Security & Saftey

Modem Line Protection Schematic Circuit Diagram

We haven’t yet experienced this first-hand, but it seems to occur more often than when some websites are visited they switch the Internet connection over to a premium rate dial-up number. This is of course very irritating because it is hardly noticeable and it certainly costs a lot more. This circuit is simplicity itself, but still offers effective protection against these practices. The circuit consists of nothing more than a push-to-make switch and a reed relay with a home-wound coil and is connected to the phone line in series with the modem. During the dial-up, the switch has to be pushed down for a while (a bit earlier really, or otherwise the modem won’t be able to detect the dial tone). Once the modem is ‘off hook’, the pushbutton can be released. The current in the phone line then keeps the reed relay energized, maintaining the connection.

Modem Line Protection Schematic Circuit Diagram

When an attempt is made to switch to a different number, the line has to be dropped first, no matter how quickly this happens. The reed relay opens at any interruption of the current, stopping these tricks in their tracks. No matter how clever the programmer is at the other side, this simple hardware protection cannot possibly be circumvented! A guideline for the construction of the reed relay coil is to wind about 200 turns of 0.1 mm or 0.2 mm copper enameled wire round the relay. The complete reed relay can also be bought from the author via his website: http://home.worldonline.dk/ ~wildsto/sdb/.

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