# Phone Line In use LED

Posted on Feb 11, 2013

If we have a lot of parallel telephones on a telephone line, we needed a unit that will have the possibility of showing if somebody of the telephone has raised the earphone. Our this possibility give the circuit. The circuit is connected at parallel with the two cables of telephone line. The bridge of rectification D1- D4, protect and ensure the circuit from error polarity connection. If no telephone is not raised, then voltage in telephone is 50V until 70V roughly. This voltage via the bridge and resistors R1-2 turn on the gate of Fet Q1, in order that this not open. As soon as is raised a telephone then the voltage in line fall, Q1 turn on and the D6 it lighten, showing the situation of line.

Because of the Q1 and R3 the current that leak Led D6 it is limited in 10mA. The diode zener D5 prevent the voltage of gate of Q1, to pass above 10V, while capacitor C1 function as filter for parasitic pulses. If they exist problems of adaptation and operation of circuit should is replaced the R1 with other of bigger price (until 220K). The himself is also in effect for R3, who can be suppressed, if the circuit work and without the R3. The circuit function with supply + 3V until + 9V, from battery, one and the consumption is enough small. Part List R1=1Mohms C1=1uF 63V D6=Led RED R2=100Kohms see text D1-2-3-4=1N4001 Q1=BF256B R3=47 ohms D5=10V 1W Zener B1=3V.....9V Battery

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