Intercity blocker. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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This device is designed to prohibit long-distance communication from a telephone set, which is connected to the line through it. The device is assembled on the K561 series IC and is powered by a telephone line. Current consumption - 100 ... 150 μA. When connecting it to the line, the polarity must be observed. The device works with automatic telephone exchanges having line voltage 48-60V. Some complexity of the circuit is caused by the fact that the algorithm of the device is implemented in hardware, in contrast to similar devices [1], where the algorithm is implemented in software using single-chip computers or microprocessors, which is not always available to a radio amateur.
The functional diagram of the device is shown in Fig.1. In the initial state, the SW keys are open. The SLT is connected to the line through them and can receive a ringing signal and dial a number. If, after picking up the handset, the first dialed digit turns out to be an index for long-distance communication, a standby multivibrator is activated in the control circuit, which closes the keys and breaks the loop, thus making the PBX clear. The intercity exit index can be anything. In this scheme, the number "8" is set. The time for disconnecting the device from the line can be set from fractions of a second to 1,5 minutes.
(click to enlarge)
Schematic diagram of the device is shown in Fig.2.
On the elements DA1, DA2, VD1 ... VD3, R2, C1, a 3,2 V microcircuit power supply is assembled. Diodes VD1 and VD2 protect the device from incorrect connection to the line. On transistors VT1 ... VT5, resistors R1, R3, R4 and capacitor C2, a telephone line voltage level converter is assembled to the level necessary for the operation of MOS microcircuits. The transistors in this case are included as micropower zener diodes with a stabilization voltage of 7...8 V at a current of several microamperes [2]. On the elements DD1.1, DD1.2, R5, R3, a Schmitt trigger is assembled, which provides the necessary steepness of the fronts of the dial pulses. Elements DD1.3, DD1.4, C3, C4, R6 form a clock generator with a frequency of about 80 Hz. On microcircuits DD2, DD3, a "sensor" of the tube position is assembled, and on DD4.1 - a dial pulse selector.
The counting of the pulses of the set is carried out by the counter DD6. ICs DD4.2 ... DD4.4, DD5 form a circuit that allows the counting of pulses when dialing the first digit of the number and prohibits counting pulses when dialing subsequent digits. On the IC DD7, DD8, a waiting multivibrator is assembled that controls the keys DA3 and DA4. Switch SA1 is used to turn off the device. Capacitor C4 is needed to improve the initial start of the generator.
Literature
- Radio, 1996, No. 10.
- Perlov V. Transistors and diodes as zener diodes. - Radio, 1976, No. 10.
- Biryukov S.A. Digital devices based on MOS integrated circuits. - MRB. Issue 1132, 1990.
Author: S.Kopylov, Tiraspol; Publication: N. Bolshakov, rf.atnn.ru
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