Phone line indicator using BF256B

With this circuit can determine whether any of the phones of the same line is busy (it's up the handset), with the help of a LED.To circuit has no measurable effect on the telephone, so that there are problems with phone line company . The circuit is connected in parallel with the two wires of the telephone line. The bridge rectifier (D1. .. D4) at the entrance to protect the circuit from incorrect polarity. When none of the phones of the same line is not occupied then the voltage on the line is about 50 ... 60 V.I trend means the bridge and the voltage divider R1/R2 influences BF256B gate so that it will not conduct. Once you get up the handset of a device then the trend line is falling rapidly, and T1 conducts and LED lights, giving an indication of `busy`. Average T1 current flowing through the LED is limited to about 10mA.
Phone line indicator using BF256B - schematic

The Zener diode D5 prohibits the gate voltage of 10V to pass while the capacitor C1 acts as a filter for the parasitic pulses. If there are problems in the circuit should be replaced by the resistance R1 (220KO to be reasonable enough to adjust the voltage on the line). The circuit operates comfortably with a 9V battery which together with the board can be placed in a small plastic box. Attention, the 50V line are true low voltage but nevertheless under certain conditions can be dangerous.

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