Telephone pirate protection device. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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The proposed device is another of the electronic means that prevent unauthorized use of the telephone line. Its scheme is shown in Fig. 1.
In the initial state, when the inputs R flip-flops DD1.1, DD1.2 high logic level, the direct outputs are low.
When the handset is picked up by the owner of the device or a "pirate" connected to the line to the resistor R3, the voltage on the line will drop, the transistor VT2 will close, VT3 will open and the R trigger inputs will go low. However, trigger switching will not occur and the line will remain unblocked. Moreover, if the “pirate” picked up the phone, then the current through the resistor R3 does not flow and a high level is held at the input D of the trigger DD1.1.
When dialing a "pirate" to the input C DD1.1 receives pulses that rewrite the information from the input D to the output of the trigger. As a result, the output of the trigger DD1.2 will be set to a high level. Transistor VT1 will shunt the line, and dialing will become impossible.
When the number is dialed by the owner of the phone, at the moment the contacts of the dialer are closed due to the current through the resistor R3, the transistor VT4 opens. With a slight delay (due to capacitor C2), a positive voltage drop comes to input C DD1. Both triggers remain in their original state and the line is not blocked.
The device is assembled on a printed circuit board (Fig. 2).
The device can be powered from the telephone line (as shown in the diagram) or from a separate source.
When properly assembled, the device does not require adjustment. It is not critical to the accuracy of the values of the resistors used, with the exception of R1, which determines the supply voltage level and affects the amount of current consumed.
Author: B.Porohnyaviy, Krasnoyarsk
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