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 does not match any single tone and that’s why DTMF sounds so ugly!T he DTMF encoder circuit show n here is based on the HT9200B tone generator device produced by Holtek and distributed by Futurlec among others. The encoder is complemented by a decoder elsew her e in this publication.
8-Channel DTMF Link: Encoder Schematic
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 . It s 8 - pin ‘ younger brother’ the HT9200A provides a serial mode only whereas the HT9200B contains a select-able 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 complementary 3-bit binary word at outputs A0, A1, A2 of IC1. IC2 then generates the dual tones accordingly to these binary codes.
Pressing S1–S8 generates the dual tones for DTMF digits C, B, A, #, *, 0, 9 and 8. By pressing and holding down S9 the DTMF digits 7, 6, 5, 4, 3, 2, 1 and D are generated.
To generate the eight single frequencies accurately a 3.58 MHz crystal quartz is connected to pin 2 and 3 of IC2. Pin 13 of the HT9200B supplies a DTMF signal of about 150 mV at a 5 KO load.
Pull-up resistor array R2 may be omitted if you substitute the 74HC148 with a 74LS148. R1 must be present in that case, otherwise it can be omitted.
The circuit consumes about 2 mA from a regulated 5 V supply. It should be easy to build on a small piece of prototyping board. link
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