In earlier days, an operator used to operate our telephone systems in the telephone exchange room manually. The callers would pick up the phone and give instruction to the operator to connect their destination line. The DTMF technology provides ultimate solutions for the telephone industries which are used to switch two lines automatically.


What is DTMF?

The DTMF is a signaling system for recognizing the keys or the dialed number of a push button or simply we can call it a DTMF keypad. It is a combination of two sine wave tones to constitute a key. The tones are called rows & column frequencies and they are represented on the DTMF keypad. DTMF switches ON the long-distance signaling to the dialed number through the voice frequency. In the dual tone multi-frequency, if we press any key on your phone, then it generates two tones of specific frequencies, the first specific frequency is a high frequency of tone and the second one is a low frequency of the tone.  


DTMF Keypad 


What is a DTMF decoder?

DTMF (Dual tone multiple frequency) is the most popular and nowadays ubiquitously used telecommunication signalling method. A DTMF decoder detects the DTMF tones and generates the binary sequence corresponding to the key pressed in a DTMF keypad. The circuit of this project presented here is a DTMF decoder. DTMF keypads are used in almost all landline and mobile handsets. The DTMF decoders, therefore, are used at the telephone switching centres to detect the number dialled by the caller. The DTMF version used in push button telephones is called touch tone and is a registered trademark of AT&T.





  • The DTMF tones are mainly used at the telephone switching centers, to detect the dialed numbers.
  • These are used in the terrestrial stations to switch on and switch off the remote transmitters.
  • DTMF is also used in the call centers, IVR systems, and security systems.
  • This system can be used in industrial applications.



  • A fast response will be achieved.
  • With a low price, the DTMF can be designed.
  • It can control the electrical devices wirelessly.
  • The consumption of power is less.
  • The construction of DTMF is very easy.


DTMF decoder with ARDUINO - Components Required:



The DTMF decoder consists of 6 pins: D0 to D3, VCC and GND. D0, D1, D2 and D3 are connected to pin 3, 4, 5 and 6 of Arduino respectively. VCC and GND are connected to +5V and GND pins of Arduino or we can provide an external power supply of 5V to DTMF decoder. Connect LEDs to pin 7, 8, 9 and 10 respectively. Connect the connector to the DTMF module as well as your mobile phone.  


DTMF decoder with Arduino:

 Power Supply - DTMF Decoder


Connect the external power supply of 5V dc to the DTMF module.


LEDs and resistors with Arduino:

Connection Diagram - DTMF



int x, y, z, t; int d0=3; int d1=4; int d2=5; int d3=6; int e=7; int f=8; int g=9; int h=10;

void setup()


pinMode(d0,INPUT); pinMode(d1,INPUT); pinMode(d2,INPUT); pinMode(d3,INPUT);

pinMode(e, OUTPUT); pinMode(f,OUTPUT); pinMode(g,OUTPUT); pinMode(h,OUTPUT);

Serial.begin (9600); }

void loop()  { x=digitalRead(d0); y=digitalRead(d1); z=digitalRead(d2); t=digitalRead(d3); Serial.print(x);


Serial.print(z); Serial.println(t); delay(1000);

if (x==1 && y==0 && z==1 && t==1)

{   digitalWrite(e,HIGH);   digitalWrite(f,LOW);   digitalWrite(g,HIGH);   digitalWrite(h,LOW);


else if (x==1 && y==1 && z==1 && t==0)

{   digitalWrite(e,LOW);   digitalWrite(f,HIGH);   digitalWrite(g,LOW);   digitalWrite(h,HIGH);   }

else if (x==1 && y==1 && z==0 && t==1)

{   digitalWrite(e,LOW);   digitalWrite(f,LOW);   digitalWrite(g,HIGH);



else if (x==1 && y==0 && z==0 && t==1)

{   digitalWrite(e,HIGH);   digitalWrite(f,LOW);    digitalWrite(g,LOW);   digitalWrite(h,LOW);


else if (x==0 && y==0 && z==1 && t==1)

{   digitalWrite(e,LOW);   digitalWrite(f,LOW);   digitalWrite(g,LOW);




Working Process:

In this project, we’ll be giving inputs to the DTMF module of different combinations through the DTMF app and correspondingly we’ll be observing output on 4 different LEDs of different combinations.


First of all, a connection is made as shown in fig. and we give i/ps as 1101 to D3D2D1D0 and it will turn on LED 1 and 3 and rest LEDs will be off and the process is repeated by giving different inputs and obtaining corresponding o/ps. We can control LEDs according to our choice for different i/ps just by changing the state of LEDs in the program.


The dual tone multi-frequency uses eight frequency signals which are transmitted in a pair to produce the sixteen numbers with the symbols & letters. The following table shows the eight numbers, symbols and the letters from A-D. Let us consider that, if we press any key it will produce the highest & a low tone for each of two frequencies. By using the HT9170D IC, digital counting technique these two tones are decoded to determine the key which should be pressed.



1209 Hz

1336 Hz


























Frequently Asked Questions

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.

Components and Supplies

    You may also like to read