indicator circuit

Posted on Feb 5, 2014

The modem off indicator is intended especially for serious Internet surfers. It will be seen that the circuit of the indicator cannot be much simpler, or there might be nothing left. In spite of its simplicity, it may prove to be a cost-saving device, since it shows at a glance whether the telephone line is free again after the modem has been used

indicator circuit
Click here to download the full size of the above Circuit.

. This obviates high telephone charges in case for some reason the modem continues to operate. The circuit depends on the fact that there is a potential of about 40 V on the telephone line when it is not busy. This voltage drops sharply when a telephone call is being made. If, therefore, the circuit is linked to telephone terminals a and b, the lighting of the green LED shows that the line is not busy in error. The bridge rectifier ensures that the polarity of the line voltage is of no consequence. This has the additional benefit that polarity protection for the LED is not necessary. To make sure that the telephone line is not loaded unnecessarily, the LED is a high efficiency type. This type lights at a current as low as 2 mA, and this is, therefore the current arranged through it by resistor R1.

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    Radio Glen Transmit Selector system
    Electronic Voltmeter Ammeter Circuit Using Single IC 741
    sleep watchdog battery
    Dynamic Microphone Preamp PCB
    Versatile Op Amps Need No Resistors
    LM2896 Car Audio Amplifier Circuit
    Switchmode Constant Current Source
    Simple Door Alarm Circuit
    Magneto Remote control switch circuit diagram
    Dimming circuit using capacitance