Telephone interface 1. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Phone interface. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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The proposed device allows single-frequency simplex radio stations to have access to the city telephone network.

The device consists of:

- receiver and converter of DTMF signals into decimal code and dialing pulses, made on DD1...DD3, DD8...DD12;
- amplifier and comparator VOX system on DA1 and DD6;
- a generator that controls the reception-transmission switching, assembled on D12.4, D13.3 ... 13.5, D14.1, D15.1;
- automatic reset circuits for DD4, DD5;
- ringing signal generator on DD7.

Interface Diagram - Figure 1
Interface Diagram - Figure 2

Let's take a quick look at how the interface works. After the radio station connected to the interface receives the sign "*" and then enters the dialed telephone number, the device switches to the two-way simplex communication mode - periodically switches to reception. When any signals (ringing, voice) appear on the telephone line, the transmission time increases, but the circuit in any case periodically switches to receive to detect a possible voice signal and control signals from the radio station. All time parameters are easily changeable. It is also possible to increase the width of the access code-password" by installing a circuit between the elements of the decoder and the trigger DD14.2, DD14.3, which gives a logical "0" at the output with a certain sequence of input signals. Resetting the telephone interface is carried out by the signal "#". Incoming telephone call switches the circuit to the transmission mode and generates a melodic sound signal into the radio channel (circuit C24, VD12 ... VD14, C25, DA2, VT4).Elements DD4, DD5, DD13.6, HDD15.2 reset the interface in case of a long absence of carrier from the receiver.

The telephone interface requires the following connections to the radio base station: - The output of the radio receiver is connected to the RX interface point. The required signal level is set in the radio station; - The low-frequency input of the transmitter is connected to the TX point. If it is not possible to adjust the signal level in the radio station, a resistive divider is connected between the interface and the transmitter; - the SQ contact is connected to the receiver in such a way that the appearance of a carrier (opening the noise suppressor) leads to a voltage transition at the SQ point from logic "1" (1 ... 3V) to "0" (0 ... 0, ZV).

It is permissible to use an optocoupler (with appropriate inclusion), the LED of which is illuminated by the SQ signal of the radio station.

The "Reception-transmission" mode is controlled, with the corresponding switching on of the contacts, by the relay K2 of the interface. If the radio station has electronic switching of operating modes, it is better to use non-contact control by replacing relay K2, for example, with an optocoupler.

The trimmer resistor R3 sets the sensitivity of the DTMF receiver, R5 is the sensitivity of the VOX amplifier, R9 is the threshold for the operation of the comparator DD6, R11 is the delay for VOX to return to its original state after the LF has stopped

signal in the line, R16 - the period of operation "for transmission" of the switching generator in the presence of a low-frequency signal in the telephone line, R17 - the period of operation "for transmission" of the generator in the absence of a low-frequency signal in the telephone line, R18 - the period of operation "for reception" of the switching generator with no active SQ signal. R35 sets the required level of the low-frequency signal from the telephone line to the input of the transmitter.

The HL1 LED indicates the reception of DTMF signals, HL2 - the reception of the low-frequency signal by the VOX system, HL3 - the operation of the squelch in the receiver.

By changing the overall division ratio of the counters DD4 and DD5, you can change the delay for resetting the interface in case of a long absence of a signal at the receiver input.

Some features of the circuit diagram:

-diode VD1 must be germanium, otherwise a resistor with a resistance of about 200 ... 300 kOhm is connected in parallel with the diode;

- transformer T1 in the circuit can be practically any, it is desirable that the transformation ratio is approximately one, and the resistance of the winding connected to the line is within 200 ... 500 ohms.

Authors: A. Dovnar, V. Rusetsky, Minsk; Publication: N. Bolshakov, rf.atnn.ru

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