DIY - DTMF Tone Generator and Decoder Circuits

The given project describes how different DTMF tones can be generated using DTMF tone generator chip 91214B and how these tones can be decoded by DTMF decoder chip MT8870 that generates 4-bit digital output. These circuits can be used to build

1. DTMF remote control transmitter in wireless control application
2. DTMF encoded numeric keypad in security applications
3. DTMF based remote control receiver in wireless control application
4. Cell phone controlled devices and applications

Using these two circuits together one can build DTMF based remote control application in which 91214B chip is used as remote control and MT8870 is used at the receiver side to decode DTMF tones. The 4 bit digital output of MT8870 can be given to micro controller that will control any device or any application as per the button pressed on 91214B based remote control.

First let us see how to build DTMF tone generator circuit

DTMF Tone Generator Circuit Description

(Check DTMF Tone Generator Circuit in the circuit diagram tab 1)

· As shown in figure, 9 different push buttons are connected in 3×3 raw column structure. All three raw terminals and three column terminals are connected with raw (R1, R2, R3) and column (C1, C2, C3) pins of IC.

· The output of IC is drives 8? loud speaker through Darlington connection of Q1 and Q2. A 470? resistor is used to limit the current to Darlington pair

· A 3.59 MHz crystal with two capacitors C1 & C2 is connected to crystal terminals 3 and 4.

· A zener diode D1 is connected across the supply terminals of IC 91214B to limit the supply voltage of chip to 3.3 V. A 330? resistor is used to limit the current to zener diode

· Whole circuit is given power from 5 V power supply.

Here is the snap of DTMF tone generator circuit setup. Main circuit of 91214B chip is built on PCB and speaker driver circuit of Q1 and Q2 is built on bread board.

Fig. 1: Prototype of 91214B IC based DTMF Tone Generator

Circuit Operation:

The circuit is build using DTMF tone generator chip 91214B. It is a single-chip, silicon gate, CMOS integrated circuit with an on-chip oscillator for a 3.58MHz crystal or ceramic resonator. It provides dual tone multi-frequency (DTMF) dialling. A standard 4 x 3 matrix keyboard can be used to support DTMF modes. Total 12 keys are arranged in 4 rows and 3 columns. Each row is assigned specific frequency from lower group and column is assign one frequency higher group. Please refer the table.

Fig. 2: Table showing frequency assignment to different row and column combinations on DTMF Keypad

When we press any key the raw and column are connected that combines two frequencies and generates Dual Tone Multi Frequency (DTMF) tone. When supply is switched on all rows are high and columns are low. When any of the switches is pressed row and column gets connected and it generates DTMF tone. This DTMF tone is amplified by Darlington pair so that it can be heard through speaker.

Circuit Testing:

Switch on the power supply. Press switches 1 to 9. For each key you hear different DTMF tone through speaker. If you connect output pin 7 of chip with CRO (or DSO) you can observe DTMF tone output on it.

Now let us see how these DTMF tones are decoded and 4-bit digital output is generated using MT8870 chip.

DTMF Tone Decoder Circuit

(Check DTMF Tone Decoder Circuit in the circuit diagram tab 2)

· DTMF tone input is given to inverting terminal IN- (2) of built in op-amp through 10 µF coupling capacitor

· Two 100 K Ohm resistors connected as shown with pins IN- (2) and GS (3) selects gain of op-amp

· A 3.59 MHz crystal is connected to crystal input pins X1 (7) and X2 (8)

· A 330 K Ohm resistor is connected between Est (18) and St/GT (17) pins for steering circuit

· tD (15) – Tone detect pin drives one red LED through NPN transistor connected in switch configuration

· Other four red and green LEDs are connected to four outputs Q0 – Q3 to indicate digital output

· OE (10) – Output enable pin is connected to Vcc to enable output

Circuit operation:

MT8870 chip is DTMF tone decoder chip that takes DTMF tone as input and generates 4-bit digital output from 0000 to 1111 for 16 different DTMF tones. Internally it consist of op-amp as an amplifier, anti aliasing filter, low-group BPF, high-group BPF, steering logic circuit and latch output circuit. Maximum it can get 16 different combinations of DTMF tones because of 4 frequencies from low group and 4 frequencies from high group. So it gives 4-bit digital output one for each combination of low and high group frequencies. Please refer the table given below.

Fig. 3: Prototype of MT8870 IC based DTMF Decoder Circuit

To test the circuit, connect output of DTMF tone generator circuit to the input of this circuit first. Give common 5 V supply to both circuits. Switch on the power supply. Now press keys 1 to 9 on DTMF tone generator circuit. The tone detect LED blinks every time when any key is pressed and four LEDs D0 – D3 glows corresponding to 4 bit digital output as 0000 to 1001 (0 means LED is off and 1 means LED is on)