Against telephone pirates. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Against telephone pirates. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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We continue the topic of protecting home phones from "pirates". The proposed device is designed to protect SANYO telephone radio extenders model CLT-55KM. It is possible to operate such a unit in conjunction with other telephones that have technical characteristics similar to the above model.

The product blocks the connection of the base unit (hereinafter WB) to the telephone line until a four-digit code is dialed from the handset keypad. In addition, the BB is protected from "freezes", which can often occur when trying to connect to a pirate. If within 30 seconds after lifting the handset the correct code is not dialed, the device will "restart" the WB. After a successful dialing of the code combination, the WB is connected to the telephone line and a constant beep appears in the handset - the PBX tone signal, suggesting to start dialing. To indicate the connection to the telephone line, the green HL3 LED on the device board is used. After the end of the conversation and pressing the "END" key on the handset, the device goes into standby mode, deletes the dialed code from the memory and disconnects the WB from the telephone line.

When an incoming call is received, the device automatically connects the WB to the telephone line for the duration of the ringing signal. An incoming call is signaled by the yellow HL2 LED. After pressing the "TEL" key, the WB remains connected to the line until the end of the conversation without first dialing the code sequence. Thus, the operation of the phone during an incoming call remains unchanged.

To provide greater security, the operation of the "FLASH" key (reset line) is blocked. When you press this key, the WB remains connected to the line, but there is no reset and short beeps will be heard in the handset. On some lines, the "FLASH" key lock may not work.

For ease of use, the code sequence can be stored in the handset's memory and called up each time before dialing.

The diagram of the protection device is shown in the figure. It is powered by the mains power supply of the radio extender. The polarity of the connection to the telephone line does not matter.

(click to enlarge)

The code variant is determined by the jumpers between the outputs of the counter DD3 and the diodes VD19-VD22 and VD24-VD27. After picking up the handset, a low level occurs at the counting input of the CP DD3, and when dialing a number or code sequence, pulses occur, and the number of pulses corresponds to the dialed digit. In standby mode, the output of the DD5.3 element should be high, and when there is an incoming call or off-hook, it should be low.

The time constant of all integrators R23C8, R24C9, R25C10, R26C11 is constant. They provide a signal delay of approximately 0,35 s. After resetting the counter DD3, a high level voltage appears at output 0, and a low level voltage is present at all other outputs (1-9). With each change in the level at the CP input from low to high, the logical unit will sequentially move from output 0 to output 9 and then again to 0, 1, 2. The integrator at the input of the DD4.1 element is responsible for the first digit in the sequence, DD4.2 - for the second, DD4.3 for the third and DD4.4 for the fourth.

Suppose that the code sequence consists of numbers 4853. Then the VD24 diode must be connected to output 4 of the DD3 counter (pin 10), the VD25 diode to output 2 (pin 4), the VD26 diode to output 7 (pin 6), and the VD27 diode - to output 0 (pin 3). Why this is so will become clear from what follows. After picking up the tube, a high level will be present at output 0 of the counter DD3 (pin 3). During the dialing of the first digit - "4", the high level will shift to output 4. After 0,35 s (during the inter-series pause), a high level will appear at the output of the DD4.1 element and at the output of the trigger DD6.1, which will allow the signal to pass to the trigger input DD6.2. After dialing the second digit ("8"), the DD3 counter will receive eight clock pulses and a high level will be present at output 2 (pin 4 of the DD3 chip). After another 0,35 s, a high level will appear at the output of the DD4.2 element. The remaining numbers are entered in the same way.

If all the numbers are entered correctly, a high level will appear at the output of the trigger DD6.4, which will disable the counter DD3 and open the optoelectronic key DA1 through the transistor VT1. Thus, the telephone set will be connected to the line.

Diodes VD19-VT22 are connected in random order to the free outputs of the counter DD3 with the condition that between the two outputs of DD3 connected to the reset system (DD5.1, DD5.2), there must be either a free output or an output connected to the input system code (DD4.1-DD4.4). So, if one of the VD19-VD22 diodes is connected to output 1, then a reset system cannot be connected to outputs 0 and 2. In the case of the selected code sequence (4853), the reset system can be connected, for example, to outputs 1, 3, 5, 8.

In standby mode, there is a low level at the output of element DD7.3, relay K1 is de-energized and power is supplied to the base unit through contacts K1.1. If the correct code sequence is not dialed 30 seconds after picking up the handset, then a pulse with a duration of 7.3 s will appear at the output of the DD1 element. Relay K1 will work and the power supply to the BB will be interrupted.

Installation of the device begins with power circuits (R12, DA2, C13, R1, HL10). By connecting an external power supply with the ability to adjust the output voltage, they change the supply voltage in the range from 14 to 2 V. At the same time, they control the voltage at the input and output of the DA8 stabilizer. At the input, it should vary from 12 to 5 V, and at the output it should be constant (XNUMX V).

Next, they assemble and configure the line control and management unit. Install the elements of the incoming call sensor (C1, VD1, VD2, R1, R2, VD3, VD4, C2, R3) and the resistor R4. Then the set sensor is mounted, assembled on the elements VD6-VD11, R8-R11, C4, C5, R15. The device is connected to a telephone line (the positive output of the line to the top point according to the diagram) and the voltage on the capacitor C2 is checked. In standby mode (on the 60 V line), it should be equal to zero, and when exposed to a ringing signal, it should be 3,5 ... 5,6 V.

Then they connect a telephone (BB) to the output and, in standby mode, check the voltage at the zener diode VD6 and the VD11 diode, first with the handset down, and then with it up. With the tube down, the voltage at VD6 should be 5,6 V, and with the tube up, VD6 should be zero. A voltage of 11 V must be present on the VD5 diode in both cases.

After that, change the polarity of the telephone line connection and again check the voltage on the capacitor C2 according to the method indicated above. In standby mode, when the tube is lowered, VD6 and VD11 should have a voltage near zero, and when raised: zero on VD6, and 11 V on VD5.

Zener diodes VD1, VD2 can be used type 2S536A, VD4 - KS156A, VD7 and VD8 - 2S524A, KS524G, KS224Zh.

Author: A.Novikov

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