components Required
1
HT9200B
1 No
2
Crystal Oscillator(3.58MHz) 2 No’s
3
IC MT 8870
1 No
4
Push button switches
4 No’s
5
LED
4 No’s
6
100Kohms resistor
2 No’s
7
10Kohms resistor
1 No’s
8
1Kohms resistor
7No’s
9
330K resistor
1 No’s
10
0.1 Microfarad capacitor
3 No’s
11
10 Microfarad capacitors
1 No’s
12
IC 555 Timer
1 No’s
13
Power supply
1 No’s
14
Cutter
1 No’s
15
Wire
50 Pieces
Basic Theory:
DTMF or Dual Tone Multi Frequency is a signaling system for identifying keys which was
designed to recognize number press over telephonic system. For every key press two specific
frequencies are transmitted over telephone line.(DTMF) is a telecommunication signaling system
using the voice-frequency band over telephone lines between telephone equipment and other
communications devices and switching centers. DTMF (dual tone multi frequency) is the signal
to the phone company that you generate when you press an ordinary telephone's touch keys.
This DTMF (Dual Tone Multi Frequency) decoder circuit identifies the dial tone from the
telephone line and decodes the key pressed on the remote telephone. Here for the detection of
DTMF signaling, we are using the IC MT8870DE which is a touch tone decoder IC. It decodes
the input DTMF to digital outputs. The M-8870 DTMF (Dual Tone Multi Frequency) decoder
IC uses a digital counting technique to determine the frequencies of the limited tones and to
verify that they correspond to standard DTMF frequencies. The DTMF tone is a form of one-way
communication between the dialer and the telephone exchange. The whole communication
consists of the touch tone initiator and the tone decoder or detector. The decoded bits can be
interfaced to a computer or microcontroller for further application.
There are multiple ways to decode DTMF tone. You can use choose any one of these.
1. The easiest way is to use Mobile Phone. All mobile phones use DTMF tone for dialing
numbers. Connect a 3.5mm jack to mobile and feed the output to DTMF decoder. Pressed
different numbers in mobile & observe the decoder LEDs.
2.
If you want to generate from your PC there are multiple free software for it. Connect the
input of DTMF decoder to PC sound output port. Some software gives keyboard
accessibility and you can use numpad on keyboard.
3.
If you want DTMF generator for your project permanently you can make by yourself
easily. There are number of DTMF generator ICs e.g. UM91214B, HT9200A, HT9200B.
This ICs need very few numbers of external component to get work.
DTMF Decoding Using Mobile Phone:





Connect a 3.5mm jack to mobile phone and feed the output to DTMF decoder.
DTMF receiver decodes dial tone and identifies which key is pressed by the user.
The input to the decoder is a DTMF tone that is generated by DTMF encoder.
Decoder has Band split filter (FIR) which is centered at the frequencies of interest for
decoding each key pressed.
Band split filter section separates lower frequency tone and higher frequency tone and
provides it to digital decoder section

Digital decoder verifies the frequency and duration of received tones and provides4-bit
output.
Table showing DTMF Low and High frequency tones and decoded output
Button
Low DTMF
frequency(HZ)
High DTMF
Frequency(Hz)
Binary coded
output
Q1
Q2
Q3
Q4
1
697
1209
0
0
0
1
2
697
1336
0
0
1
0
3
697
1477
0
0
1
1
4
770
1209
0
1
0
0
5
770
1336
0
1
0
1
6
770
1477
0
1
1
0
7
852
1209
0
1
1
1
8
852
1336
1
0
0
0
9
852
1477
1
0
0
1
0
941
1209
1
0
1
0
*
941
1336
1
0
1
1
#
941
1477
1
1
0
0
Circuit Diagram:
Pin Diagram of MT8870 DTMF Decoder
Pin diagram of DTMF Encoder
Pin Description of IC 9200B (DTMF Encoder)
Pin Name
I/0
Internal
Connection
Description
CE
I
CMOS IN
Chip enable, active low.
No internal pull-high resistor
X2
O
Oscillator
The system oscillator consists of an inverter, a bias
X1
resistor, and the required load capacitor on chip.
I
The oscillator function can be implemented by
Connect a standard 3.579545MHz crystal to the X1
and X2 terminals.
VSS
-----
------
Negative power suppl, ground
NC
-----
------
No connection
D0-D3
I
CMOS IN
Data inputs for the parallel mode
Pull-high
or Floating
When the IC is operating in the serial mode, the data
input terminals (D0~D3) areincluded with a pullhigh resistor. When the IC is operating in the
parallel mode,these pins become floating.
