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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Telephone microprocessor switch 1x5. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony The proposed telephone switch (otherwise known as micro-PBX) is a device that allows you to connect up to five devices to one telephone line (without their mutual influence on each other), which have a set of additional service capabilities. Without a certificate of conformity, this switch can only be connected to those telephone lines that do not belong to public networks (for example, to departmental PBXs). To connect the switch to the public telephone network, you must obtain a certificate. The basis of the device is the microcontroller PIC16F84-04/P from Microchip, the program of which determines the operation of all other components of the switch. The device blocks phones that are not involved in a conversation, indicates a busy phone, allows you to randomly set calling numbers and forward incoming calls with music to the line. It is also possible to restrict access to the telephone line by setting a three-digit password separately for each phone (the "anti-piracy" function). For phones set as do not ring, you can set the number of missed calls before they start ringing. The switch can be reprogrammed from any telephone connected to it, with all settings saved in the electrically reprogrammable memory (EEPROM) of the controller. Power is supplied from the telephone line, current consumption is up to 200 µA. The device diagram is shown in Fig. 1. In the initial state, i.e., with the phones on-hook and in the absence of an incoming call signal, the processor program monitors the states of the R15R16 call sensor and the R18VT16 telephone current sensor. At this time, telephone sets are connected to the line, since a high level is applied to the gates of current switches VT9-VT13. The gates of the remaining field-effect transistors are low. For example, telephone set TA1 is connected via the following circuit: positive terminal of the telephone line, diode bridge VD18-VD21 with an open transistor VT8 diagonally, resistor R11 and LED HL2, transistor VT9, resistor R18 and base-emitter junction VT16, common wire (minus line ).
When you pick up the handset from one of the phones, current begins to flow through the base-emitter junction of transistor VT16 and it opens. The voltage at the VT16 collector changes from high to low, which is recorded by the processor program. Then the controller turns off the phones one by one, monitoring the level on the VT16 collector. As soon as the off-hook phone is disconnected, the voltage on the VT16 collector will again change from low to high and the program will be able to identify this phone. Then he reconnects to the line, and the rest of the phones are disconnected. If the device operates in the “anti-piracy” mode (this depends on the processor EEPROM settings), then when the handset is lifted, a high level is supplied to the VT14 shutter. Transistor VT14 opens, thereby connecting the zener diode VD27 to the line. In this case, dialing a number becomes impossible, since during dialing pulses the line current will flow through VD27, and the switching relays on the telephone exchange will not operate. However, the controller still has the ability to read the dialed number from the telephone current sensor, i.e., from the collector of the VT16 transistor. Thus, the user is given the opportunity to enter a three-digit password, which the controller program will compare with the values stored in the EEPROM, and if a match, the high level on the VT14 gate will change to low. To inform the user that the password was entered correctly, the device emits a confirmation sound signal into the line (through transistor VT15 and resistor R19). If the entered password does not match the required one, dialing into the line continues to be blocked. The processor waits until the handset is hung up, monitoring the state of the phones' current sensor. Switching to the settings programming mode is possible if, before picking up the handset, switch the SA1 toggle switch to a position in which pin 3 of the DD3 controller will be high. In essence, this is exactly the same mode as the “anti-piracy” mode, except that instead of the line access password, the corresponding programming codes must be dialed. An incoming call sensor is assembled on a resistive divider R15R16. When a call appears, a high level appears on resistor R16, which is monitored by the processor. Then the phones set as non-calling (if any) are switched off and the calling phone rings. During pauses between calls, the state of the telephone current sensor is monitored. After picking up the handset, the phone involved is determined, and the remaining devices are turned off. Next, the program waits for a number to be dialed, which is the phone number to which the call needs to be forwarded. Let's say an incoming call was answered from TA2 and needs to be forwarded to TA4. In this case, you need to dial the number 2 on TA4. After reading this number, the controller emits an audio signal into the line, turns off TA2 and