The indicator of listening of conversation on parallel THAT
<= = "2"> the Device is intended to font face by "Arial" size for its use on the telephone line with connected by two parallel THAT and allows to have indication that one more tube, except that through which you speak is removed.
the Principle of work of this circuit is based by font on use of change of tension in the telephone line at the removed tube on two parallel THAT at the same time. If the tube only on one THAT is removed, the circuit works
the same as described above — HL1 light-emitting diode is shone, and VT1 and VT2 transistors can be absent (fig. 2.4). At tube removal for one THAT tension in the line will change approximately on 1... 5 In (it depends on internal resistance of the established telephone sets).
the Fig. 2.4
the VT1 Transistor is the analyzer of level of tension in the line, and its mode is established by the R2 resistor (rough control is made by the R3 resistor) so that at a voltage reduction to lines on 0,5 In it from a mode of saturation passed to the locked condition that should lead to saturation of the VT2 transistor and the termination of a luminescence of a light-emitting diode. It also will be the indicator of that your conversation, probably, is listened on parallel THAT. In a normal state at the removed one tube HL1 should be shone.
the Topology of the printed-circuit board and an arrangement of elements on it are given by font on fig. 2.5. The design of a payment is calculated on that it was located in a standard telephone nest (see the case shown on fig. 2.3). R2 resistor small-sized, SPZ-19a type.
the Device previously adjusted by a technique for the circuit on fig. 2.1, is connected to the line, observing polarity, and final control carry out on T the R2 resistor during the moment when two receivers are removed at once. Thus the light-emitting diode of HL1 should die away.