DTMF (Dual Tone Multi Frequency) decoder circuit identifies the dial tone from the telephone line and decodes the key pressed on the remote telephone. Here for the detection of DTMF signalling, we are using the IC MT8870DE which is a touch tone decoder IC. It decodes the input DTMF to 5 digital outputs. The M-8870 DTMF (Dual Tone Multi Frequency)decoder IC uses a digital counting technique to determine the frequencies of the limited tones and to verify that they correspond to standard DTMF frequencies. The DTMF tone is a form of one way communication between the dialer and the telephone exchange. The whole communication consists of the touch tone initiator and the tone decoder or detector. The decoded bits can be interfaced to a computer or micro-controller for further application (For example, remote control of home /office electrical appliance using a telephone network, cell phone controlled home appliances, Mobile phone controlled robot, etc.)
In the early days, our phone system used to be operated by human operator in a telephone exchange room. The caller will pick up the phone, giving instruction to the operator to connect their line to the destination over the other end of the telephone. As more and more people find phone technology a useful communication tools, line connection use human operator has become a tedious task.
As technology matures, pulse/dial tone method was inverted for telephony communication. It uses electronics and computer to assist in the phone line connection. Basically on the caller side, it is a dial tone generator. When a key is being pressed on the matrix keypad, it generate a unique tone consisting of two audible tone frequency. For example, if the key '1' is being press on the phone, the tone you hear is actually consist of a 697hz & 1209hz sine signal. Pressing key '9' will generate the tone form by 852hz & 1477hz. The frequency use in the dial tone system is of audible range suitable for transmission over the telephone cable.
This project article focus on a simple DTMF (dual tone multi-frequency) decoder circuit. This circuit can be interface to a computer, allowing caller to computer interaction. Many communication application can be build for example, a computerize call receiving/diverting phone network system. Remote control to Home/Office electrical appliances using a telephone network.
DTMF is a popular project especially in DSP (digital signal processing) subject. DSP software algorithm can be implement to generate as well as to decode DTMF tone. It is very interesting, and I will try to cover that aspect in near future. For now we do the hardware way.
DTMF Circuits
Very small, roughly about my thumb size.
Output Logic behavior from the DTMF decoder IC.
no button press
Q1: Logic 0
Q2: Logic 0
Q3: Logic 0
Q4: Logic 0
'1' press and hold
Q1: Logic 0
Q2: Logic 0
Q3: Logic 0
Q4: Logic 1
release from button '1'
Q1: Logic 0
Q2: Logic 0
Q3: Logic 0
Q4: Logic 1
'2' press and hold
Q1: Logic 0
Q2: Logic 0
Q3: Logic 1
Q4: Logic 0
release from button '2'
Q1: Logic 0
Q2: Logic 0
Q3: Logic 1
Q4: Logic 0
'0' press and hold
Q1: Logic 1
Q2: Logic 0
Q3: Logic 1
Q4: Logic 0
Detection of dial tones is reflected on the bit TOE, while the output Q4, Q3, Q2, Q1 indicate the dial tone that is being detected on the telephony system. A complete table of the decoded digital output for individual dial tone is available in the coming section.
Key Tone Output Logic
Q1 Q2 Q3 Q4
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
0 1 0 1 0
* 1 0 1 1
# 1 1 0 0
A 1 1 0 1
B 1 1 1 0
C 1 1 1 1
D 0 0 0 0
These are the decoder output table for the given dial tone detected. Notice that there are key tone for A B C and D. These are special tone which are normally not found on our telephone. It is a common standard build into the decoder chip.
The circuit is relatively simple and straight forward, and all components can be easily found
FOR MORE DETAILS WATCH THIS VEDIOS
https://www.youtube.com/watch?v=c61ZU8NEs9oTO BUY THE DTMF MODULE FOLLOW THE LINK GIVEN BELOW
http://www.amazon.in/Robomart-DTMF-Module-Version-3/dp/B00QUY4WAO/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1427033341&sr=1-1-catcorr&keywords=dtmf+module
http://www.amazon.in/DTMF-Module-MT8870-Decoder-Auxiliary/dp/B00RD55FNM/ref=pd_sim_sbs_t_1?ie=UTF8&refRID=09MTAMWT08NVY9XAE45E
http://www.robomart.com/index.php?route=product/search&filter_name=dtmf
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