telephone in use indicator

Posted on Feb 4, 2014

Please note that it is illegal to make a physical permanent connection to your telephone linein some countries (this includes the UK and Ireland). If building this circuit it is advisable touse a plugin cord so that the unit can be unplugged should a fault occur. If in doubt consult either your telephone or cable operator. If all extension phones

telephone in use indicator
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are on-hook and the line voltage is around 48V, Q1 will conduct thus effectively shorting the gate of Q2 to itssource, so it will be off and the LED will be disabled. Lifting the handset of any phone on the line causes the line voltage todrop to 5-15 V. The gate voltage of Q1, equal to some 6% of the linevoltage, will then be too low and Q1 will be turned off. So Q2`s gateis now biased at approximately 1/2 of the line voltage, Q2 turns on andthe LED indicates that the line is in use. The circuit itself ispractically invisible to the other telephone devices using the sameline. LED1 must be low-current and its current-limiting resistor must be 2k2or more. The other components` ideal values may vary slightly, depending on the local telephone line parameters. The circuit ispowered off the telephone line. If other types of MOSFETs are used, the 500k trimmer can be adjusted toensure that Q1 is biased fully on while the line is not in use (LED1off), and vice versa. If Q2 is not a BS108 but some other 200 V MOSFETwith a higher G-S threshold voltage, it might be necessary to increasethe value of the lower (or decrease the value of the upper) one of thetwo resistors connected to the gate of Q2. Plain (bipolar junction) transistors can be used instead and thecircuit also works fine, but the resistor values are then much lower -letting ten times more microamps of current pass through while the lineis not in use, and even this MOSFET...

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