Phone detector used on the master phone no effect, when the owner was suspected of tapping during a call, you can press the phone key spring, if found call indicator light on the detector, then was tapped, then, the phone surveillance inspection devices will sound an alarm to warn of the wiretapping, and make it unusable.
Phone detector circuit as shown below. When you hear a 48V voltage by R1, DW turns on the BG1, BG2 close to 0V and the potential base closing, BG3, BG4 are cut-off does not work.
When the unauthorized use and machine use in the outside line when the line voltage less than 12V, DW can not be breakdown, BG1 end, BG2 conduction by BG3, BG4, C2, R6 oscillation circuit composed to work, when light emitting diode LED , indicating that the phone was stolen, an alarm tone oscillator on the line. Pick up the telephone in the master, though also outside the voltage drop, but because the phone to generate current in R8 voltage of about 1V, so BG5 conduction through R7, BG2 close to 0V and the potential base closing, BG3, BG4 not oscillation, which does not affect the normal use of the owner.
This circuit is very simple to install, you can not debug the work should be noted that BG1, BG2 should be used in a small silicon tube leakage. use the line-side and the telephone line connecting the positive terminal, and the negative terminal phase, phone users can be terminated.