Telephone Free Indicator


Posted on Feb 4, 2014

Depending on local regulations and the telephone company you happen to be connected to, the voltage on a free telephone line can be anything between 42 and 60 volts. As it happens, that`s sufficient to make a diac conduct and act like a kind of zener diode maintaining a voltage of 38 V or so. The current required for this action causes the green h


Telephone Free Indicator
Click here to download the full size of the above Circuit.

igh-efficiency LED in the circuit to light. Line voltages higher than about 50 V may require R1 to be changed from 10 k to a slightly higher value. When the receiver is lifted, the line voltage drops to less than 15 V (typically 12 V) causing the diac to block and the LED to go out. The circuit diagram indicates + and with the phone lines. However, in a number of countries the line polarity is reversed when a call is established. To make sure the circuit can still function under these circumstances, a bridge rectier may be added as indicated by the dashed outlines. The bridge will make the circuit independent of any polarity changes on the phone line and may consist of four discrete diodes, say, 1N4002`s or similar. Finally, note that this circuit is not BABT approved for connection to the public switched telephone network (PSTN) in the UK.




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    RF data link and remote control
    Pseudorandom-sequencer
    Oscilloscope triggered sweep circuit
    Measure your stress level Tension meter Schematic Diagram
    Baxandall Tone Control Circuits Using Two Transistor
    24V flasher circuit
    lm324 wave generator vco schematic
    Electronic Lock
    sonar
    ATMEGA16 8-bit AVR Microcontroller with 16K Bytes In-System Programmable Flash
    Complete Ar/Kr Ion Laser Power Supply Schematics
    Propeller-powered IN-12 Nixie Clock
    NAD C 520



    Top