The DTMF generator comprises 3 main components.

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1 Make a DTMF generator with an Arduino board This article is for absolute beginners, and describes the design and construction of a DTMF generator. DTMF generators are often used to signal with equipment at the remote end of an RF link. EMDRC use them to control their repeater and ATV link. DTMF signalling is explained at Recent years has seen the emergence of very cheap, small and powerful microcomputers. Arduino and Raspberry Pi are but 2 examples. Whilst the hardware and software associated with these systems might seem daunting, I would recommend having a crack at something Arduino based. They are very much simpler than Raspberry Pi to program. The DTMF generator comprises 3 main components. 1. A keypad. These are normally a 4x3 matrix, emulating the keypad on a telephone. There are also 4x4 variants available, and there are DTMF tones available for the 4x4 keypad. 2. A microcontroller. The basic Arduino UNO is used. It has more than enough input/output capacity and processing power. 3. A small speaker to emit the multi tones. Keypad The keypad is arranged as a matrix, i.e. 12 keys arranged in 4 rows and 3 columns. Each row/column is made available at the 7 pin edge connector. There seems no sense to the pinout it is whatever was convenient to the PCB designer. Pressing of a key will cause a short circuit on the intersecting row and column. To detect a keypress, all that is required is to scan every row/column combination and detect the short circuit. Conveniently, the IO pins on the Arduino can be set to be either INPUT or OUTPUT. And further, INPUT s can be set to have an internal pullup resistor. This means that we can drive the columns as outputs, and read the rows with an input that is normally HIGH. This is done by driving each column LOW in turn, and scanning each row for an input which is also LOW. If one is found then the key on the intersecting row/column has been pressed. As with any mechanical system, there is the possibility of contact bounce, but this is easily eliminated by requiring that the row input remain LOW for at least 50mS. Microcontroller (Arduino) The microcontroller runs the code that performs the function of the DTMF generator. The language is C. The code must contain 2 modules setup which is run once when power is applied, and

2 loop which is run continuously until power is removed. The code is mostly self documenting, but some elements require a little further explanation. The Arduino has a sound generator, but it can only play one sound at a time. To be a DTMF generator, it is necessary to play 2 tones simultaneously. The required tones are generated by two independent timers. Each timer is set to one half of the period of the required tone. When each timer expires, the tone output is inverted, generating a square wave of the required frequency. The two tones are mixed using a pair of 240Ω resistors. A 4.7 mf capacitor is also used to round off the generated square waves. The source code is listed below. I can it to you if you wish to build this device. Arduino code (they call it a sketch) /* DTMF encoder (Dual Tone Generator) Created by Marc Hillman, VK3OHM Released into the public domain. SETUP: - Connect a 4x3 (7-pin), or 4x4 (8-pin), keypad matrix between D2-D9 - Connect Pins 12 and 13 to the + speaker terminal, each through their own 240 Ohm resistor - Connect a 4.7 uf capacitor between the + and - terminals of the speaker - Connect speaker GND to Arduino GND */ // Pin allocation for keypad columns #define Col1 4 // pin for Col 1 #define Col2 2 // pin for Col 2 #define Col3 6 // pin for Col 3 #define Col4 9 // pin for Col 4 (only present on 4x4 keypad) // Pin allocation for keypad rows #define Row1 3 // pin for Row 1 #define Row2 8 // pin for Row 2 #define Row3 7 // pin for Row 3 #define Row4 5 // pin for Row 4 // pin allocations for tone outputs #define tone1pin 12 // pin for tone 1 #define tone2pin 13 // pin for tone 2 #define NoKey 0xff // no key pressed const byte columns[] = {Col1, Col2, Col3; // list of all columns const byte rows[] = {Row1, Row2, Row3, Row4; // list of all rows const byte NumCols = sizeof(columns) / sizeof(byte); // calculate number of columns const byte NumRows = sizeof(rows) / sizeof(byte); // calculate number of rows byte LastKey = NoKey; // last key pressed // frequencies adopted from: int DTMF[][2] = { {697, 1209, // frequencies for key 1 {770, 1209, // frequencies for key 4 {852, 1209, // frequencies for key 7 {941, 1209, // frequencies for key * {697, 1336, // frequencies for key 2 {770, 1336, // frequencies for key 5 {852, 1336, // frequencies for key 8

