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Circuit Diagram of adding visual judgment function to teleph

2014-11-02 10:47  
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Circuit Diagram of adding visual judgment function to telephone ringing
HA868(Ⅲ) P/ TSD telephone, adds a resistance of 10K Ω indirectly between C301 and R301, and connects the anode of diode IN4004 with the other side of the resistance while connects the cathode of the diode with the anode of LED. It is shown by dotted line in the picture. When the telephone works, the current goes through R201 and make the LED lighted. Because IN4004 is reversely biased, and prevents the current from flowing to ringing circuit, the revised part has no affection towards working indication light. When it starts ringing, the ring flows through C301, resistance of 10KΩ and diode IN4004, and becomes a loop via LED and D304. LED then twinkles and gives out light. Since the shunt of the ringing indication is very small, the ring is obviously affected.