Sunday, October 7, 2012

The Surfer’s Preserver


The Surfer’s Preserver is a simple device that prevents other phones in the house from disrupting your critical Internet session by disconnecting them from the line while you surf! The circuit is also useful in preventing eavesdropping from other extensions since other phones are "dead" until you hang up.  The circuit wires in series with either of the offending phone’s wires (red or green) and it is small enough to tuck behind the wall cover plate.

Circuit Description: Due to the resistor divider, the SCR will not fire unless there is at least 17 volts across the bridge. When the receiver is lifted, the full line voltage appears across the circuit and the SCR triggers. The SCR will remain triggered since the DC phone current is about 25 mA and the SCR holding current is only about 5 mA. If the phone line is in use when the receiver is lifted, the line voltage is insufficient to trigger the SCR and the phone remains disconnected. When the phone rings, the 17 volt threshold is quickly passed and the SCR triggers early in the ring voltage cycle, supplying a nearly full amplitude ring voltage to the phone.



A momentary push-button switch may be added across the 33k resistor to manually trigger the SCR so that the phone can connect when another phone is off-hook. This push-button could be mounted in the wall plate if the plate is in a convenient location or the circuit could be built into the telephone itself with a small switch added on the side. This push-button is handy if more than one telephone is on the line.

Other SCRs may be substituted as long as their working voltage is above 150 volts and their holding current is well below your phone’s current. Connect a current meter in series with your phone to determine your current - expect about 25 to 30 mA.

The circuit requires that the modem or other phones pull the line voltage below 17 volts when off-hook. A simple voltage check will determine if the voltage is dropping low enough. It will typically drop to 5 volts. If you must raise the trigger voltage, increase the 33k resistor. A very high resistor value may reduce the ringing volume on older phones or prevent normal phone use.

A separate circuit may be constructed for each phone or one circuit may be used to disconnect several phones. To use one circuit for several phones, make sure that they share a common wire not shared with the modem. Place the circuit in series with the common wire. The advantage of this connection is that the push-button is not needed to transfer a call from one phone to another but some custom wiring may be necessary.

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