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.