Receiver-decoder of DTMF signals. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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Tone (frequency) dialing - DTMF - is used in telephones, radio stations and other devices. This article describes a receiver-decoder that can be used in various designs. This device can be used for remote control of various devices, for transmitting small amounts of information by telephone or via radio stations, in devices for diagnosing the state of objects, etc.

A two-tone DTMF signal is well defined in the presence of interference in the transmission channel, so the reliability of such remote control systems is very high. If all 16 codes are involved, it is quite easy to implement a unidirectional telephone bridge, a device that allows you to link two telephone lines. In this case, you can, by calling one phone, dial the number on the second, connected to the second line. To do this, it is necessary to supplement the decoder with an "auto-hook" device and connect the outputs of the decoder to the keyboard of the second phone through optocouplers. Four "extra" codes can be used to control the second line and to "combine" the lines.

The scheme of the device is shown in fig. one.

(click to enlarge)

Chip DD1 KR1008VZH18 (import analogues - MV8870DP, MV8870-1DP, MT8870, M9270, AKT3170) is a DTMF signal receiver-decoder. The design and operation of the microcircuit are considered in some detail in [1, 2].

In the described design, a typical switching circuit is used. According to [2], the KR1008VZh18 microcircuit is not a complete analog of the MV8870 prototype. The latter has two options for the encoding table, which can be selected depending on the logic level at input 5. In this design, this feature is provided by jumper X2. Chips KR1008VZH18 and HM9270 have only one version of the table, in which the tonal combination corresponding to the number "0" gives the binary combination 10102=10. In this case, jumper X2 must be set to the "2-Z" position (at pin 5 of the DD1 microcircuit - low level).

In the most accessible book [1] on p. 160 encoding data in table. 8.7 are given with errors, both in the frequency column and in columns Q1-Q4 (output binary code). The correct version of the tables of correspondence between DTMF signals and the output binary code is given in [2] (see p. 50).

The DD2 chip converts the four-bit binary code from the DD1 output into sixteen signals that can be used to control various devices. After the receiver DD1 has received a two-tone message, the corresponding binary combination appears at the outputs Q1-Q4, which remains until the next message arrives. This allows you to implement two modes of operation of the decoder DD2. In the upper position of the jumper X4 ("2-3"), the signal at the corresponding output DD2 (low level) is present only during the action of the tonal message. If you set the jumper X4 to the lower position ("1-2"), the signal at the output of DD2 will be present indefinitely until the next tone message arrives.

LED НL1 is used to indicate the device is on and to control the recognition of the tonal message. In the position of the HZ jumper "1-2" the LED lights up permanently and briefly goes out for the duration of the tone signal. If you set the jumper to the "2-Z" position, the LED will turn on only when a two-tone message is received at the DD1 input.

The printed circuit board (Fig. 2) is made of one-sided foil fiberglass. The DD2 chip can be replaced with KR1533IDZ, but you need to consider that it has a different case.

Chip inverters DD3 - DD5 are used to control transistor switches (Fig. 3). As a buffer (without changing the pattern of the printed circuit board), you can use the K155LN2, K155LNZ, K155LP9 microcircuits (repeater, Fig. 4). The output transistors of the K155LNZ and K155LP9 microcircuits can operate at voltages up to 30 V and currents up to 30 mA [3]. If the board has open-collector chips at the output (LN2, LNZ, LP9). the second row of holes in the X5 output connector can be used to install "pull-up" resistors.

To power the device, any (including unstabilized) DC source with an output voltage of 8 ... 15 V is suitable. If K155 series microcircuits are used, the current consumption is about 90 ... 100 mA. It will be significantly less when installing microcircuits of the KR1533, K555 series.

The device can be connected to a telephone conversation node or directly to a telephone line. In the latter case, capacitor C1 must have an operating voltage of at least 160 V. A device that is correctly assembled from serviceable parts does not require adjustment.

The easiest way to check the device is to call someone you know who has a telephone set with the ability to switch to tone dialing mode. It is even better to use a "beeper" on the remote phone. The sample made by the author normally determined the signals of the "beeper", which was installed at a distance of 10 cm from the microphone of the handset. Of course, this test is purely "qualitative" in nature, since it does not take into account the frequency response of the emitter, microphone, telephone line. In most cases, only 12 tones can be checked in this way ("0"-"9", "#", " " ).

It should be noted that in [1] in Fig. 8.9 (p. 160) and fig. 8.13, 8.14 (p. 162) an inaccuracy was made in the switching circuit of the KR1008VZH18 microcircuit. True, at the same time, the microcircuit works, but the resistance to chatter and interference worsens. Resistor R3 = 300 kΩ (Figure 8.9) should be connected to pin 16, and the connection point R4-C17 to pin 8.10 (by the way, Figure XNUMX in this book shows the correct connection).

According to [2], the internal delays in determining tone bursts in a DTMF decoder are within 10...15 ms. In other words, with the corresponding values ​​of C5, R4, the maximum repetition rate of tone bursts is approximately 20...50 Hz. If we take into account that four bits are transmitted at once in one package, then for many applications a completely satisfactory speed is obtained.

Literature

1. Kizlyuk A. I. Handbook on the device and repair of telephone sets of foreign and domestic production. 3rd ed. - M.: Biblion. 1997. p. 149-162.
2. Chips for telephony and communications (Chips for telephony. Vol. 2). - M.: DODEKA. 1998. p. 45-52.
3. Biryukov SA Digital devices on integrated circuits. 2nd ed. - M.: Radio and communications. 1987. p. 9-10.

Author: O. Fedorov, Moscow

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