The circuit shown in the figure can be useful in cases where a common telephone extension is used. This circuit activates a relay when any extension is off the hook. The circuit can be used to disconnect the other extensions of the line to avoid listening. Another possibility is to control automation, such as turning on a light, when the phone is answered. In this circuit it may be necessary to adjust the values of the components to obtain the best performance. Thus, the voltage in the MOSFET gate must be between 1 and 3 V negative when the telephones are on the hook and must rise to values between 6 and 10 V when they are removed. The relay used must be 12 V with a current of 50 to 100 mA and the source used must have good stabilization. Any N-channel MOSFET can be used in this application.
