DTMF Generator/Decoder

The photo depicts a DTMF generator/decoder pair you can build in an afternoon or two. Dual-tone-multi-frequency (DTMF, also known as touch-tone) are the audible sounds you hear when you press keys on your phone.

The tone generator (top) uses the 5589 chip and a DIP switch. You can actually hear the tones through the speaker. The bottom circuit uses the 8870 to decode a tone and display its associated number on the 7-segment LED.

Touch-tone is familiar to many (telephone), it is a mature technology, and readily available with off-the-shelf, single-chip, low-cost components. For these reasons DTMF is often used in remote control applications that typically use telephones (e.g. accessing your messages from an answering machine, retrieving your account balance info from your bank's database).

This tutorial will not discuss telephone interfacing. Rather it will give you a basic working foundation which you can build upon. The generator/decoder above are tethered together by a single wire. But you can expand upon this foundation for wireless remote control using a microphone. For longer distances maybe you can add a pair of walkie-talkies, generating audible tones into one, and decoding with the other.

Another possibility is to use infrared (IR). Since tones are just electrical pulses, you can replace the speaker with an IR emitter and add an IR detector to the decoder.

Yet another experiment is to interface either the generator or the emitter or both to a PC or embedded microprocessor (e.g. 8051, PIC or Stamp). In this scenario, the PC or a peripheral, through touch-tones, can respond and control.

If you are familiar with how telephones work, the basic circuit might also help you to build devices the respond to your call. For example, you can build upon the decoder and add relays to control household devices that respond when you call your home.

Well hopefully I got you motivated. The bottom line is that DTMF was designed for optimal performance with each tone being very distinct. This makes decoding the tone very easy even in surrouding noise. It is this performance that makes DTMF ideal for clear transmission and reception in remote control (wireless or through phone lines) applications.