connects TA4. Then a call signal is received on TA4. There are several known ways to generate ringing voltage while maintaining the connection. The simplest of them is the following. The called telephone is connected to the line, and the line itself is closed and opened to a small load (about 50 Ohms) using a current switch with a frequency of 25 Hz. In this case, during the opening, a voltage surge occurs, the amplitude of which depends on the inductance of the ATS relay and on the reactance of the connected telephone. Thus, with a line voltage of 60 V, it is possible to generate a ringing signal with an amplitude of 60...80 V. However, with this ringing voltage, not all telephone sets will ring. For example, telephones with an electronic ringing circuit may not ring at all or may ring quietly. If you increase the frequency of the ringing signal to 70...80 Hz, then most electronic phones will ring, but phones with a mechanical bell will stop ringing. This method also has two more big drawbacks: firstly, the subscriber at the other end of the line will hear a very strong and unpleasant sound, and the other drawback is significant interference that can be emitted during a call. The ringing voltage can be increased if an inductor is connected in series with the line through limiting and decoupling circuits. In this case, all phones will ring, but this method is not without these two disadvantages. In the proposed design, a simple single-cycle converter is used to generate the call voltage, switched by diode bridges with transistors in the diagonal. The converter consists of a master oscillator based on elements DD1.1, DD1.2, key transistor VT2, coil L1, diode VD6, storage capacitor C7 and limiting zener diodes VD9-VD12. When a high level is applied from the controller to the gate of transistor VT3, the converter is connected to the line. The generator on DD1.1, DD1.2 starts operating with a frequency of about 25 kHz. Key transistor VT2 switches coil L1, and a voltage of 7 V appears on storage capacitor C120, which is limited by zener diodes VD9-VD12. If we consider the example of forwarding an incoming call from TA2 to TA4, then the controller’s operating algorithm will be as follows. After TA4 has been connected to the common wire through an open transistor VT12, a high level is supplied to the gate of VT7. Transistor VT7 opens and VT8 closes. Thus, TA4 is disconnected from the positive terminal of the telephone line and remains connected to the storage capacitor of the C7 converter through the diode bridge VD14-VD17 and transistor VT6. At the same time, the converter is turned on by applying a high level to gate VT3. Then the controller produces two common-mode meanders with a frequency of 25 Hz to the gates of transistors VT5 and VT15. When their level is low, TA4 is connected to the converter; when it is high, it is disconnected from it and shunted by resistor R19 to discharge the bell circuit capacitor. This creates a call with a duration of 1 second. The current conversation is held by the current flowing through the converter. The voltage in the line is about 15 V, no interference is allowed through. The telephone current is monitored during pauses between calls. As soon as the handset is picked up from TA4, the ringing signal will stop and the converter will turn off. If the handset is not picked up, then after about 30 seconds the call will be redirected back. The absence of interference during forwarding made it possible to provide musical accompaniment to the line. To do this, the DD2 microcircuit, a melody former, is connected together with the converter. Music enters the line through elements VT4, R6. The processor is powered through the current stabilizer VT1. The voltage on it is about 5 V. The HL1 LED is used to indicate the polarity of the connection. If connected correctly, it should not light up. The switch is assembled on a printed circuit board made of double-sided foil fiberglass (Fig. 2). Coil L1 is wound on a B18 armored core with PETV-2 wire with a diameter of 0,1 mm; it contains 100 turns.
The assembled device should start working immediately, no settings are required. Reprogramming is performed as follows. With the handsets on all phones, the SA1 toggle switch must be set to a position in which a high level will be applied to pin 3 of the DD3 processor. Then you need to pick up the handset from any telephone and dial the password to access the programming mode. In the supplied firmware, this password is 21534. If it is entered correctly, a confirmation beep will be heard. Next, dial one of the codes given in table. 1, you can change any function and, after a confirmation beep, hang up. "xxx" means any three-digit number. It is possible to set the number of rings after which even non-ringing phones will ring. To do this, you need to dial 43x in reprogramming mode, where x is the number of missed calls. If x=0, non-ringing phones will not ring. Table 1
The switch is operational on PBXs with voltages of 48...60 V, on most paired lines, and on electronic PBXs (when working with pulse dialers). Author: V.Kulakov, Rostov-on-Don
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