Advanced Phone in use circuit

Posted on Jul 11, 2012

This project has two features. It will let you know when a single phone is using the line and also when two phones are on the line! The green LED indicates the first phone and the red LED indicates the second phone! There are many times when you need to know the state of the phone line and this project will prove to be invaluable. It will prevent you picking up the phone when the extension is being used but most important it will indicate when the extension has been picked up when you are on the line. In some cases the extension will make no difference to the sound level on the main handset and the extension party can listen in without being detected.

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There are quite a number of PHONE ALERT products on the market and although I have not seen them all, the ones I have tested failed to work. They failed because no one has ever tried them! The phone system is much more complex than you think and when a mechanical and electronic phone is connected to the same line (the same two wires), it is not possible to detect the electronic phone when the mechanical is in use. The reason is the mechanical phone drops the line voltage to about 3-5v while the electronic phone drops the line voltage to about 15v. (when they are on the line separately). This means they are different impedances (different resistance) and when they are on the line at the same time, the electronic phone drops the line voltage only about a further .5v over that of the mechanical phone. This is a very small change and since the voltage across the mechanical phone is not always the same, there is no way of setting a detection point. It is not possible to detect the extension by using a series LED or parallel LED to cover all the possible combinations where the detection unit is placed with respect to the high and low impedance phones. Furthermore, any circuit using the phone line to drive the indicator LEDs will result in very poor brightness as the line does not have enough energy in reserve, when the phone is being used. With a mechanical phone there is only about 3-5v available and you can only draw...

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