What is DTMF? What can I use it for?

DTMF stands for Dual Tone Multi Frequency. It is the basis of the touch tone telephone system.
When you press the buttons on a touch tone telephone you can hear the DTMF Tones.

Eight different audio frequencies are transmitted in pairs to represent sixteen different numbers, letters and symbols:

The table shows the eight different frequencies used and the keys on a sixteen key Keypad.

When a key is pressed on the Keypad two audio tones are generated together.

For example pressing  5 would generate 770Hz and 1336 Hz  together. The waveform generated is similar to this:

Two tones together are used to avoid accidental operation which could occur if a single frequency were used.

By using a DTMF Encoder and a DTMF Decoder you can control almost anything you want to remotely. As the tone pairs are audio you can send them over a pair of wires, over phone lines, radio links (wlalkie talkie, PMR, CB, Amateur) or even across a room or further using a speaker and microphone.

You can generate the tones from a mobile / cellphone or a DTMF Encoder and keypad. (A, B, C and D are not available on a telephone).  At the receive end you need a DTMF Decoder which you can configure to switch things on and off, lights, pumps, motors, repeaters, alarms, cameras; it’s only limited by your imagination.

cstech.co.uk have a DTMF Encoder kit which encodes a sixteen key Keypad (0 -9, A -D, *, #) .

Also a range of DTMF Decoder kits and Modules with various output and other options.

Photo shows a cstech.co.uk DTMF Decoder being used to switch a  Ham Repeater.