Low frequency telephony pulse detector - detects test signal in synchronism with strong active signal presence

摘要

The circuit is used in telephony when line checking at 50Hz and 450Hz. The locally generated signal at high level is connected to a bridge rectifier (D4-7) and it provides the supply to a transistor detector stage (T5). The test signal after attenuation through line equipment is amplified and squared (T1, T2) and slaves a monstable multivibrator (T3, T4). This square-wave passes to the detector and if the stage is active (i.e. the high level signal is present) a diode and capacitor circuit (D2, C7) links it to an output transistor that provides an output signal and energises a light-emitting diode. This circuit protects against false readings caused by parasitic voltages by its system of coincidence detection.