Remote Circuit DiagramsTelephone Related

8-channel DTMF Link: Encoder Schematic Circuit Diagram

DTMF Frequencies: A Daily Occurrence

Every day, our telephone keypads generate millions of DTMF frequencies. These eight frequencies were carefully selected to minimize in-band signal levels caused by harmonics and intermodulation. The signal is encoded as a pair of sine waves, deliberately designed so that no frequency is a multiple of the other. Additionally, the sum and difference between two frequencies are carefully calculated to ensure they don’t match any single tone. This meticulous design choice is the reason why DTMF sounds discordant and distinctive.

8-channel DTMF Link Decoder Schematic Circuit Diagram

DTMF Encoder Circuit with HT9200B Device

The DTMF encoder circuit presented here is built around the HT9200B tone generator device developed by Holtek and available through distributors like Futurlec ( This encoder is part of a larger system that includes a decoder discussed in another section of this publication. The HT2900B is a traditional 14-pin device that can be controlled by a microcontroller to generate 16 dual tones and, in serial mode only, 8 single tones from the DTMF pin output. Its 8-pin counterpart, the HT9200A, operates exclusively in serial mode, while the HT9200B offers a versatile serial/parallel mode interface, suitable for applications like security systems, home automation, remote control via telephone lines, and communication systems.

Generating DTMF Tones with 74HC148 Priority Encoder

To convert information from switches S1–S8, which represent the keypad, into the 3-bit tone selection codes required by the HT9200B, an essential component is the 74HC148 8-to-3 priority encoder. The ninth switch, S9, is linked to input D3 on the encoder chip. When any of the switches S1–S8 is pressed, a corresponding 3-bit binary word is generated at outputs A0, A1, and A2 of IC1. IC2 then produces the dual tones based on these binary codes.

DTMF: Signaling System in Telephone Technology

Dual-tone multi-frequency (DTMF) refers to the sounds or tones produced by a telephone when numeric keys are pressed. These tones are transmitted within the voice channel and are vital for controlling automated equipment and indicating user intent, such as the desired number to dial. Each key on the phone generates two tones at specific frequencies.

DTMF Technology: Enabling Touch-Tone Phone Operation

Dual-tone multi-frequency (DTMF) technology is utilized in touch-tone phones, recognized by users as the audible feedback when pressing a number key. This technology signals the phone company, indicating the user’s intention to initiate a call and sending a command to the switch for further processing.


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