DTMF tone decoder

How it works

A dedicated DTMF decoder recognizes pairs of audio tones and presents a four-bit result. A data-valid or strobe output tells a microcontroller when it can read the decoded value.

DTMF supports sixteen symbols, including A–D in addition to the familiar twelve-key telephone keypad. The numeric code is device-defined, so read the exact IC’s truth table rather than assuming it equals the printed key value.

On the bench

Use a known audio source, appropriate coupling, and the required clock components. The original table’s bit labels were ambiguous; the restored description deliberately defers to the device datasheet.

Resources & provenance

Rewritten from the original circuits/tel/nu4p01x_tel003.php page. Component values and historical performance claims have not been independently bench-tested. The circuit figures have had their watermarks removed.