Phone dial blocker. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Phone dial blocker. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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A telephone in the house is a necessity and a boon. But one day in the mailbox you find a bill for calls from ... Los Angeles or Tel Aviv, where you never called. Everything is explained very simply - some lover of free calls connected to a pair of wires on your telephone line (this can be done outside your apartment). And you will have to pay for the negotiations.

There are several ways to prevent such cases. The easiest of them is to pick up the phone while you are away. True, PBX services do not welcome such a measure and may even turn off the line. The second way is to apply to the PBX with a request to block your line for long-distance and international calls. But at the same time, you yourself will be deprived of such an opportunity. And the third is to connect a dial blocker, such a device is also called a "telephone watchman". It is connected either instead of the phone or in parallel with it and does not allow you to dial any number at all. In standby mode, the watchman practically does not load the line.

(click to enlarge)

The diagram of the device is shown in the figure. In standby mode - the handset is not picked up either on the main or on the "parallel" (in our case, unauthorized) phones - when the device is connected to the line, the capacitor C1 is charged through the resistor R1 and the rectifier bridge diodes VD1 - VD4 to the stabilization voltage VD5. The voltage at the inputs of the element DD1.1 (two elements of the DD1 chip are connected in parallel) corresponds to log.1, and at the outputs - log.0. Capacitor C2 is discharged, at the output of element D1.3 log.0 - the key on the unipolar transistor of the DA1 microcircuit and the transistor VT1 are closed. The current consumption from the line does not exceed 200 μA at the time of charging the capacitor C1 and decreases to 185 μA when it is fully charged.

Blocking mode. When the handset is picked up on the main or parallel device, the voltage at the inputs of the DD1.1 element decreases to a value less than the log.0 level, and the log.1 voltage appears at the outputs. The charging of the capacitor C2 begins with a time constant (t = C2-R9) of about 47 s. In the process of charging C2, the charging current creates a voltage across resistor R9 that is greater than the response level of element DD1.2. As a result, the output of the element DD1.3 (output 4) is set to log.1. This opens the key DA1 and the transistor VT1. When VT1 is opened, the line is shunted by resistor R2 and LED HL1, the voltage in it decreases and dialing becomes impossible. The LED indicates when the handset is lifted on the main or extension telephone.

After a certain time, the current in the circuit of the resistor R9 drops and the elements DD1.2 and DD1.3 switch to a state in which the key DA1 and the transistor VT1 are closed. The shunt load is removed from the line. If the handset is still off-hook, nothing else will happen until another attempt is made to dial the number. In this case, the first two interruptions of the dialer lead to the charging of the capacitor C2 and the process of turning on the shunt load is repeated. If the tube is lowered, then the voltage on the line exceeds 60 V and the device goes into standby mode. Capacitor C2 is discharged through resistor R8 and diode VD6 with a time constant of about 47 ms.

Normal operation of the device is possible only when the blocker is turned off, which protects the phone in your absence from attempts to "pirate" use of the line.

In the device, as a digital microcircuit DD1, you can use 564TL1, KR1561TL1, inverters without hysteresis KR561LA7, KR561LE5, as well as inverters with increased load capacity K561LN2.

Transistor VT1 and key DA1 must be designed for the action of the line voltage during a ringing signal (more than 90 V), therefore, it is unacceptable to use the indicated microcircuit with the letter index A. The transistor can be replaced with KT851A, KT851B, KT9115A.

The blocker is assembled by volumetric mounting without the use of a board and is placed in a telephone plug-adapter.

Literature

S. Yankovsky. Phone watchman. - RadioAmator, 1996, No. 7, p.5

Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Telephony.

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