Dial Tones

Introduction

Telephones (not mobiles) now use DTMF to dial numbers. This involves outputting two different frequencies of beep at once (a different combination for each key). The wikipedia page linked above has a better description, but the frequencies are as follows:

We're going to emulate this with Espruino. It won't sound just like your normal phone's DTMF (because Espruino outputs a square wave rather than a sine wave), but it does work.

How do we mix the two tones?

Espruino can't output two tones at once, but it can output one tone on each pin. By connecting the speaker to two signal pins and putting a different tone on each pin, we can use the Speaker itself to mix the two different tones together!

You'll Need

• An Espruino Board
• A KeyPad (not required - see the last part of the tutorial)
• A Speaker

Wiring Up

• Wire up the two wires of the Speaker to pins C6 and C9
• Wire up the KeyPad as described here

Software

Just copy and paste this into the right-hand window, then click the Send to Espruino button.

var SPEAKERA = C6;
var SPEAKERB = C9;
var silenceTimeout;

function onKeyPad(key) {
  // tones for the row and column
  var tone_col = [1209, 1336, 1477, 1633];
  var tone_row = [697, 770, 852, 941];
  // our key is a number between 0 and 15
  // work out row and column
  var col = key&3;
  var row = key>>2;
  // now output two tones, one on each pin
  analogWrite(SPEAKERA,0.5,{freq:tone_col[col]});
  analogWrite(SPEAKERB,0.5,{freq:tone_row[row]});
  // finally make sure we get rid of the beeping after a second
  if (silenceTimeout!==undefined) clearTimeout(silenceTimeout);
  silenceTimeout = setTimeout(function() {
    silenceTimeout = undefined;
    // make sure we turn out beeping off
    digitalRead(SPEAKERA);
    digitalRead(SPEAKERB);
  }, 100);
}

require("KeyPad").connect([B2,B3,B4,B5],[B6,B7,B8,B9], onKeyPad);

Now, just press the number keys on the Key Pad and the corresponding tone will be output. If you pick up your landline and press the microphone against the speaker you wired up, you'll actually be able to dial numbers with it!

Automatic dialling

In fact there's no need for the keypad - you could just dial automatically. For instance:

function fastDial(num) {
  var dial = function() {
    // dial the first digit of the number
    onKeyPad("123A456B789C*0#D".indexOf(num[0]));
    // cut the first digit off and go again 0.5 sec later
    num = num.substr(1);
    if (num.length>0) setTimeout(dial, 500);
  };
  dial();
}
fastDial("01234123123");

Note: If you don't have a keypad and want to leave out that code, just delete the line beginning require("KeyPad") from the original code.

And if you want to dial the number when you press the button on Espruino, just do the following:

setWatch(function() {
  fastDial("01234123123");
}, BTN, {edge:"rising", repeat: true});