CMOS IN
Operation mode selection input
S/P=H: Parallel mode
S/P
I
S/P=L: Serial mode
CLK
I
CMOS INPull- Data synchronous clock input for the serial mode
highor Floating When the IC is operating in the parallel mode, the
input terminal (CLK) is includedwith a pull-high
resistor. When the IC is operating in the serial mode,
this pin becomes
floating.
DATA
I
CMOS INPull- Data input terminal for the serial mode
highor Floating When the IC is operating in the parallel mode, the
input terminal (DATA) is included
with a pull-high resistor. When the IC is operating in
the serial mode, this pin becomesfloating.
DTMF
OUT
O
CMOS OUT
Output terminal of the DTMF signal
VDD
------
------
Positive power supply, 2.5V~5.5V for normal
operation
Procedure:
1. Construct astable multivibratorusing 555 timer as per the above circuit diagram.
2. Connect the o/p of the astable multivibrator to Chip Enable input of IC9200B.When the
Chip Enable input is active low, DTMF Encoder IC generates DTMF tone based on
pressed switch.
3. Observe the O/P of astable multivibrator using Digital Oscilloscope.
4. Construct DTMF encoder circuit as per the above circuit diagram.
5. Connect 4 push button switches to D0,D1, D2, D3 of IC 9200B as per the diagram .Instead
of mobile Key pad, here push-button switch is used for dialing no. When D0=1 and
D1=0, D2= 0, D3 =0 means digit 1 is pressed and DTMF tone for digit 1 will be generated.
6. Connect Oscilloscope to observe the DTMF Tone.
7. Plot DTMF generated Tone.
8. Construct DTMF decoder circuit.
9. Connect DTMF tone from the encoder to the input of decoder circuit.
10. Connect the O/P of DTMF decoder using LEDs as shown in fig.
11. Verify the LED display based on the key pressed. For Example if you pressed D0=1 and
D1=0, D2= 0, D3 =0, the same sequence will be displayed in the LEDs means the LED
which is connected to LSB is ON and remaining LEDs will be OFF.
Precautions:
1. Check the connections before switching ON the power supply
2. Observations should be done carefully
Results:
Post- Lab Requirements
1. Create a video of the total project with good audio (max 3 minutes)
2. Compare the results obtained in hardware lab with that of computer simulations.
3. Submit your video to the lab professor
Viva Questions:
1. What is DTMF Encoder?
2. How does DTMF decoder works?
3. What is MT 8870 Decoder?
4. What are DTMF tone used for?
5. Why does DTMF uses two frequency?
6. What is DTMF dialing?
7. What is the DTMF tone for mobile phone?
8. What are the chip available in market for DTMF Decoder?
9. What is the pin diagram of HT9200B IC?
10. What is the pin diagram of MT8870 IC?
11. What is chip enable line for IC 9200B?
12. Why push button is used?
13. When you pressed 1, what are the frequencies should be generated?
14. What is the difference between tone and pulse dialing?
15. What are the chip available in market for DTMF Encoder?
16. What is the uses of encoder and decoder?
17. What is DTMF and why it is used?
18. What are the applications of DTMF Decoder?
19. Why crystal oscillator is used?
20. What is pull high resistor?
21. What are the mode IC 9200B can able to operate?
22. In this project which mode IC 9200B is operating?
23. Why Pull high resistor is connected to Pin no 11 and 12 of IC9200B?
24. Why astable multivibrator is used?
25. When you pressed 5, which are the frequencies will be generated?
26. What is Low DTMF frequency?
27. What is high DTMF frequency?
28. What is the frequency of crystal oscillator?
29. Why pin 10 of IC9200B is connected to high?
30. How DTMF decoder detects tone from mobile phone?
31. You have MT 8870 IC, which pin you should give DTMF tone?
32. What is timing diagram of IC9200B?
33. What is the difference between IC 9200A and IC9200B?
34. What is the use of Decoder and Encoder?
35. What is the difference between Encoder and Decoder?
36. What is duty cycle in 555 timer?
37. What do you mean by Astable multivibrator?
38. What are the application of Astable multivibrator?
39. How many states has a stable astable multivibrator?
40. Why do we use multivibrator?
41. What are the application of LED?
42. What is the threshold voltage and current of LED to be ON?
43. Why push button switch is used in this project?
44. How to generate DTMF tone?
45. Why switches are connected to pull high resistors?
46. What is 4 X 3 key pad?
47. How to generate DTMF Tone using IC9200A?
48. Is it possible to generate DTMF tone using microcontroller?
49. Why DTMF generates two frequencies?
50. Is there any polarity for crystal oscillator?