Phone-Use Indicator Design

Related documents: Project Overview, Final Report

As a member of the Honors College at Michigan State each student must complete eight honors classes or honors options. In my ECE 302 class (Electronic circuits), I completed an honors option which consisted of re-designing a phone in-use indicator illustrated in a popular electronics article from 1998. You can read the full overview and my final report by clicking on the links under related documents above or continue reading a shortened version below.


A phone-use indicator or phone busy indicator is a simple circuit which identifies whether or not a phone line is currently in use. When the phone line is currently being used a red LED is lit. When the phone line is not in use a green LED is lit. This circuit requires no external power and is often mounted inside of the phone, but it can be connected anywhere on the phone line.

Inital Schematic

Design Phases

  1. Modify the circuit to work with the instructor's phone

    For the circuit to work with the instructor's panasonic phone the in-use voltage must be raised to at least 13V, and the circuit cannot draw more than 3 mA of current or else the operator will come online. A quick inspection of the circuit reveals the voltage divider between R3 and R5 determines the cutoff voltage of Q2. In order for the ciruit to work with the instructor's phone, Q2 must cutoff below 13V. Letting R5 stay at 4.7kΩ the equation, 4.7k/(R3 + 4.7k)*13 = 0.7 (Assmuing a 0.7V BE drop) can be solved for R3 = 82.5kΩ. Using the nearest standard value of 100kΩ for R3 raised the cutoff voltage to 14.05V thus satisfying the requirements.

  2. Simulate the circuit using PSpice and compare with measured results

    My PSpice simulation files and results can be seen in the final report.

  3. Total re-design of the circuit using only one transistor.

    In the new circuit the one transistor must be thought of as a toggle switch. One LED must light when the transistor is in saturation and the other must light when the transistor is in cutoff. This is acheived by removing Q1 from the initial schematic, moving the red LED into parallel witht he remaining transistor and adding a resistor in series and one in parallel with the green LED all while keeping the same voltage divider between R3 and R5. This voltage divider keeps the cutoff voltage at 14.05V satisying the requirements in phase one. The final schematic is shown below.

Final Schematic


This project required knowledge on how BJT transistors work which is a topic covered in ECE 302. Overall, this project was simple armed with that knowledge, but it did require a little bit ingenuity to put it all together. It was a good experience and one of the better honors options I completed.