Schematics > Telephone related > Telephone line monitor
Telephone line monitor
If you feel that somebody is tampering with your telephone line you might
find this little circuit useful. It detects if there is another telephone
connected to the line, if there is a short or an open line. Sound and a flashing
light will tell you which is the current situation. The speaker is practically
cut out during a normal conversation thus preserving privacy, only the LED will
flash occasionally. The circuit does not require any battery and takes the
supply from the telephone line itself. The transistors used are wired in a
reversed biased fashion thus behaving as oscillators. You might try the 2N2222A
as an alternative (not tested). This monitor is, of course, suitable only for
analogue lines. Watch the polarity of the input line: the circuit will not be
damaged by a polarity reversal but it will not operate correctly.
Title: Telephone line monitor
Source: smartkit
Published on: 2005-02-10
Reads: 300
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