The indicator of listening to the conversation on the parallel telephone set. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The indicator of listening to the conversation on the parallel telephone set. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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The device is designed to be used on a telephone line with two parallel SLTs connected and allows you to have an indication that another handset is off-hook, in addition to the one you are talking through.

The principle of operation of this scheme is based on the use of voltage changes in the telephone line when the handset is off-hook on two parallel SLTs simultaneously. If only one SLT is off-hook, the circuit works in the same way as described above, the HL1 LED is lit, and transistors VT1 and VT2 may be absent (Fig. 2.4). When you pick up the handset on another SLT, the voltage in the line will change by about 1 ... 5 V (this depends on the internal resistance of the installed telephones).

Listen-in indicator on an extension telephone set
Fig. 2.4

Transistor VT1 is an analyzer of the voltage level in the line, and its mode is set by resistor R2 (coarse adjustment is made by resistor R3) so that when the voltage in the line drops by 0,5 V, it goes from saturation mode to the locked state, which should lead to saturation of the transistor VT2 and stop the glow of the LED. This will be an indicator that your conversation may be monitored on a parallel SLT. In the normal state, when one tube is removed, HL1 should light up.

Listen-in indicator on an extension telephone set
Fig. 2.5

The topology of the printed circuit board and the arrangement of elements on it are shown in fig. 2.5. The board is designed to fit in a standard telephone jack (see the housing shown in Figure 2.3). Resistor R2 is small-sized, type SPZ-19a.

The device is pre-configured according to the method for the circuit in fig. 2.1, is connected to the line, observing the polarity, and the final setting is performed on the TL with resistor R2 at the moment when two handsets are off at once. In this case, the HL1 LED should go out.

Publication: cxem.net

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