cameroon gce advanced level 2025 design of electronic systems 2

TITLE: AUTOMATIC SWITCH OVER OF TELEPHONE RECEIVERS

I- DESCRIPTION OF THE SYSTEM

Two telephone receivers connected to the same telephone line are installed in the master’s room and in the parlor of an apartment in Buca. The landlord seeks your expertise to prioritize one of the telephone receivers. When a call comes, telephone A should ring. If no body picks the call after four rings, the system automatically switch the call to telephone B located in the parlor.

II- OPERATING PRINCIPLES

The input from the telephone line drives a bridge rectifier whose output drives the following inputs

• It directly connects the first telephone receiver.
• A voltage divider circuit where one – sixth of the telephone line voltage is used to drive the inverting input of an operational amplifier that operates as a comparator. The non inverting input of the comparator is connected to a voltage divider circuit that fix the voltage at this input at half the supply voltage. When the telephone line is idling, a dc voltage of 50V is found on the line. When a call comes, the 50V dc and a 50Vac of frequency 50Hz are found on the line. When a receiver is picked, the telephone line voltage drops to 15V. The output of the comparator is used as one of a inputs to a NOR gate.
• The base of an NPN bipolar junction transistor through a voltage divider circuit. One – twenty of the telephone line voltage is used to drive the base of the transistor through a capacitor.

The collector of the transistor is used to drive a two inputs NOR gate. The output of the NOR gate drives an RC integrator circuit. The output of integrator is used to drive two NOR gates connected in series. The output of the two NOR gates connected in series drives the following:

• A Schmitt trigger whose output serve as the clock to a counter
• Another integrator circuit whose output passes through another Schmitt trigger and the output of the Schmitt trigger serve as one of the inputs to the NOR gate that the comparator output is connected to. The output of this NOR gate serve as the RESET signal to the counter.

After four counts of the counter, a relay is energized and causes a switch over of the receiver.

III-FUNCTIONAL SPECIFICATION

The device is powered from a 12V, 1A transformer regulated power supply. A red LED indicates the presence of power supply.

The comparator is designed with µA741

The first RC integrator circuit has a charging time constant of 2.2 seconds and a discharging time constant of 1 second.

The second integrator circuit has a charging time constant of 8.46 seconds and a discharging time constant of 2 seconds.

The Schmitt triggers are designed with NOR gates.

The system uses 4017 as the counter.