Another phone Guard
In recent years, the magazine "Radio" several times referred to the topic "telephone piracy". The magazine published a description of several relatively simple designs, allowing you to identify unauthorized connection to a telephone line, or to lock the dial with the "pirate" phone. In the article describes another device.
The scheme of the device for locking the telephone line in the illicit connection and emitting an audible alarm signal, shown in Fig. 1. In case of disconnection the line going into the apartment, the device also gives a warning. The attacker does not can call as a "long distance" and on the landline. To the disconnection of the line did not go unnoticed when the owner is not home, advise, for example, to negotiate with the neighbors, to repeat an alarm device.
The guard can be made in two versions: with audible alarm and without such. In an embodiment without the audible alarm device is powered by the telephone line, and with alarm - battery of electrochemical cells. Elements, necessary in the case of an alarm, shown in Fig. 1 on a colored background.
First, consider the case without alarm sound - powered telephone line. In this case, the device provides only a line lock when unauthorized connection. The current stabilizer DA1 and the Zener diode VD4 assembled the node power from the line. The key DA2 is designed to block the line. When the device is connected to the line with the correct polarity diode VD1 current flows on the resistive divider R1R2.
At pin 1 of the element DD1.1 there is a high level, since the voltage in telephone line 60 To the voltage across resistor R2 will be about 2.1 V, and voltage IC supply (set by the Zener diode VD4) - 3.2 V. the Diode VD2 protects the input circuit of high voltage at the time of the call. At the output of the element DD1.1 - low level, the output DD1.3 - high, and the multivibrator elements DD1.2, DD1.4 does not work. When you remove the handset from the telephone, located in the apartment, through a resistor R4, a current flows. Because of this, at the entrance 2 item DD1.1 is high level, and input 1 is low, as it falls the voltage on the telephone line. The multivibrator also does not work, because the output element DD1.1 will still be low.
When removing the tube from the apparatus connected to the device (pirate connection), both the inputs of the element DD1.1 will be low, therefore, the output is high. The capacitor C3 through the resistor R7 begins charge and upon reaching the threshold level at the output of the element DD1.3 will occur the low level. Starts running multivibrator DD1.2, DD1.4, the period of oscillation which is approximately equal to four pulse dialing. The key DA2 periodically closes the telephone line, preventing the pirate to dial the number. When stop pirated connection on the output element DD1.1 will periodically to experience a low level (with frequency circuit line key DA2) and the capacitor C1 will be discharged through the circuit R8VD5. The device returns to the original mode.
Led HL1 performs the function of indicating the correct polarity.
To implement the option watchman with an alarm you want to add the items shown on a colored background, remove elements of the food chain (DA1, VD3, R6) and indication (R11, HL1).
For signaling in isolation from the line you must have an external power. The device incorporates a battery voltage of 9 V ("Crown"). As alerter you can use electronic siren industrial manufacturing or any generator loaded on the piezoceramic transducer. So the power to choose options based on signaling device, but the current consumption should not exceed the maximum current key KRKT.
Led HL2 performs the function of the battery indicator. When very small the current consumption of the device is continuously enabled, the led would be almost the only consumer of battery power. Therefore, the display operates as follows: when hook on "legitimate" phone opens the transistor VT1, the led turns on HL2, fed by the battery.
In the absence of the battery or its discharge below the threshold level (set Zener diode VD6) led turns on, which will signal the need replacement battery. Conventional battery lasts approximately eight months from calculate four hours on the phone per day.
Printed circuit Board for option devices without an audible indication shown in Fig. 2.
Note that the watchman reliably indicates the presence of pirated connection, but not always able to prevent it. It is therefore necessary to try to eliminate free access to a telephone line, switching the shield.
Author: M. Pashkov, St. Petersburg