3 {941, 1336, // frequencies for key 0 {697, 1477, // frequencies for key 3 {770, 1477, // frequencies for key 6 {852, 1477, // frequencies for key 9 {941, 1477, // frequencies for key # {697, 1633, // frequencies for key A {770, 1633, // frequencies for key B {852, 1633, // frequencies for key C {941, 1633, // frequencies for key D ; int ReadKeyPad() { /* Reads first key pressed. If no key pressed, returns 0xff The keypad is wired so that D2-D5 can be used to drive individual columns (Col1-Col4), whilst any key pressed can be read in the row (Row1-Row4) in D6-D The value returned will be (column-1) * 4 + (row-1), i.e. a number 0- The table below shows the keypad layout, with the value returned in brackets. Col 1 Col 2 Col 3 Col 4 Row 1 1 (0) 2 (4) 3 (8) A (12) Row 2 4 (1) 5 (5) 6 (9) B (13) Row 3 7 (2) 8 (6) 9 (10) C (14) Row 4 * (3) 0 (7) # (11) D (15) */ byte result = NoKey; // no key pressed for (int c = 0; c < NumCols; c++) digitalwrite(columns[c], HIGH); // set all columns HIGH for (int c = 0; c < NumCols; c++) { // scan each column digitalwrite(columns[c], LOW); // set column LOW for (int r = 0; r < NumRows; r++) { // scan each row for LOW on any row if (digitalread(rows[r]) == LOW) { delay(50); // in case of bounce if (digitalread(rows[r]) == LOW) return c * 4 + r; digitalwrite(columns[c], HIGH); return result; void setup() { // initialise row readers for (int i = 0; i < NumRows; i++) pinmode(rows[i], INPUT_PULLUP); // Set row drivers to INPUT // initialise column drivers

4 for (int i = 0; i < NumCols; i++) { pinmode(columns[i], OUTPUT); // Set column drivers to OUTPUT digitalwrite(columns[i], HIGH); pinmode(tone1pin, OUTPUT); // Output for Tone 1 pinmode(tone2pin, OUTPUT); // Output for Tone 2 Serial.begin(9600); // Set serial port speed void loop() { byte key = ReadKeyPad(); // read any key being pressed if (key!= LastKey) // key has changed switch (key) { case NoKey: break; // nothing to do default: // play tones Serial.print("Pressed "); Serial.println(key); // Debug only playdtmf(key, 500); break; LastKey = key; // remember last key pressed void playdtmf(byte digit, unsigned long duration) { // Play the DTMF digit for duration millisecs unsigned long tone1delay = ( / DTMF[digit][0]) - 10; // calculate delay (in microseconds) for tone 1 (half of the period of one cycle). 10 is a fudge factor to raise the frequency due to sluggish timing. unsigned long tone2delay = ( / DTMF[digit][1]) - 10; // calculate delay (in microseconds) for tone 2 (half of the period of one cycle). 10 is a fudge factor to raise the frequency due to sluggish timing. unsigned long tone1timer = micros(); unsigned long tone2timer = micros(); unsigned long timer = millis(); // for timing duration of a single tone while (millis() - timer < duration) { if (micros() - tone1timer > tone1delay) { tone1timer = micros(); // reset the timer if (digitalread(tone1pin) == HIGH) digitalwrite(tone1pin, LOW); else digitalwrite(tone1pin, HIGH); // toggle tone output if (micros() - tone2timer > tone2delay) { tone2timer = micros(); // reset the timer if (digitalread(tone2pin) == HIGH) digitalwrite(tone2pin, LOW); else digitalwrite(tone2pin, HIGH); // toggle tone output digitalwrite(tone1pin, LOW); digitalwrite(tone2pin, LOW); Construction The Arduino has a number of headers on each side. These are designed to accommodate plugin boards called shields. To get a pleasing result, if you are just plugging discrete components in, it is recommended to use Plug to Plug jumper leads, usually with one end removed. On the far left you will note that the first 2 header positions (0 & 1) are unused. These are reserved for serial IO. Even though we don t use them in this project, they do get used for debugging purposes, so best to avoid them. Using 7 jumper leads, make an interface cable to the keypad. The Arduino ends can be superglued together to make a single plug. The keypad ends are soldered. Plug the 7 leads into pins 2 8.

5 Butchering two of the cutoff plug ends, insert a 240Ω resistor in both, and twist the flying ends together. Use a third jumper to make an earth wire. Solder a 4.7mF capacitor across the speaker (-ve to ground) and connect speaker to resistors/gnd using pins 11, 12 & 13. Loading the software onto the Arduino is simplicity itself. Download the Arduino IDE from There are versions for Windows, Mac and Linux. Install, connect the USB cable, and then upload the sketch above. The sketch is now permanently loaded on the board. All you need to do is to apply power to the board, and you will have a functioning DTMF generator. To make it look pretty, you could put it in a project box. I will get around to that one day

6 Testing There is a Smartphone app DMF Decoder. Play the generator into your Smartphone and verify that the keys generate the right tones. Parts list Item Jaycar part # 4x3 keypad matrix LF-1290 Duinotech UNO r3 XC-4410 Duinotech 150mm Plug to Plug Jumper leads WC x 240 Ω resistor RR mF capacitor RY-6806 Speaker USB B cable WC-7700 Post build note someone has pointed out to me there is a free smartphone app that does the same thing. Yes there is, but this was fun to build anyway. Marc Hillman VK3OHM/VK3IP June 2017