DTMF (Dual Tone Multi Frequency) decoder Circuit schematic using M8870

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 5 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 (For example, Remote control of home/office electrical appliances using a telephone network, Cell Phone controlled home appliances, Mobile phone controlled robot, etc.).Recommended for better understanding:

Circuit Diagram

DTMF decoder circuit
(Click on the image for enlarged view)

Components required

  1. DTMF decoder IC (M-8870)
  2. Resistors (100kΩ; 70kΩ; 390kΩ)
  3. Capacitors (0.1µFx 2)
  4. Crystal oscillator (3.579545MHz)

What is the Need of DTMF Decoding?

In the premature days, our telephone systems were operated by human operators in a telephone exchange room. The caller will pick up the phone, giving instruction to the operator to connect their line to the destination. It is a kind of manual switching. As more and more people entered in the telephone technology as useful communication gear, manual switching becomes a time consuming tedious task.

As technology established, pulse or dial tone technique were invented for telephone communication switching. It employs electronics and computers to support switching operations. DTMF is the ultimate technique used in any of the Mobile, Telephone communication systems.

The operation of DTMF method are as follows:

  • Caller generates a dial tone consisting of two frequencies. It is transmitted via the telephone line (communication media).
  • Telephone exchange consists of a DTMF decoder, which decodes the frequencies in to digital code.
  • These codes are the address of destination subscriber; it is read and processed by a computer which connects caller to the destination subscriber.

Working of DTMF decoder circuit

  • DTMF keypads are employed in almost all landline and mobile handsets. Thus this technology is used in the telephone switching centers to identify the number dialed by the caller.
  • The decoder distinguishes the DTMF tones and produces the binary sequence equivalent to key pressed in a DTMF (Dual Tone Multi Frequency) keypad.
  • The circuit uses M-8870 DTMF decoder IC which decodes tone generated by the keypad of cell phone.
  • DTMF signals can be tapped directly from the microphone pin of cell phone device. Cut the microphone wire and you will get two wires red and green. The red wire is the DTMF input to the circuit.
  • The signals from the microphone wire are processed by the DTMF decoder IC which generates an equivalent binary sequence as a parallel output like Q1, Q2, Q3, and Q4.

Table showing DTMF Low and High frequency tones and decoded output

DTMF decoded frequency output table

  • There is an inbuilt Op amp present inside the M-8870 decoder IC. The electrical signals from microphone pin are fed to inverting input of the Op Amp via a series of resistance (100kΩ) and capacitance (0.1 µF).
  • The non inverting input of Op-amp is connected to a reference voltage (pin4 -VREF). The voltage at VREF pin is Vcc/2.
  • Pin 3 (GS) is the output of internal Op Amp, the feedback signal is given by connecting the output pin (pin3- GS) to inverting input pin (pin2- IN-) through a resistor (270kΩ).
  • The output of Op Amp is passed through a pre filter, low group and high group filters (filter networks). These filters contain switched capacitors to divide DTMF tones into low and high group signals (High group filters bypass the high frequencies whereas low group filter pass low frequencies).
  • Next processing sections inside the IC are frequency detector and code detector circuits. Filtered frequency passed through these detectors.
  • At last the four digit binary code is latched at the output of M-8870 DTMF decoder IC.

Uses of other pins:

  • The entire process from frequency detection to latching of the data, is controlled by steering control circuit consisting of St/GT, Est pins, resistor (390kΩ) and a capacitor (0.1µF).
  • 5th Pin, INH is an active high pin, inhibits detection of A, B, C, D tones of character.
  • 6th Pin, PWDN is an (active high), inhibits the working of oscillator thus stops the working of our circuit.
  • The 10th pin 10; TOE is the output enable pin which is active high logic and enables the latching of the data on the data pins Q0, Q1, Q2, and Q3.
  • 15th Pin StD is the Data valid pin, turn out to be high on detection of valid DTMF tone or else it remains low.
  • Pins 7 (OS1) and 8 (OS2) are used to connect crystal oscillator. An oscillator of frequency 3.579545 MHz is used here.

Pin out of DTMF decoder IC M-8870

33 thoughts on “DTMF (Dual Tone Multi Frequency) decoder Circuit schematic using M8870

  1. i designed a dtmf decoder which initially worked very nicely but now whenever i connect it to the +5v supply then the led connected at pin 5 starts glowing . any suggestions ?

  2. Hi.

    I tried to build a DTMF decoder using the IC MT8870DE but it didn’t work. All components are the same as above except that I used 100k for R2 in place of the 70k. Could this be the reason?

  3. I ‘ve been doing cellphone controlled home appliance without microcontroller .I used 1600 Nokia as dtmf receiver …I unable to glow 20w cfl bulb …I used diagram of this website… plz help I need to submit on 20th Feb

  4. I build this but I get all bad 3.58 Mhz Xtals from local supply

    When try with Xtal from circuit I purchased from market, it worked.

    Where to buy reliable 3.579545 Xtals for this project ?
    My money keeps wasted on bad components I get from market 🙁

  5. sir,

    we are in spy business in india and sell various equipments…….your circuit makes me feel that i should ask you…..
    can we have a small battery operated type of device with a screen and a small inbuilt mic…..now when ever we play a recorded audio clip which has a dtmf tone ( like a recorded phone call(………….can the system read it and display the number on the screen as to what was dialled in the audio clip……
    thanks and regards

  6. Hi,
    I Tested this circuit with GSM Module as well as the Samsung Galaxy fame,But I’m not getting the decoded output.. any Suggestions.?

  7. i am intersted in doing project like mobile control robot vehicle, but withouT microcontroller …so please suggest me how can i make this project by using ic HT9107B and L293D and how can i provide input tone to the ic HT9107B

  8. I build this but I get all bad 3.58 Mhz Xtals from local supply

    When try with Xtal from circuit I purchased from market, it worked.

    Where to buy reliable 3.579545 Xtals for this project ?
    My money keeps wasted on bad components I get from market 🙁

  9. Can I know how the input is fed to the decoder from the mobile. And how much volt will we obtain at the output terminals. And which is the msb and lsb from Q1 and Q4…

  10. Please explain to me what I have to do if I want to change x’tal 3,5mhz with lower or higer value. Give me formula to calculate x’tal for this case. Thank’s

  11. hello sir
    how can i measure the output signal of this circuit.does this mean at the outputs pin will be zero voltage unless i input the signal from dtmf.??

  12. How can I connect smat phone to DTMF thru 3.5 jack pin..please can you provide circuit diagram for connection between mobile and DTMF.?

  13. hello sir , may i get the full designing steps, i mean why we are using resistor\capacitor with particular value ?

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