Electronic subscriber telephone lock - has dialling pulses fed to rectifier bridge to retain uniform polarity in event of line reversal and permits use of local or trunk calls

摘要

The electronic lock for a subscriber telephone controls the time and type e.g. local, trunk, of calls which can be made and is switched on or off by the subscriber by lock. The lock is connected to the telephone lines through a full-wave bridge diode rectifier to receive the pulses generated by the phone dial during dialling. The bridge provides the same polarity signal even when the line polarity is invented. The pulses are rectified and integrated by the capacitor coupled diode resistor integrators. The result is amplified and applied to a NAND gate integrated circuit (2A) in turn coupled to a counter (2B) whose output diodes (D25-28) decode the resulting count of the NAND gates signal. The counter output is fed back to the circuit input so that, depending on the count, the output of the counter is blocked or not to preset or allow dialling to continue.