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.

On the telephone exchange side, it has a decoder circuit to decode the tone to digital code. For example, the tone of 941hz + 1336hz will be decoded as binary '1010' as the output. This digital output will be read in by a computer, which will then act as a operator to connect the caller's telephone line to the designated phone line. The telephone exchange center will generate a high voltage signal to the receiving telephone, so as to ring the telephone bell, to notified the receiving user that there is an incoming call.

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

This the first DTMF circuit I build. Very small, roughly about my thumb size.


Output Logic behavior from the DTMF decoder IC.

no button press
TOE: Logic 0
Q4: Logic 0
Q3: Logic 0
Q2: Logic 0
Q1: Logic 0

'1' press and hold
TOE: Logic 1
Q4: Logic 0
Q3: Logic 0
Q2: Logic 0
Q1: Logic 1

release from button '1'
TOE: Logic 0
Q4: Logic 0
Q3: Logic 0
Q2: Logic 0
Q1: Logic 1

'2' press and hold
TOE: Logic 1
Q4: Logic 0
Q3: Logic 0
Q2: Logic 1
Q1: Logic 0

release from button '2'
TOE: Logic 0
Q4: Logic 0
Q3: Logic 0
Q2: Logic 1
Q1: Logic 0

'0' press and hold
TOE: Logic 1
Q4: Logic 1
Q3: Logic 0
Q2: Logic 1
Q1: 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
Q4 Q3 Q2 Q1
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.

Schematics and BOM is provided below for your reference.

Bill of Material (BOM)
Part# Description Value Qty
C1, C2 Capacitor should be 100nF not 100pF.
100pF, 250V x2
D1 High Conductance Fast Diode 1N4148 x1
D2, D3 1.3 Watt Power Zener Diode Zener 5.6V x2
DS1, DS2, DS3, DS4, DS5 Typical RED GaAs LED x5
R1, R2 Resistor 56KΩ x2
R3 Resistor 150KΩ x1
R4 Resistor 47KΩ x1
R5 Resistor 330KΩ x1
R6 Resistor 100KΩ x1
R7 Resistor 68KΩ x1
R8, R9 Resistor 56KΩ x2
R10, R11, R12, R13, R14 Resistor 470Ω x5
U1 Integrated DTMF Receiver MT8870DE x1
U1 IC holder 18 pins x1
Y1 Crystal Oscillator 3.5795Mhz x1
JP1, JP2 Header, 2-Pin x2

Telephone Ring Detector Circuit Interface to Microcontroller.

Other DTMF Tone Generator or dialer chips, NTE1690, TCM5589N, TP5089N, 5088, 4089 and LC7366.
Other DTMF Tone Decoder chips 8870, SSI-202, LC7385 and KT3170.
Other DTMF Tone Generator + Decoder, 8888, 8889, 8880, HT9170C.

You may purchase MT8870 chips and 3.5795MHz crystals at http://electronics-diy.com/store.php?sel=ics&sub=remote_control