DIY – Circuitos Geradores e Decodificadores de Tons DTMF

DIY – DTMF Tone Generator and Decoder Circuits

June 18, 2024 Roberto Magalhães

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 which generates 4-bit digital output. These circuits can be used to build

1. DTMF Remote Control Transmitter in Wireless Control Application
2. DTMF coded numeric keypad in security applications
3. DTMF based remote control receiver in wireless control application
4. Devices and applications controlled by cell phone

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

First, let's see how to build a DTMF tone generator circuit

Description of the DTMF tone generator circuit

(Check the DTMF tone generator circuit in tab 1 of the circuit diagram)

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

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

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

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

· The entire circuit receives power from a 5V power supply.

Here is the snap of DTMF Tone Generator Circuit configure. The main circuit of 91214B chip is built on PCB, and the speaker driver circuit of Q1 and Q2 is built on circuit board.

Figure 1: DTMF tone generator prototype based on IC 91214B

Circuit Operation:

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

Tabela mostrando atribuição de frequência para diferentes combinações de linhas e colunas no teclado DTMF

Figure 2: Table showing frequency assignment for different combinations of rows and columns on the DTMF keyboard

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

Circuit Test:

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

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

DTMF Tone Decoder Circuit

(Check the DTMF tone decoder circuit in tab 2 of the circuit diagram)

· DTMF tone input is given to the inverting terminal IN- (2) of the integrated operational amplifier through the 10 µF coupling capacitor

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

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

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

tD (15) – The tone detection pin drives a red LED through the NPN transistor connected in the switch configuration

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

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

Circuit Operation:

The MT8870 chip is a 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 consists of op amp like amplifier, anti-aliasing filter, low group BPF, high group BPF, steering logic circuit and latch output circuit. At most you can obtain 16 different combinations of DTMF tones due to 4 frequencies in the low group and 4 frequencies in the high group. Therefore, it provides a 4-bit digital output, one for each combination of low and high group frequencies. See the table below.

Figure 3: DTMF decoder circuit prototype based on IC MT8870

To test the circuit, connect the output of the DTMF Tone Generator Circuit to the input of this circuit first. Provide common 5V power to both circuits. Turn on the power supply. Now press keys 1 to 9 on the DTMF tone generator circuit. The tone detection LED flashes whenever any key is pressed and four LEDs D0 – D3 shine corresponding to the 4-bit digital output as 0000 to 1001 (0 means the LED is off and 1 means the LED is on)