This circuit continuously checks the tel. lines and when it detects a misuse, it sounds an alarm. Additionally it transmits a loud disturbing tone through the telephone lines to prevent further misuse. The circuit does not require any external power supply.
When the switch S1 is at the on position, the line voltage at the output of the rectifier diodes is approximately 48V which is enough to exceed the zener's (D5) break down voltage. So the T2 gets forward biassed. As a result the base of T1 is grounded and the remaining part of the circuit doesn't get any supply. In this condition, circuit draws so small current that is negligible and does not effect the normal operation of the telephone line.
When handset of any telephone connected to the line is lifted, the line voltage drops to 10V so T2 is switched off and T1 gets in forward biassed mode. The astable multivibrator circuitry starts oscillating and the speaker starts sounding. The output of the astable multivibrator is also connected to the base of T1 so a loud sound is heard in the ear piece of the unauthorized telephone instrument.
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