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Off-Hook Indicator For Telephone Lines

A "busy" indicator to prevent annoying interruptions to internet access.

By John Clarke

How many times have you been on the phone and some one else has picked up another phone and begun dialling? It’s pretty annoying, isn’t it?

How many times have you done the same thing to them? That’s not so bad, is it? Or how many times have you been almost finished down-loading a large file from the Internet and someone has picked up an extension and started dialling? Now that’s really frustrating.


Please note that the Off-hook Indicator is NOT an Austel-approved device. The penalty for using such a non-approved device, if detected and subsequent prosecution took place, could be a heavy fine, up to $10,000.

Click for larger image
Shown here bigger than life-size, the Off-Hook Indicator is designed to connect "in line" between the wall socket and the phone via a pair of RJ-12 modular (American-style) connectors. Most modern phones now have these connectors.

Of course some people may argue that this is a good reason to get a second phone line and indeed it is but the same thing still occurs in homes with two phone lines. We can vouch for that. It happens in offices too, where a fax machine can be connected to the same line as a computer modem. It’s certainly happened in our office and there have been howls of anguish when the victim has realised what has been done.

Mind you, in our office there’s no excuse because our telephone system indicates which lines are busy. Wouldn’t it be good if there was a similar doodad you could fit to any phone line? Well, now there is and you can build it. It will not affect telephone, fax or modem operation in any way and draws negligible current from the phone lines.

Called the "Off hook Indicator", it is a little plastic box which flashes a LED whenever your phone line is in use. As the name suggests, it also solves another problem with tele-phones and that is when they are left off-hook. The flashing LED will remind you that the phone is off-hook and will be "engaged", preventing outside callers from reaching you.

The Off-hook Indicator is housed in a small plastic case with a US modular phone socket (RJ12 6P6C) at each end. These enable the Off-hook Indicator to be connected in-line with your telephone, modem or fax machine. The indicating LED can be seen at the top of the lid.

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