Remote Circuit DiagramsTelephone Related

8-channel DTMF Link: Encoder Schematic Circuit Diagram

Generated millions of times every day by our telephone keypads, the eight DTMF frequencies were chosen so that the harmonics and intermodulation do not generate significant in-band signal levels. The signal is encoded as a pair of sine waves, ensuring that no frequency is a multiple of the other and the sum and difference between two frequencies do not match any single tone — and that’s why DTMF sounds so ugly!

8-channel DTMF Link Decoder Schematic Circuit Diagram

The DTMF encoder circuit shown here is based on the HT9200B tone generator device produced by Holtek and distributed by Futurlec (w w among others. The encoder is complemented by a decoder elsewhere in this publication. The HT2900B is supplied as a nice old-fashioned 14-pin device. It can be instructed by a microcontroller to generate 16 dual tones and (in serial mode only) 8 single tones from the DTMF pin output. Its 8-pin ‘younger brother’ the HT9200A provides a serial mode only whereas the HT9200B contains a selectable serial/parallel mode interface for various applications such as security systems, home automation, remote control through telephone lines, communication systems, etc.

A 74HC148 8-to-3 priority encoder is used to convert the ‘keypad’ information from S1–S8 into 3-bit tone selection words the HT9200B wants to see at its input. The ninth switch, S9, is connected to input D3 on the encoder chip. Pressing one of the switches S1–S8 generates a complimentary 3-bit binary word at outputs A0, A1, and A2 of IC1. IC2 then generates the dual tones accordingly to these binary codes.

Dual-tone multi-frequency (DTMF) is the sounds or tones generated by a telephone when the numbers are pressed. These tones are transmitted with the voice channel. DTMF is used to control automated equipment and signal user intent, such as the number they wish to dial. Each key has two tones at specific frequencies.
Dual-tone multi-frequency (DTMF) is a technology used with touch-tone phones, best known to users as the sound made when pressing a number key. It signals the phone company that you want to make a call and sends a command to the switch.

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