Phone Line In Use Indicator

Introduction

The function of this fun electronic project design is to indicate whether a phone line that is connected to many phone extension is in use or otherwise. It is an interesting project for beginners to electronics hobby as there are only a few commonly available parts that are used in this circuit.

Circuit Description

The main part of this project is the use of an N channel power MOSFET IRF511 that has a current rating of 5.0 A at room temperature and drain to source voltage of 60V. If the phone is offhook, it will have a tip to ring voltage of 48V DC. When this voltage is applied to the gate of Q1, it will turn Q1 ON which in turn will bias the Q2 transistor to turn OFF. When Q2 is OFF, LED will also be OFF which means that the phone line is not in use.

When the phone hook is remove, the voltage will drop and cause Q1 to turn OFF. When Q1 is OFF, the gate of Q2 will be biased ON and thus transistor Q2 will turn ON. When Q2 turn ON, LED will also turn ON indicating that the phone line is in use.

Phone Circuit Parts List

Source : Extracted from Popular Electronics Feb 1993, By Charles D. Rakes

ref: electronics-project-design.com/fun-electronic-project.html