Modem Off Indicator


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. 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.

Circuit diagram:

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.

WARNING.
In spite of the liberal age we live in, it is highly probable that in
many countries it is not allowed to connect the indicator across the
telephone lines. Seek advice of your local telephone company that owns
or operates the telephone network.


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