Telephone in use indicator


Posted on Feb 6, 2014

Here is a simple circuit that can be used as a telephone status indicator. When the telephone is in use (off hook) the transistor Q1 switches ON making the red LED D2 glow. When the telephone is not in use (on hook) the Q1 turns OFF and Q2 turns ON. This makes the red LED D2 off and green LED D3 ON. The circuit is powered from the phone line itsel


Telephone in use indicator
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f and no external power supply is required. That is a nice circuit. Also a telephone line checker. In North America telephone system, red wire is negative and green wire is positive. ( Use a multimeter ) In-hook ( no phone is pick up ) The tel line content a DC AND AC. DC about -48 to -60, AC is about 90 to 120. On-hook, the DC is -10 V and no AC, just a phone signal ( talk, dial-tone, modem signal ). The circuit simply use the DC part of the phone line. Useful to check the phone line at your home. I don`t know about European system or Asia system.




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