DTMF Decoder

      While calling to someone, if we press any button in the cell phone a tone produced and the receiver can heard.This tone is called DTMF (Dual Tone Multi Frequency). DTMF assigns a specific frequency (consisting of two separate tones) to each key so that it can easily be identified by the electronic circuit. The signal generated by the DTMF encoder is a direct algebraic summation, in real time, of the amplitudes of two sine (cosine) waves of different frequencies, i.e., pressing ‘5’ will send a tone made by adding 1336 Hz and 770 Hz to the other end of the line. The tones and assignments in a DTMF system are shown below.

  

Frequencies	1209 Hz	1336 Hz	1477 Hz	1633 Hz
697 Hz	1	2	3	A
770 Hz	4	5	6	B
852 Hz	7	8	9	C
941 Hz	*	0	#	D

      An MT8870 series DTMF decoder is used here. All types of the MT8870 series use digital counting techniques to detect and decode all the 16 DTMF tone pairs into a 4-bit code output. The built-in dial tone rejection circuit eliminates the need for pre-filtering. When the input signal given at pin 2 (IN-) in single-ended input configuration is recognised to be effective, the correct 4-bit decode signal of the DTMF tone is transferred to Q1 (pin 11) through Q4 (pin 14) outputs.The circuit is shown below.

Click the circuit for enlarging  

           Below shows the DTMF data output of MT8870. 

Low Group (Hz)	High Group  (Hz)	Digits	Led  (L1)	D3  (Q4)	D2 (Q3)	D1 (Q2)	D0 (Q1)
697	1209	1	1	0	0	0	1
697	1336	2	1	0	0	1	0
697	1477	3	1	0	0	1	1
770	1209	4	1	0	1	0	0
770	1336	5	1	0	1	0	1
770	1477	6	1	0	1	1	0
852	1209	7	1	0	1	1	1
852	1336	8	1	1	0	0	0
852	1477	9	1	1	0	0	1
941	1209	0	1	1	0	1	0
941	1336	*	1	1	0	1	1
941	1477	#	1	1	1	0	0
697	1633	A	1	1	1	0	1
770	1633	B	1	1	1	1	0
852	1633	C	1	1	1	1	1
941	1633	D	1	1	0	0	0

0 (Zero) indicate low O\P

1 (One)  indicate high O\P

        While pressing the buttons on the cell phone the decoder ic decodes as above in the table. For example, when 7 is pressed the O\P's Q2, Q3, Q4 are high and the O\P Q1 is low.

        In the circuit, cell phone is connect through the headphone. For the more information about the headphone please see my 3.5 mm Headphone Jack connection post. In the real world the DTMF is useful for creating various types of circuits like cell phone controlled car, cell phone controlled home appliances, cell phone controlled relay. (click on the links)