The proposed device allows single-frequency simplex radio stations to have access to a local telephone network.
The device consists of:
receiver and transmitter DTMF signals into decimal code and pulse dialing, made on DD1 DD3..., DD8...DD12;
amplifier and comparator VOX system on DA1 and DD6;
generator control switch sending and receiving harvested on D12.4,D13.3 13.5..., D14.1, D15.1;
schema auto reset on DD4, DD5;
- shaper ringing on DD7.
The interface circuit - figure 1
The interface circuit - figure 2
A brief look at the work of the interface. After taking the radio connected to the interface, the symbol "*" and then enter the telephone number being dialed, the device enters the sleep mode two-way simplex communication is periodically switched to the reception. With the advent of the telephone line of any signals (call, voice) transmission time increases, but the scheme in any case periodically goes to the reception for the detection of possible voice signal and control signals from the radio station. All time parameters are easy to change. Also may increase the code length of the access password by setting between the elements of the decoder and the trigger DD14.2, DD14.3 circuits that output a logical "0" when a certain sequence of input signals. Reset phone interface signal is "#". An incoming phone call switches the circuit mode of transmission and generation in the radio channel melodic sound signal (chain C24, VD12...VD14, C25, DA2, VT4). Elements DD4, DD5, DD13.6, HDD15.2 drop interface in case of prolonged absence of the carrier from the receiver.
For operation of the telephone interface requires the following connections with the base station: - the output of the radio receiver is connected to the point the RX interface. The required signal level is set in a radio station; - a low input of the transmitter is connected with the point TX. In the absence of the possibility of adjusting the level of the signal between the radio interface and the transmitter is connected to a resistive divider; - contact the SQ is connected to the receiver so that the appearance of the carrier (open squelch) led to the transition of the voltage at the point SQ from a logical "1"(1...3V)to"0"(0...0,SV).
Allowed the use of optocouplers (with the proper inclusion), the led which illuminates the signal SQ station.
Mode "Reception-transfer controls, with appropriate inclusion of the contacts of relay K2 interface. If your radio has an electronic switching modes, it is better to use touchless control, replacing the relay K2 such as optocouplers.
A trimming resistor R3 sets the DTMF receiver sensitivity, R5 - sensitivity of the VOX amplifier, R9 - threshold operation of the comparator DD6, R11 - delay on the return VOX to its original state after the cessation of LF
dial tone, R16 - work period "for the switching of the generator in the presence of the bass signal on the telephone line, R17 - work period "for the generator in the absence of a bass signal on the telephone line, R18 - period of work on the practice of switching the generator when there is no active signal SQ. R35 installs the necessary level of the LF signal from the telephone line to the input of the transmitter.
HL1 led indicates the reception of DTMF tones, HL2 - bass signal reception system VOX, HL3 is triggered in the receiver squelch.
Altering the overall division ratio of counters DD4 and DD5, you can change the delay on reset interface with a long the absence of a signal at the receiver input.
Some features of schematic diagrams:
-diode VD1 should be germanium, otherwise, the diode is connected parallel to the resistor of the order of 200...300 kω;
the transformer T1 in the circuit can be almost anyone, it is desirable that the ratio was approximately unity, and the resistance of the winding connected to the line, was in the range of 200...500 Ohms.
Authors: A. Dovnar, V. Rusetsky, Minsk; Publication: N. Bolshakov, rf.atnn.ru